Mutation

Mutation
Definition
Any change in the amount, structure and content of genetic material is called Mutations.
Mutations can appear in both sex chromosomes as well as in autosomes.
Types of Mutations
There are two main types of mutations.
1. Chromosomal Mutation
2. Gene Mutation
1. Chromosomal Mutation
The change in amount arrangement and the nature of genetic material on a chromosome is called Chromosomal mutations. It is also called Chromosomal aberration.

• This mutation is visible under the microscope.

Types of Chromosomal Aberration
There are following types of this mutation.
i. Deletion
ii. Duplication
iii. Inversion
iv. Translocation
i. Deletion
Definition
When a small portion of a chromosome is missing the situation is called Deletion.
Effects of Deletion
Pseudo-Dominance
Deletion may cause Pseudo dominance in heterozygous condition.
Lethal Effect
If deletion takes place in both homologous chromosomes then it has the lethal effect on the organism.
ii. Duplication
Definition
The repetition of a segment on a chromosome is called Duplication.
Effects of Duplication
Due to the duplication different physiological and morphological functions are disturbed.
iii. Inversion
Definition
When the arrangement of genes on a chromosome is changed then the mutation is called Inversion.
Effect of Inversion
Inversion reduced crossing over.
iv. Translocation
Definition
The transfer of a chromosomal segment to a non-homologous chromosomes is called Translocation.
Effect of Translocation
Translocation may give rise to varieties within species.
2. Gene Mutation
Definition
When small changes occur in the molecular structure of DNA, these are called Gene-Mutations.

• This mutations can not be detected by the microscope.
• These changes can produce drastic changes in the expression of the genetic messages.

Types of Gene Mutations
There are following types
i. Point Mutation
ii. Transposition
i. Point Mutation
Definition
The change of the sequence of one or a few nucleotides is called Point Mutation.
ii. Transposition
Definition
Individual genes may move from one place to another place on their own chromosome which is called Transposition.
Effects
This chromosomal rearrangement often brings alternation in the expression of the genes or that of neighboring genes.
DNA Damage (Causes of Mutation)
There are three major important causes of DNA damage, they are
1. Ionizing Radiation
2. Ultra Violet Radiation
3. Chemical Mutagens
1. Ionizing Radiation

• High energy radiations such as X-rays and Gamma rays are highly mutagenic Nuclear radiation is also of this sort.
• These radiations release unpaired electrons which are called free radical.
• These free radicals are highly reactive chemically, reacting violently with the other molecules of the cell including DNA.

2. Ultra Violet Radiation

• Ultra violet radiation is the component of sunlight.
• When molecules absorb UV radiation little damage is produce in these molecules.
• Mostly certain organic ring compounds are affected by UV-radiation.

3. Chemical Mutagens

• The chemicals which are capable of damaging DNA are called Mutagens.
• There are three main types of mutagens.
• Chemicals resembling DNA nucleotides but pair incorrectly when they are incorporated into DNA.
• Chemicals that remove the amino group form Adenine or cytosine, causing them to pair wrongly.
• Chemicals that add hydrocarbon group to nucleotide bases also causing them to pair wrongly.

Hereditary Diseases Due to DNA Damage
The two main hereditary disease due to DNA damage are
1. Sickle Cell Anaemia
2. Phenylketonuria
1. Sickle Cell Anaemia
An inherited autosomal recessive condition that causes abnormal haemoglobin in blood cells, leading to defective oxygen carrying, infections or organ damage is called Sickle Cell Anaemia.
Cause
The disorder occurs due to presence of abnormal haemoglobin (Glutamic acid is replaced by valine in β-chain at 5th position). It is produced due to mutation in gene. By the activity of changed gene abnormal haemoglobin is formed and sickle cell disease is produce.
Symptoms

• The abnormal haemoglobin has loco binding capacity with oxygen.
• The R.B.C containing the defective haemoglobin deform to a sickle shape during relative oxygen scarcity, haemoglobin molecules become insoluble and combine with one another forming stiff, rod like structures. Because of stiffness and irregular shape, these R.B.Cs may form clots blocking the small vessels.

2. Phenylketonuria
An inborn error of metabolism caused by the lack of an enzyme which breaks down amino acid phenylalanine, resulting in conversion of phenylalanine to other chemicals is called Phenylketonuria.
Causes
Phenylketonuria is a ressisive disorder caused by a mutant allele of the gene encoding the enzyme that normally breaks down phenylalanine. Only individual homozygous for the mutant allele development the disorder. It is because of the point mutation.
Symptoms

• The abnormal derivatives of phenylalanine are harmless because they interfere with the development of brain cells causing irreversible brain damage to the infants and can lead to severe, progressive mental retardation.
• Affective individuals rarely live for more than 30 years.

Treatment
It can be treated, if early detected by restricting the diet and avoiding phenylalanine.

Decoding

Decoding
Definition

Messenger RNA (m-RNA) contains gentic code in three nitrogen bases and t-RNA contains anticodon triplet and it transfers amino acids to the ribosome, if anticodon triplet is attached the codon triplet of m-RNA. This process is called Decoding.

Gene Expression

Gene Expression
Definition
All functions in the body of an organism are controlled by genes. A function expressed or performed by a gene is called gene expression.
Process of Gene Expression
The process of gene expression occurs in two phases
1. Transcription
2. Translation
1. Transcription
Definition
The process in which an RNA copy of the DNA sequence encoding the gene is produced with the help of an enzyme RNA polymerase is called Transcription.
Step of Transcription

• Transcription is initiated when a special enzyme called RNA polymerase binds to a particular sequence of nucleotide on one of the RNA strands. This strands is known as template strands or Antisense strands while the other strand is called coding or sense strand.
• RNA polymerase proceeds to assemble to assemble a single strand of RNA with a nucleotide sequence complementary to that of the DNA pairing Adenine to Uracil and Guanine to Cytosine and vice versa.
• Only one strand of DNA is transcribed and when the RNA polymerase reaches a specific stop sequence at the far end of the gene, it disengages itself from the DNA and release the newly assembled RNA chain.
• This RNA chain is called the primary RNA transcript (copy) of the DNA nucleotide sequence of the gene or simply mRNA.

Translation
Definition
The process of formation of the polypeptide chains using the messenger RNA is called Translation.
Steps of Translation
1. Binding of mRNA
2. tRNA Binds Amino Acids
3. Reading Or Decoding of mRNA
4. Polypeptide Chain Synthesis
1. Binding of mRNA
The process of translation begins with the binding of one end of the mRNA with a rRNA on a ribosome.
2. tRNA Binds Amino Acids
A tRNA molecule possessing the complementary three nucleotide sequence or anticodon, binds to the exposed codon on the mRNA, because this tRNA molecule bind with a particular amino acid and put amino acids at correct place on the elongated polypeptide chain.
3. Reading Or Decoding of mRNA
The ribosome then starts to move along the mRNA molecules in increment of three nucleotides, adding a specific amino acid at each step through tRNA.
4. Polypeptide Chain Synthesis
It continues until it reaches the stop sequence, after which it stops the process. It then disengages itself from the mRNA and releases the newly assembled polypeptide.

RNA

RNA
Definition
The single stranded helical polynucleotide contain ribose sugar and uracil instead of thymine is called RNA.
Location
RNA is found in the nucleus (in nucleolus 10%) as well as in the cytoplasm (90%).
Types of RNA
There are three types of RNA.
1. Ribosomal RNA (rRNA)
2. Transfer RNA (tRNA)
3. Messenger RNA (mRNA)
1. Ribosomal RNA (rRNA)
The class of RNA found in ribosomes is called ribosomal RNA.
Function
During polypeptide synthesis it provides the site on the ribosome where the polypeptide is assembled.
2. Transfer RNA (tRNA)
A second class of RNA is called transfer RNA is much smaller. Human cell contains more than 40 different kinds of tRNA molecules.
Function
During polypeptide synthesis tRNA molecules transport the amino acid into the ribosome for the synthesis of polypeptide chain.
3. Messenger RNA (mRNA)
It is long strand of RNA that passes from the nucleus to the Cytoplasm.
Function
During polypeptide synthesis, mRNA molecules brings information from the chromosomes to the ribosomes to direct the assembly of amino acids into a polypeptide.

One Gene One Enzyme Hypothesis

One Gene One Enzyme Hypothesis
Introduction
George Beadle and Coworker Edward L.Tatum proved that the information coded within DNA of a chromosome is used to specify particular enzymes.
Method of Study

• Beadle and Tatum created Mandelian in the chromosomes of the fungus call Neurospora by the use of the x-rays.
• They studied the effect of the mutations caused by them and suggested “one gene one enzyme hypothesis”

Choice of Material

• They choose the bread mold, neuropora crassa as an experimental organism. It had a short life cycle and was easily grown on a defined medium, containing known substances, such as glucose and NaCl.
• The nutrition of Neurospora could be studied by its ability to metabolize sugars and other chemicals the scientist could add or delete from the mixture of the medium.

Production of Mutations

• The induced mutations in Neurospora spores by using x-rays.
• The mutated spores were placed on complete growth media enriched with all necessary metabolites, so keeping the strains alive because the strains were deficient in producing certain compounds necessary for fungus growth due to damaged DNA by earlier irradiation, hence called Mutants.

Identification of Mutant Strains

• To test the mutations, they grew the mutated strains on the animal media containing sugar, ammonia, salt, a few vitamins and water.
• A strain that had lost the ability to make a necessary metabolite, failed to grow on such media.
• Using this approach they succeeded in identifying and isolating the different mutants.

Identification of Specific Mutations

• To determine the specific nature of each mutation, they added various chemicals to minimal media, to make the strains grow.
• Using this technique, they were able to pinpoint the biochemical problem and thus the genetic deficiency of the mutants.
• Many of the mutants were unable to synthesize a single amino acid or a specifc vitamin.
• If a spore lacked the ability to synthesize a particular amino acid, such as Arginine, it only grew if the Arginine was added in the growth medium Such mutants were called as arg mutants.
• Chromosomes mapping studies on the organism facilitated their work and they mapped three areas clusters of mutant Arginine genes.
• For each enzyme in the arginine biosynthetic pathway, they were able to isolate a mutant strain with a defective form of that enzyme and mutation always proved to be located at one of a few specific chromosomal sites, different for each enzyme.

Conclusion
They concluded that genes produced effects by specifying the structure of enzymes and that each gene encodes the structure of a single enzyme. This was called “One gene one enzyme hypothesis”.

The Meselson Stahl Experiment

Introduction
Mathew meselson and Frank Stahl performed experiments to test the semi conservative method of DNA replication.
Experiment

• They grew bacteria in a medium containing Nitrogen-15 (N15), a heavy isotope of the nitrogen.
• The DNA after several generations became denser than normal because the entire bacterial DNA now contained Nitrogen-15 (N15).
• They then transferred the bacteria into a new medium containing lighter isotope Nitrogen-14 (N14) and analyzed the cultures for changes in the DNA.
• At first DNA, which the bacteria synthesized, was all heavy.
• After one round the density of the DNA fell exactly to the value one half between the all heavy isotope DNA and all light isotope DNA.

Result
This showed that after one round of replication, each of the daughter DNA duplex contained one strand of heavy isotope, after two rounds half contained none of the heavy isotope strand to form light duplex and half contained one of the heavy strand isotope.
It was now confirmed that the semi conservative method of the replication of DNA replication was true.

Semi Conservative Replication

Semi Conservative Replication
Definition
The type of replication in which new daughter double helical duplex contain one strand old and another newly synthesized is called Semi Conservative Replication.

Replication of DNA

Replication of DNA
Definition
The mechanism in which DNA prepares its copies is called DNA replication.
OR
When the formation of new DNA molecule takes place in the cells without any change, it is known as Replication of DNA.

Genome

Genome
Definition
The total genomic constitution of an individual is known as Genome.
Example
In a bacterial cell, a single chromosome along with plasmid is genome of bacteria, while in a human being all twenty two pairs of autosome along with a pair of sex-chromosomes constitute genome.

Genes – The Unit of Hereditary Information

Genes – The Unit of Hereditary Information
Introduction
Archibald Garrod discovered in 1902, that certain diseases were more prevalent among some families and were inherited as a recessive Mendelian trait.
Alkaptonuria
Alkaptonuria is a disease in which the urine contained a substace called “Alkapton” now known as “Homogentisic acid” which was immediately oxidizes to black when exposed to the air.
Causes

• He suggested that this disease occurred due to absence of an enzyme, which could break the “Alkapton” down to other products so it would not build up in the urine.
• He proposed that the condition was “inborn error of metabolism” which is occurring due to changes in the hereditary information, which must have occurred in one of the ancestors of a affected families.

Conclusion
He concluded that the inherited disorders might reflect enzyme deficiencies

Watson and Crick’s Model of DNA

Watson and Crick’s Model of DNA
Introduction
James Watson and Francis Crick, in 1953 proposed structure of the DNA molecule.
Structure of DNA
Watson and Crick suggested a ladder like organization of DNA.
i. Double Helix
ii. Backbone of DNA
iii. Pairing of Bases
i. Double Helix
Each molecule of DNA is made up of two polynucleotide chains which twisted around each other and form a double helix.
ii. Backbone of DNA
The uprights of the ladder are made up of sugar and phosphate parts of nucleotide and the rungs are made up of a paired nitrogenous bases.
iii. Pairing of Bases
The pairs are always as follows

• Adenine always pairs with thymine and cytosine with Guanine.
• The two polynucleotide chains are complimentary to each other and held together by hydrogen bonds.

Hydrogen Bonding
There are two hydrogen bonds between Adenine and Thymine (A=T) and three between Cytosine and Guanine.
Distance

• Both polynucleotide strands remain separated by 20 A⁰ distance.
• The coiling of double helix is right handed and complete turn occurs after 34 A⁰. In each turn 10 nucleotide pairs are present, therefore the distance between two pairs is about 3.4 A⁰.

Hershey and Chase’s Experiment

Hershey and Chase’s Experiment
Introduction
1n 1952, Hershey and chase performed experiment to proof that DNA is a hereditary material.
Experiment at Material
Hershey and chase labeled protein coat and DNA of Bacteriophage separately. Protein coat was labeled with radioactive sulphur and DNA with radioactive phosphorus. These two viruses used to attack bacterial cells.
Step of Experiment
1. Hershey and chase observed that if cultures of bacteriophage are labeled with radioactive phosphorus or with sulphur.
2. Bacteriophage is ruptured, the DNA is released and treated with deoxyribsonucleus, the DNA breaks up into fragments in the solution.
3. The empty protein coats of the ruptured membrane appear as coats all the phosphorus and sulphur were made to inject bacteria and multiply by the help of special technique, all the sulphur labeled protein were removed.
4. The new phage formed contained only phosphorus indicating the presence of DNA molecule.
Conclusion
The conclusion appears similar to the transforming principle in bacteria, showing that DNA is the genetic material in phage, transmitted from one generation to the next.

Avery, Macleod and McCarty’s Experiment

Avery, Macleod and McCarty’s Experiment
Introduction
In 1944, after a decade of research, Oswald Avery, Maclyn McCarty and Colin Macleod discovered that the transforming agent had to be DNA.
Experiment
They performed various experiments and found out that the only substance, which carried the transforming capability was DNA because if the enzyme deoxyriba-nuclease was added to the bacteria, the transforming capability was lot.

Fredrick Griffith’s Experiment

Fredrick Griffith’s Experiment
Introduction
Fred Griffith in 1928 provided the evidence of hereditary material in bacteria.
Experimental Material
He was working on strains of streptococcus pneumoniae, which occurs in two distinct different forms.
R-Type
Rough surfaced, non-capsulated bacteria, without the capability of producing pneumonia.
Example
Smooth surfaced, capsulated bacteria with the capability of producing pneumonia i.e. virulent.
Steps of Experiment
1. He observed that, when he injected R-type bacteria in the mice, there was no ill effect.
2. When he injected the S-type, they proved to be fatal.
3. He also observed, when he injected both the bacteria separately after killing them by heating under high temperature, the mice did not develop the disease.
4. He also observed that when the injected the living R-type with heat killed S-type there was high mortality among the mice.
Conclusion
Fred Griffith concluded that the R-type bacteria gained genetic property of S-type inactive bacteria when they kept together, so R-type bacteria converted into virulent S-type by the activity of DNA. Hence by this experiment, it can be proved that DNA is a genetic material.

Chromosomal Theory of Hereditary

Chromosomal Theory of Hereditary
Introduction
The chromosomal theory of inheritance was first formulated by the American Biologist “Walter Sutton” in 1902.
Postulates
The main postulates of this theory are as under
i. Hereditary Material
ii. Segregation of Chromosomes
iii. Number of Chromosome
iv. Independent Assortment
i. Hereditary Material
Reproduction involves the initial union of only two cells, egg and sperm If Mendel’s model is correct then these two gametes must make equal hereditary contributions. Sperm, however contains little cytoplasm, therefore the hereditary material must reside within the nuclei of the gametes.
ii. Segregation of Chromosomes
Chromosomes segregated during meiosis in a manner similar to that exhibited by the elements of Mendel’s model.
iii. Number of Chromosome
Gametes have one copy of each pair of homologous chromosomes, diploid individuals have two copies.
iv. Independent Assortment
During meiosis each pair of homologous chromosomes orients on the metaphase plate independent of any other pair.

Types of Chromosome on the Bases of Centromere

Types of Chromosome on the Bases of Centromere
There are four types of chromosome according to the position of centromere are as follows.
i. Telocentric Chromosome
ii. Acrocentric Chromosome
iii. Sub-Metacentric Chromosome
iv. Metacentric Chromosome
i. Telocentric Chromosome
When the centromere is placed at the extreme end of the chromsome, the chromosome is called Telocentric Chromosome.
ii. Acrocentric Chromosome
When the centromere is placed near the end of the chromosome, the chromosome is called Acrocentric Chromosome.
iii. Sub-Metacentric Chromosome
Chromosome with unequal arms (chromatids) that resembling the “j” shape are called Sub-meta Centric chromosome.
iv. Meta Centric Chromosome
Chromosome with equal arms and centromere in center is called a Metacentric Chromosome.
There are three more types of chromosomes
i. Homologous Chromosome
ii. Autosomes
iii. Sex Chromosomes
i. Homologous Chromosomes
Those chromosomes which are morphologically similar with same set of genes in which one chromosome comes from male and another from female is called as Homologous Chromosome.
ii. Autosomes
The somatic paired chromosome which contain the genes of various characters except reproductive organs are called Autosomes.
These are same in male and female both.
iii. Sex Chromosome
Those chromosomes in a cell by which the sex of organism can be determined are called Sex-Chromosome.
There are different types of distribution of Sex-Chromosome.
i. XY type distribution
ii. ZW type distribution
i. XY Type Distribution
Females have a pair of similar sex chromosomes called X but males have a mismatch set and X and another chromosome called a Y, so the male XY and the female is XX.
ii. ZW Type Distribution
In some fishes, moths and birds, the male have similar chromosome called Z and the female has different so the male is ZZ and the female is ZW.
Chromosomes as Carrier of Genes
Genes are small bodies found in the chromosomes.
Chromosomes are considered as the carrier of genes.

• The chromosomes can be separately identified visually but the genes are very small units. And so far have not been seen even with the best microscope.
• The chromosomes and gene behave as hereditary units but the genes can not be considered outside the chromosome.
• At the time meiosis, the separation of homologous chromosomes takes place which result in the segregation of gene pairs.
• In the genotype of every individual one member of each pair of genes is contributed by one parent and the other by the other parent.

Ultra Structure of Chromosome

Ultra Structure of Chromosome
Eukaryotic chromosomes are composed of chromatin, a complex of DNA and protein and less amount of RNA.
Deoxyribonucleic Acid (DNA)
The DNA of a chromosome is one very long double stranded fiber, duplex which extend unbroken through the entire length of chromosome.
A typical human chromosome contain about 140 million nucleotide in its DNA.
Size
If a strand of DNA from a single chromosome were laid out in a straight line, it would more than 7 feet long (2 meter). It is too much long to fit into a cell. In the cell, however, the DNA is coiled, thus fitting into a much smaller space.
Nucleosome
DNA resemble a string of beads. Even 200 nucleotides, the DNA duplex is coiled around a core eight histone proteins forming a complex known as Nucleosome.
Protein
Histones are positively charged proteins due to an abundance of the basic aminoacids, arginine and lysine. They are thus strongly attached to the negatively charged phosphate groups of the DNA.
Further coiling occurs when the string of nucleosomes wraps up into higher order coils called Super Coils.
Heterochromatin
Highly condensed portions of the chromatin are called Heterochromatin.
Euchromatin
The reminder of the chromosome called Euchromatin is condensed only during the cell division.
Karyotype
The particular array of chromosome which an individual possess is called the Karyotype in which chromosomes are paired by matching bonding and arranged by size and shape.
Applications

• Karyotypes show marked differences among species and sometime even among individuals of the some species.
• It help in determining many issue as the culprits of difference offences and settle the disputed paternity.
• It help in detecting genetic abnormalities, arising from extra or lost chromosome.

Composition of Chromosome

Composition of Chromosome
Chromosome are complex structure made up of Deoxyribonuleo protein. This protein composed of
1. DNA
2. Protein
3. RNA
1. DNA
Chromosome have 30-40% DNA. DNA is made up of billion of units called Nucleotide and the nucleotide made up of three components.

• Phosphate group
• Deoxyribsoe sugar
• Nitrogenous base

There are two groups of nitrogenous bases.
i. Purines
ii. Pyrimidines
i. Purines
Nitrogenous base that have double ringed structure called Purines.
Example

• Adenine (A)
• Guanine (G)

ii. Pyrimidines
Nitrogenous structure that have single ringed structure called Pyrimidines.
Example

• Cytosine (C)
• Thymine (T)

2. Protein
Chromosome have 50-65% protein. The most abundant chromosomal protein is called Histones. It is a basic protein.
3. RNA
Chromosome have very less amount of RNA about 1-10%

Structure of Chromosome

Structure of Chromosome
Chromosomes can only be seen when the cells are divide. A chromosome is made up of
1. Chromatid
2. Centiomere
1. Chromatids
Each chromosome consists of two very fine thread like structure called Chromatids.
Chromonema
Each chromatids of a chromosomes consists of one or more fine thread called Chromonema.
Chromomere
Chromonema contain deeper staining regions along their length, giving the threads like appearance of strings of beads. These regions are called Chromomeres.
Types of Chromotids
There are two types of chromatids.
i. Sister Chromatids
2. Non-Sister Chromatids
i. Sister Chromatids
The two chromatids of same chromosome are called Sister Chromatids.
ii. Non-Sister Chromatids
The chromatids of different chromosomes are called Non-Sister Chromatids.
2. Centromere
The region or point in which the chromatids are linker together is called Centromere.
Kinetochore
Centeomere is small spherical zone on the chromosome, within centromere a disc shape protein structure called Kinetochore is present to which the spindle fibres attached during cell division.

Chromosomes and DNA

Chromosomes
Definition
The thread like structure present inside the nucleus bearer of hereditary character in the form of genes is called Chromosomes.
Discovery
Chromosomes were discovered by a German Biologist Walther Flaming in 1882. He placed the larva of salamander in a chemical Perkins Aniline, then he observed that the chromosomes become darker than other organelles of a cell.
Number of Chromosomes
The number of chromosomes varies from species to species.
For Example

• Penicillin => 2 Chromosomes
• Mosquito => 6 Chromosomes
• Drosophila => 8 Chromosomes
• Pea => 14 Chromosomes
• Frog => 26 Chromosomes
• Human => 46 Chromosomes
• Sugar Cane => 80 Chromosomes
• Fern => 100 Chromosomes

Apoptosis

Apoptosis
Definition
Apoptosis means dropping off or falling off.
OR
The naturally cell death programme is called apoptosis.
Mechanism of Apoptosis

• Apoptosis is thought to require the activation of a specific set of genes.
• Death by apoptosis is characterized by over all compaction of the cell and its nucleus by the orderly dissection of the chromatin by special DNA splitting endonuclease and the rapid engulfment of the dying cells by phagocytes.
• Apoptosis appears to be mediated primarily by the release of ca++ and the activation of certain protein kinesos.

Example

• Apoptosis is common during the embryonic development the growth of the cells between finger that are no longer needed undergoes apoptosis.
• It also occurs in adult tissues where it destroys the cells that have potential to develop the malignant tumor.

Meiotic Error – (Non-Disjunction)
Definition

The abnormalities in the process of meiosis are termed as meiotic error.
Non-Disjunction
The failure in the separation of the homologous chromosome due to meiotic error is known as non-disjunction.
Heteroploidy
A change in the normal chromosome number is called heteroploidy.
There are two types of Heteroploidy
1. Aneuploidy
2. Polyploidy

Cell Death

Cell Death
Cell death in multicellular organisms is controlled by two ways.
1. Autophagy
2. Necrosis
1. Autophagy
Auto => itself, Phagy => eat
The cell commits suicide carries self destruction in the absence of survival signals is called autophagy.
2. Necrosis
The cell dies due to injury of tissues is called necrosis.
During this, cell swells and bursts while releasing toxic substance which can damage neighbouring cells and cause inflammation.

Cancer

Cancer
Definition
A process of uncontrolled cell division, cells divide rapidly and abnormally and caused cancer.
Types of Cancer
There are two types of cancer.
1. Malignant
2. Benign
1. Malignant
A tumor can invade adjacent tissues and destroy them. Such tumors are called malignant.
2. Benign
A tumor is said to be benign if it does not attack the other tissues.
Causes of Cancer
The main causes of cancer are
1. Mutation
2. Hereditary
3. Virus
1. Mutation
The cancer is caused by mutation of cellular gene that control cell growth and cell division.
The mutation in cellular gene may be due to the X-rays, Gamma Rays and radiation from radioactive substance.
2. Hereditary
There may be a hereditary tendency for cancer.
3. Virus
In animals so many viruses can cause some kinds of caealike leukamia. In this case DNA of virus is inserted directly in chromosome and caused gene mutation.
Cancer Cell kills Other Cells
Cancer cells multiply their number day by day and required all essential nutrients available in the body due to which normal tissues suffer regularly and after some time they become dead.

Amitosis

Amitosis
Definition
A cell division without spindle formation is called Amitosis.
Mechanism of Amitosis
Amitosis is also called the direct cell divisions. In this cell division first the nucleus of the cell become dumbbell shaped and finally it splits into two nuclei. In this way karyokinesis is completed.
Then an invagination appears in the centre of the cell which becomes deep and at the end it divides the parent cell completely into the two daughter cells.
Nuclear Budding
When nuclear portion unequally divide is called nuclear budding.
Nuclear Fragmentation
When nuclear portion divides more than two in number is called nuclear fragmentation.
Example
This type of cell division usually occurs in bacteria, protozoa, abnormal and diseased tissues such as cancer and tumor.

Types of Cell Division

Types of Cell Division
There are three types of cell division.
1. Mitosis
2. Meiosis
3. Amitosis
1. Mitosis (Somatic Or Vegetative Division)
The type of cell divides in which a parent cell divides into two daughter cell and each daughter cell gets same number of chromosomes from parent cell is called mitosis.
Phases of Mitosis
It can be divides into two phases.
1. Karyokinesis
2. Cytokinesis
1. Karyokinesis
Karyokinesis is the division of nucleus and occurs in four different phases.

• Prophase
• Meta phase
• Ana phase
• Telophase

1. Prophase
(Pro=before, phase=appearance)
Phrophase is the main phase of nuclear division it is the longest stage the following main change occur in nucleus and cytoplasm.
Changes in Nucleus

• The chromatin material in nucleus break into thread like structure called chromosome.
• As the prophase further proceeds Chromosomes become thick and short.
• Each chromosomes consist of two thread known as chromatids. These chromatids are attached together by a spherical body is known as Centromere or kinetochore.
• In the nucleus chromosome move to its peripheral, part and central part of nucleus remain empty.
• Nucleoli disappear.
• Nuclear membrane disappears and nuclear material is released in the cytoplasm.

Change in Cytoplasm

• The most important change which take place in cytoplasm is formation of mitotic apparatus.
• The formation of mitotic apparatus varies in plant cells and in animal cells.

In Animal Cell

• In animal cell two rod like structure found in cytoplasm cell as centrosome, each centrosome divides into two centriole.
• One centriole remain at its place while another one moves towards the opposite pole.
• Between the centriole thread like structure arise known as spindle fibre or nuclear spindle, which is of three types.

Continues Spindle Fibres
These fibers continuously running from one pole to other pole.
Half Or Discontinuous Spindle Fibers
These fibers running one pole to the broader region of cell.
Astral Fibers
These fibers arise from to Centriole and give the position to Centriole in the cytoplasm.
In Plant Cell
In higher plant and insects the Centriole is absent and spindle fibres are formed without it. In them the spindle appears with its fibers converging to piole regions comparable to the area occupied by Centriole in the animal cells.
2. Meta Phase
(Meta = after)

• At this stage centromere of chromosome attached with the equator of half or discontinuous fiber by kinetochore.
• All chromosomes arrange an alignment line and formed equatorial plane.
• Chromatids of chromosome attain their maximum thickness.
• The faces of chromatids are towards the opposite poles as they are ready for separation.

3. Ana Phase
(Ana = upper)

• Anaphase begins when centromere divide, separate the sister chromatids from each other.
• Two set of chromatids migrates toward the opposite pole due to the contraction of discontinuous spindle fibers.
• At the end of the anaphase, the two groups of chromatids reach the opposite pole.

4. Telo Phase
(Telo = end)

• When two sets of chromosome reach to opposite poles telophase begins.
• The daughter chromosomes become thin and long.
• The coil with each other forming network again.
• Spindle fibers start to disappear.
• Nuclear membranes reappear around each set of chromosome.
• Nucleolus reappears.
• Now at this time two daughter nuclei are formed in a cell.
• The process of karyokinesis completed.

2. Cytokinesis
The division of cytoplasm is called cytokinesis. There are two types of cytokinesis.
Cell – Plate Formation
In plant cell the division of cytoplasm begin with the formation of a structure called cell – plate at the equator. It grows outward dividing the mother cell into two daughter cells.
Cleavage
In animal cell construction or depression arise in the cell membrane from outer side to the inner side due to which cell divides into two daughter cell is known as cell-cleavage.
Important of Mitosis
i. No Change in Hereditary Materials
As there is no crossing over during this cell division, the genetic information remains unchanged generation after generation.
ii. Asexual Reproduction
All plants and some animals show asexual reproduction with the help of mitosis such as regeneration in starfish.
iii. Development
New organs are formed with the help of controlled and planned mitosis.
iv. Growth
Young individuals grow with the help of mitosis.
v. Healing of Wounds
Formation of lost organ (regeneration), healing of wounds and replacement of older cells takes place due to mitosis.
vi. Tissue Culture and Cloning
Tissue culture in plant and cloning in animal is carried out through mitosis.
2. Meiosis (Reduction Division)
(Meiosis => meioum => to reduce)
The type of cell division in which one cell divide into four daughter cells and each daughter cell gets half number of chromosome from parent cell called meiosis.
OR
Cell division in which one diploid cell divide into four haploid cells called meiosis.
Phases of Meiosis
Meiosis consists of two phases.
1. Karyokinesis
2. Cytokinesis
1. Karyokinesis
Karyokinesis is the divisions of nucleus. Karyokinesis in meiosis consist of two divisions.
i. First Meiotic Division
ii. Second Meiotic Division OR Mitotic Division
i. First Meiotic Division
In first meiotic division, one nucleus divides into two nuclei and number of chromosome reduce to half. It is consist of following stages.
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
a. Prophase I
Prophase I is the longest phase of meiosis consisting of a complicated chain of events. It further divide into following sub stages.

• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

Leptotene (Slender)

• This is the first stage of meiosis.
• In this stage, cell is larger in size and has large nucleus.
• The chromosome become more uncoiled and seems as thin, long thread like shape.
• Beaded appearance found on chromosome at irregular interval is known as chromosome.

Zygotene (Joining)

• In this stage chromosome are shorter and thicker.
• The homologous chromosome come very close to each other and make their pair. This pairing of chromosome is called synapsis.
• The pair of homologous chromosome are called bivalents.

Pachytene (Thick)

• After pairing chromosome become thick and short.
• Each chromosomes of a bivalent forms two sister chromatids. Thsi called duplication.
• Now at this time, bivalent has four complete homologous chromatids called tetrad.

Diplotene (Double)

• The force of repulsion arise between the homologous chromosomes of bivalent and they start to separate from each other.
• But they remain attached at one or more point. Their point of contact are called chiasmata.
• By chiasmata process of crossing over occurs (crossing is a process by which transfered of genetic material occurs between non-sister chromatics of tetrad).

Diakinesis

• The separation processes of bivalent continue by a process called terminazation.
• In this process the chiasmala move from Centromere towards the end of the bivalent.

b. Metaphase I

• The discontinuous spindle fibers are attached to the Centromere of homologous chromosomes.
• All chromosomes arranged an alignment line and formed equatorial plane.

c. Ananphase I

• In this stage each chromosome of bivalent move toward the opposite pole due to contraction of half spindle fibers.
• At the end of this phase the chromosome are separated into haploid sets, on set being present at each pole.

d. Telophase I

• Nuclear membrane reorganized around each set at two poles.
• Nucleoli reappear thus two nuclei each with haploid number of chromosome are formed.

ii. Second Meiotic Division OR Mitotic Division
In second meiotic division, one nucleus further divide into two nuclei and number of chromosomes remain same. It consists of following stage.
a. Prophase II
b. Metaphase II
c. Anaphase II
d. Telophase II
a. Prophase II

• Nucleolus and nuclear membrane disappear.
• Centrioles duplicate and move towards the opposite pole.
• Spindle fibres are formed.

b. Metaphase II

• The discontinues spindle fibres get attached to centromere.
• Chromosomes are arranged at the equatorial plane.

c. Anaphase II

• The centromere divides and chromatide are separated from each other.
• Chromatids move to opposite poles on spindle fibre.
• Each chromatid is now called monad.

Importance of Meiosis
i. Crossing Over
Crossing over during meiosis brings about re-shuffling of genes. As a result new combination of character in the off spring occurs.
ii. Gamet Formation
Meiosis is responsible for the formation of haploid gametes and spores. Thus after fertilization, the constant diploid number of chromosomes is restored.
iii. Random Assortment
Separation of homologous chromosomes and their random distribution to different daughter cells, also bring about genetic variations, which are raw material for evolution.

Cell Cycle

Definition
An orderly sequence of events in which one cell divides and forms daughter cells and the daughter cell again divides is called cell cycle.
OR
Round of cell growth and cell division is known as cell cycles.
Phases of Cell Cycle
Cell cycles consist of two phases
1. Inter Phase Or Resting stage or (non-division)
2. Division Phase
1. Inter Phase
The period of the cell cycle between two consecutive divisions is called Interphase.
OR
The phase of cell cycle in which the cell cannot be divided but perform its metabolic function for the preparation of its division are called Interphase.
Sub – Stages of Interphase
Inter phase divide into three sub-stages.
i. G1 – Stage
ii. S – Stage
iii. G2 – Stage
i. G1 – Stage

• The inter phase begins with this phase.
• Enzyme for the formation of DNA is produced.
• During this stage cell grows rapidly by synthesis of protein and cytoplasmic organelles.
• Duration of G1 is about 25-50% of Interphase.

ii. S – Stage (synthesis stage)

• In this stage synthesis of DNA occurs and numbers of chromosomes become double.
• Duration of s-phase is 35-40% of Interphase.

iii. G2 – Stage

• It is the end stage of Interphase.
• In this period, production of protein and microtubules occur that are required for cell division.
• Numbers of mitochondria also increase for energy production.
• The size of cell becomes doubled and cell starts division.

Duration of Cell Cycle

• Human Cell (animal cell)
• 18-24 hours for cell cycle
• Mitosis 30 minutes
• G1 phase 9 hours
• S phase 10 hours
• G2 phase 4.5 hours

Plant Cell

• 10-30 hours for cell cycle

2. Division Phase
This phase involves actual cell division and consists of two stages.
Karyotinesis
Cytokinesis
Karyokinesis
Division of nucleus is called as karyokinesis.
Cytokinesis
Division of cytoplasm is called as cytokinesis.

Sex Determination and Sex Linkage

Sex Determination and Sex Linkage
Introduction
In somatic cells of most animals the chromosomes occur in homologous pairs. The two chromosomes of a homologous pair exactly match up and are morphologically similar. However, in many cases, there is an exception to this rule. The two chromosomes of one of the pairs differ morphologically from each other and are not homologous. This non-homologous pair of chromosomes is responsible for determining the sex an individual and is, therefore, called “sex chromosomes” or “heterosomes”. All the other chromosomes pairs are known as autosomes.
Example
In man, there are 22 pairs of autosomes and one pair of sex chromsomes.
Pattern of Sex Determination
The exact pattern of differences of chromosomes between the male and female of different species vary a great deal. Some patterns are discussed below:
1. Sex Determination in Grass Hopper (X-O System)
2. Sex Determination in Drosphila (X-Y System)
3. Sex Determination in Humans (x-Y System)
4. Sex Determination in Birds and Fishes (Z-W System)
1. Sex Determination in Grass Hopper (X-0 System)
The simplest pattern of chromosomal difference between the sexes is found in grass hopper. in these insects the male has one less chromosome than the female. The female has 24 chromosomes (11 pairs of autosomes and one pair of sex chromosomes) and the male has 23 chromosomes (11 pairs of autosomes and single sex chromosomes). The genotype of female is designated as XX and that of male as XO; the O stands for absence of sex chromosome. The male forms two types of sperms, one type containing 12 chromosomes (11 + X chromosomes) and the other type with 11 chromosomes (without any sex chromosome). Thus the male is said to be heterogametic. On the other hand the female produces one type of ova only containing 12 chromosomes (11 + X chromosomes). Therefore the female is termed as homogametic. If a sperm with 12 chromosomes fertilizes the ovum, a female is produced and if a sperm with 11 chromosomes fertilizes the ovum, a male is produced.
2. Sex Determination Drosphila (X-Y System)
T.H. Morgan, during his genetic experiments on fruit fly Drosphila, noted that male and female flies show difference in the chromosomes. Drosphila contains 4 pairs of chromosomes, out which three pairs are same in male and female and are homologous. These chromosomes are called autosomes. The fourth pair shows difference in male and female and is termed as the sex chromosomes, because it determines the sex. In female both sex chromosomes are rod shaped (morphologically similar). In male one sex chromosomes is rod shaped and the other is hook shaped (morphologically different). The rod shaped chromosomes, which are similar in both male and female are called X chromosomes. The hook shaped chromosomes present in male only is known as the Y chromosomes. Thus the genotype of female Drosphila is XX and that of male is XY. In other words in Drosphila the female is homogametic and the male is heterogametic.
3. Sex Determination in Humans (X-Y System)
In humans there are 23 pairs of chromosomes in a somatic cell out of which 22 pairs are similar both in males as well as females and are known as the autosomes. The 23rd pair constitutes the sex chromosomes and differ in the male and female. In female the two sex chromosomes are similar but in male the two sex chromosomes differ in size and shape. They are termed as X and Y chromosomes. The X chromsomes in humans is long and contains full compliment of the genes, but the Y chromosomes is short and carries lesser number of genes. It does not bear alleles of most of the genes present on the X chromosomes. An individual which inherits two X chromosomes becomes female (XX), while the one which inherits one X and one Y chromosomes becomes male (XY). Thus human female possesses genotype of 44 A + XX and the male has genotype of 44 A + XY.
It is obvious that the female produces one type of gametes only each containing one X chromosomes. Therefore, the human female is said to be homogametic. In contrast the male produces two types of gametes and is heterogametic. Half of the sperms formed contain X chromosomes and other half contain Y chromosomes. If a sperm containing X chromosomes fertilizes with the ovum, the zygote contains two X chromosomes and the resulting child would be female (XX). On the other hand if the fertilizing sperm contains Y chromosomes the zygote contains one Y and one Y chromosomes and the offspring would be male (XY). All mammals show X-Y system of sex determination and therefore the sex is determined by the fertilizing sperm.
4. Sex Determination in Birds and Fishes (Z-W System)
In birds, moths and some fishes it is the female which contains two different sex chromosomes and is heterogametic. The male is homogametic and contain two similar sex chromosomes. As a result two types of ova and only one type of sperms are produced and the ovum determines the sex of the individual. The sex chromosomes in these animals are designated as Z and W to avoid confusion with X-Y system. The genotype of male is ZZ and that of female is ZW.
Sex Linked Inheritance
Definition
All genes located on the sex chromosomes are said to be sex-linked. Inheritance of sex linked genes or any genetic trait which is transmitted through sex chromosomes is called the sex linked inheritance.
Sex Linked Inheritance in Drosphila
The sex linked inheritance was first discovered by Morgan (1910) in Drosphila during breeding experiments. In Drosphila the normal wild type has real eyes and a eye colour is dominant over white eye colour. When Morgan crossed a white eyed male with a red eyed female, the F1 and F2 generation showed simple Mendelian ratios. However when a red eyed male was crossed with white eyed female the results were different. From his experiments Morgan concluded that the gene controlling the eye colour in Drosphila is located on the X chromosomes and the Y chromosomes does not carry any allele for the eye colour.
Sex Linked Inheritance in Man
In humans there are about 200 genes present on the X chromosomes and therefore many sex linked traits are found. The two important sex linked traits are colour blindness and haemophilia. Pattern of their inheritance can be found by studying family histories (pedigrees) only because breeding experiments cannot be conducted on humans. The alleles for these traits are located on the x chromosomes and have no corresponding alleles on the Y chromosomes. Consequently, in the male even a single recessive X-linked allele can express itself in the phenotype, while in females such alleles are expressed only in double dose. Therefore, recessive sex-linked traits, such as colour blindness and haemophilia are relatively more common in male.
Inheritance of Colour Blindness
Colour blind persons cannot distinguish between red and green colours. This is a sex linked trait and is controlled by a recessive allele, which is present on the X chromosomes only the Y chromosomes does not bear any allele for colour blindness. The gene for normal vision (N) is dominant over the gene for colour blindness (n). Therefore a female can be colour blind only in homozygous condition (nn) and for that she needs to inherit two recessive alleles. A male needs to inherit only one recessive allele to be colour blind because the Y chromosome does not bear any allele for this trait. This explains why colour blindness is more common in men than in women. A bout 4% males are colour blind against less than 1% females. Heterozygote (Nn) are normal but are called carriers because they carry the defective allele and can transmit it to the next generation. The pattern of inheritance of colour blindness can be easily studied by making some theoretical crosses.
Haemophilia
Haemophilia is a human genetic disease caused by a sex-linked recessive allele, located on the X chromosomes. Haemophiliacs lack a blood clotting factor and their blood does not clot easily. Therefore haemophiliacs bleed excessively even after a minor injury. They may also suffer from internal bleeding and the severely affected individuals may bleed to death after relatively minor injuries. This disease is also called the bleeder’s disease and was common in royal families of the Europe. About 1 in 15000 males are born with haemophilia, but the disease is very rare in women. The reason is that a single recessive allele for haemophilia can express itself in male because the Y chromosomes is inert for this trait. However, a women will need to have two recessive alleles to suffer from the disease.
Pattern of Inheritance of Haemophilia
It is same as that of other sex-linked diseases controlled by recessive allele, such as colour blindness. However, the occurrence of haemophilia is rare, because the haemophilia usually die before marriageable age and therefore cannot transmit their defective alleles to next generations.
Linkage
Definition
The genes located on the same chromosome are said to be linked, and are expected to be distributed to gametes together during meiosis. The tendency of two or more genes in a chromosome to remain together during inheritance is called linkage.
However linkage is not absolute and in most cases, linkage is broken due to crossing over and some some recombinant forms appear. The linkage in such cases is termed as incomplete.
Crossing Over
The crossing over is defined as mutual exchange of chromosome parts between two non-sister chromatids of a homologous pair of chromosomes. After crossing over, chromosomes carry some genes that were earlier located on the other member of the pair of homologous chromosome. Due to crossing over lined genes are separated and enter different gametes. Therefore, in the next generation, some new phenotypes can be formed which are different from the two parents and are called recombinant.
Crossing over takes place during gamete formation at diplotene stage of meiosis and occurs randomly at one or more than one points called chiasmata along the whole length of homologous pair of chromosomes.
Example
The most through studies of linkage and crossing over have been made in fruit fly Drosophila. The wild type of Drosophila flies show following pairs of contrasting characters.
The genes for grey colour (B) and normal (V) are dominant over black colour (b) and vestigial wings (v). When a homozygous grey fly with normal wings (BBVV) is crossed with black fly with vestigial wings (bbvv), all F1 individuals are grey with normal wings, as expected. When the F1 heterozygous fly (BbVv) is test crossed with double recessive (bbvv), following results are obtained.
Gray normal …….. 40%
Black Vestigial …….. 40%
Gray Vestigial …….. 10%
Black Normal …….. 10%
Following conclusions can be drawn from above results
In case of complete linkage, no recombinants are expected and only the parental types would have been obtained.
In case of independent assortment, a ratio of 1:1:1:1 or 50% parental forms and 50% recombinants would have been obtained.
The cross does not show the above two results. It is therefore concluded that the two pairs of genes in question are linked together but get separated due to crossing over. Because crossing over takes place in about 20% of the cells undergoing meiosis, nearly 20% offspring, known as recombinants, show recombined characters in phenotpe.
Mechanism of Crossing Over
During their meiotic division, the homologous chromosomes pair together and this pairing process is called synapsis. Later on, during diplotene stage, the two non-sister chromatids of the homologous pair mutually exchange chromosomal parts and the crossing over takes place. The sites on the chromosomes where crossing over occurs are called chiasmata. After separation, the chromosomes carry genes that were previously located on the other chromosome of the homologous pair.
Terms Used in Genetics
Gene
Gene is a basic unit of hereditary information and consists of a sequence of DNA bases. Each gene occupies a specific position called locus on a chromosomes.
Gene Locus
It is the specific location of a particular gene along the length of a certain chromosome. In other words it is the position of alleles on homologous chromosomes.
Alleles
Alleles are alternative forms of teh same gene, controlling a give trait. Alleles of a gene are present on the same relative locus on a homologous pair of chromsomes. Each individual inherits two alleles for a given trait, one form each parent.
Example
in pea plant, the gene for seed coat has two alleles; one for smooth seed and the other for wrinkled seeds.
Homozygous
When a diploid organism contains two identical alleles for a particular character, it is said to be homozygous for that character.
Example
Genes TT and tt are homozygous for length of stem.
Heterozygous
When a diploid organism contains two different alleles for a particular character, it is termed as heterozygous for that character.
Example
Genes Tt are heterozygous for length of stem.
Dominant
In a heterozygous condition, the allele which expresses itself morphologically and masks the appearance of other allele is said to be dominant.
Example
In pea plant allele for tall stem is dominant over allele for dwarf stem.
Recessive
In a heterozygous condition, the allele which does not express itself morphologically and remains masked by the other allele is called recessive.
Example
Allele for dwarf stem is recessive in pea plant.
Genotype
The description of genetic make up of an organism for a particular trait.
Example
In pea plant genotype for tall stem can be TT (homozygous) or Tt (heterozygous).
Phenotype
It is the visible expression of genes, both physical and physiological.
Example
Phenotype for both TT and Tt genotype will be tall stem.
Hybrid
The product obtained as a result of crossing two pure breeding varieties of an organism with contrasting characters is known as hybrid.
Example
Individuals obtained as a result of crossing a pure breeding tall pea plant with dwarf pea plant will be a hybrid.
Test Cross
It is a cross which is used to determine the genotype that is homozygosity or heterozygosity of an organism. In this cross a phenotypically dominant individual, whose genotype is to be determined is crossed with a recessive homozygous individual (known genotype). If F1 shows all dominant individual the tested individual is homozygous. On the other hand, if F1 produces half dominant and half recessive offspring the tested organism is heterozygous.
Multiple Alleles
A gene controlling a trait may have more than two alleles in a population. In such cases the various allele forms are together called as multiple alleles. Since only two alleles can occupy the same locus in a homologous pair of chromosomes, a diploid individual can posses only two out of a set of multiple alleles.
Examples of Multiple Alleles
A well known example of multiple alleles in humans is the inheritance of blood groups. The classification of ABO blood group system is based on presence or absence of a certain substance known as antigen on the surface of the RBCs. There are two types of these antigens; antigen A and antigen B. A person with antigen “A” has blood group A, with antigen “B” has blood group B with both antigens has blood group “AB” and if there is none of the two antigens, the blood group will be O.
The production of these antigens is controlled by a gene known as “I” gene. The gene has three alleles represented by the symbols I(A), I(B) and i. The allele I(A) is responsible for production of antigen A; the allele I(B) produces antigen B, while the third allele i does not produce any of the antigens. The allele I(A) and I(B) are equally dominant (co-dominant) while allele i is recessive to both.
As only two of the three alleles can be present in any one individual, a person may posses on of the combination of the alleles and the resulting blood group.
Incomplete Dominance
Mendel’s experiment with Garden pea showed that in heterozygous condition, one allele is completely dominant over the other and expressed itself morphologically. Therefore, in Mendel’s crosses with pea plant, F1 off spring always looked like one of the two parental types. In other words heterozygous and homozygous dominants exhibited the same phenotype.
However, post-Mendelian studies have shown that this mode of inheritance is present in many organism but not in all the organisms. There are many instances where the dominance is partial and the heterozygous show phenotype which is intermediate between the two homozygous parents. An inheritance where neither of the two alleles is completely dominant over the other and the heterozygous show phenotype which is intermediate between the two homozygous parents is called incomplete dominance. It can also be said that in incomplete dominance blending of the characters occur.
Examples of Incomplete Dominance
Four O’clock plant (Mirabilis jalapa) shows two true breeding varieties for flower colour; red flower and white flowers. A cross between the true breeding red and white flowers produces pink flowers in F1 generation. On selfing the F2 generation produces Red, pink and white flowers in the ratio of 1:2:1.
Co-Dominance
It is a type of inheritance where in a heterozygous condition, the two alleles are equally dominant and both express themselves and separately in the phenotype. As a result the heterozygous offspring displays the phenotype of both the homozygous parents and none of the two alleles masks the appearance of the other.
Examples of Co-Dominance
In short horned cattle there are two true breeding varieties red and white for skin colour. When the two are crossed, the heterozygous offspring shows roan colour. However there is no hair of roan colour or any intermediate shade; only red hair and white hair are present on the skin. The appearance of roan colour is due to mixture of red and white hair. It is evident that both the alleles (for red and white hair) are equally dominant and have equally and separately expressed themselves in phenotype.
Epistasis
(Genetic Interaction)
Epistasis is a condition in which the expression of a gene depends on expression of another gene located at a different locus. The second gene, termed as epistatic gene, suppresses or modifies the expression of the first gene.
Epistasis is different from dominance in which one allele (dominant) masks the expression of other allele (recessive) located at the same locus. The term epistasis was coined by Bateson.
Inheritance of fur colour in mice is an example of epistasis. Normal wild fur colour is gray, but black and white colours are also found. There are two genes (two pairs of alleles) involved in determining the colour of the fur. One gene controls the synthesis of the pigment; the dominant allele “B” produces gray colour and the recessive allele “b” forms black colour. The second gene at a different locus, is responsible for deposition of the pigment. It has a dominant allele “C” and a recessive allele “c”. Expression of dominant allele is necessary for deposition of the pigment. Therefore, if a mouse has genotype cc for this gene, it will become white regardless of the genotype of the other gene (gray/black). It is evident that the gene responsible for deposition of the pigment controls the expression of the gene for synthesis of the pigment.
To illustrate further let us study results of a dihybrid cross between two gray coloured mice, both heterozygous for the two genes.
Pleiotropy
It is a condition in which a single gene can affect many phenotype characteristics of an individual. Such a gene with multiple effects is called pleitropic gene.
Example of Pleiotropy

• In Drosophila the gene for white eye also influences the colour of testes and shape of spermatheca.
• In cats the gene responsible for white fur and blue eyes also causes deafness.
• Another example of pleiotropy is a human genetic disease called phenylketonuria which produces severs mental retardation. However the affected children also have light hair and light skin colour. This shows that same gene is responsible for more than one phenotypic characters.

Continuously Varying Traits
(Polygenic Inheritance)
Some traits, such as colour of human skin are controlled by two or more than two separate pairs of genes (alleles), located on different gene loci. These genes express themselves in additive manner producing continuous variations in phenotype. Such traits are known as continuously varying traits (polygenic traits) and their inheritance is called polygenic inheritance.
Polygenic traits show a range of intermediate phenotypes between the two parental phenotypes. These intermediate phenotypes appear because each allele produces a small effect on the phenotype and each additional allele increases the effect gradually. Thus the final phenotype is determined by the additive or cumulative effect of all the genes controlling that trait.
Example
The simplest polygenic situation occurs when there are at least two gene pairs which affect the same trait. In a certain wheat variety colour of the kernel is a polygenic trait and is determined by two pairs of genes (alleles). There is a dominant allele for red and a recessive allele for white. Each dominant allele produces a dose of red pigment, while the recessive allele does not produce any pigment. Therefore, the colour of kernel depends on the number of dominant alleles present.

Law of Independent Assortment

Law of Independent Assortment
Mendel summed up the observation and conclusions of the dihybrid cross in the form of law of independent assortment which states
Members of one pair of factors (genes) segregate (assort) independently of members of another pair of factors (genes). In other words when there are two pairs of contrasting characters in the parents, then their distribution to the gametes is independent of each other. Therefore, all possible combination of factors (genes) can occur in the gametes.

In the above cross, it is observed that during gamete formation, genes for round, wrinkled, yellow and green are distributed to the gametes quite independently of each other. Thus four types of gametes are formed. The gene for round seed is combined with gene for yellow colou as well as with green colour. Similarly gene for wrinkled seed combined with gene for yellow as well as green colour. In this way gene did not stay in their original combination, but assortedproducing new combinations.

Inheritance of Two Traits (Law of Independent Assortment)

Inheritance of Two Traits (Law of Independent Assortment)
The study of inheritance of two traits is based on dihybrid cross. It is a cross made between two parents which differ in two pairs of contrasting characters. For example Mendel made a cross between a pure breeding pea plant having round and yellow seeds wiwth a pure breeding plant with wrinkled and greeen seeds. The result showed that in F1 generation all plants had round seeds with yellow colour (Double dominant). When these plants were allowed to self fertilize, the F2 generation showed 4 different varieties in the following ratio
F2
Round Yellow …….. 9
Round Green …….. 3
Wrinkled Yellow …….. 3
Wrinkled Green …….. 1
This F2 phenotypic ratio of 9:3:3:1 is called dihybrid ratio.

Law of Segregation (Mendel’s First Law)

Law of Segregation (Mendel’s First Law)
Mendel’s results and conclusions from his experiment on monohybrid cross are now recognized as a law called the law of segregation. It states that
When pair of contrasting characters are brought together in a diploid individual, they neither mix up nor affect each other and remain intact. During gamete formation genes (factors) for each contrasting character separate or segregate and pass into different gametes. Thus each gamete contains only one factor (allele) for a particular character and is said to be pure for that trait. This separation of genes is called law of segregation. The law is also known as law of purity of gametes.
Single Trait Inheritance
The study of the inheritance of single trait is based on monohybrid cross. It is a cross in which the two parents differ in only one pair of contrasting characters. The results obtained in monohybrid cross are called single trait inheritance.

Method of Study (Mendel’s Experiment)

Method of Study (Mendel’s Experiment)
Mendel planted seeds and grew plants of different varieties of pea. He then made crosses between plants with contrasting characters such as green seeds colour versus yellow seed colour, wrinkled seed coat versus smooth seed coat, etc. Mendel observed pattern of inheritance of characters in succeeding generations and maintained complete statistical record of each cross.
In one of his experiments, Mendel made a cross between pure breeding pea plant having yellow seeds with a pure breeding pea plant with green seeds. He found that in F1 generation (first filial generation) all plants had yellow seeds. These plants were then allowed to be self fertilized to produce the F2 generation. The F2 generation contained both plants with green seeds in the ratio of 3:1. This ratio is called the monohybrid ratio. Same results were obtained for crosses involving six other pairs of contrasting characters and the ratio of dominant to recessive was 3:1.
Conclusions and Results
From the above experiments, Mendel made following conclusions
1. For each trait an organism inherits two factors (genes), one from each parent.
2. Out of a pair contrasting characters, only one appeared morphologically in F1 generation and the other did not show itself. The character taht expressed itself morphologically is called dominant and the other character, which did not appear is known as recessive. This fact is also referred as the Law of Dominance. Thus in pea plant, tall stem is dominant over dwarf stem, round seed coat is dominant over wrinkled coat, yellow seed colour is dominant over green seed colour etc

Mendel’s Law of Inheritance

Mendel’s Work
Gregor Mendel was the first worker who performed real meaningful research on hereditary. Earlier workers who had worked on genetic experiments on plants had failed because they did not use proper method and wrong choice of material.
Choice of Experiment Material
Mendel conducted genetic experiments on garden pea plant, Pisum Sativum. Choice of this material contributed greatly to the success of his experiments. It is a very suitable material for genetic experiments because of following reasons.
Pisum Sativum has several varieties showing distinct pairs of contrasting characters some which are
(a). Length of stem …. Tall or dwarf
(b). Shape of seet coat …. Round or wrinkled
(c). Colour of cotyledons …. Yellow or greeen.

• This plant is naturally self pollinating; thus accidental cross pollination is ruled out. Cross pollination can take place only artificially.
• Artificial cross breeding is easy because flowers are large.
• Resulting hybrids are fertile.
• Life cycle is short, therefore results are known in a short period.
• Mendel studied one trait at a time and ignored complexities that created troubles for earlier workers.

Based on his experiments and observations, Mendel described patterns of inheritance and formulated some laws of hereditary. These laws are
Law of Segregation and Law of independent assortment.

Denitrification

Denitrification
The loss of soil nitrogen is called denitrification and mostly takes place through the activity of denitrifying bacteria such as Pseudomonas. They convert nitrites and nitrates back to molecular nitrogen and return it to atmosphere. These bacteria are anaerobic and use nitrates as oxidizing agent instead of oxygen. Therefore, in well acrated soils their activity is restricted and soil nitrogen is not lost. The lost nitrogen of the soil is made up by fixation of nitrogen from atmosphere.

Nitrogen Cycle

Nitrogen Cycle
The nitrogen is an essential part of protoplasm, proteins, nucleic acids, chlorophyll etc and is therefore, necessary for living organisms. The exchange of nitrogen between soil and atmosphere and its circulation through bodies of living organisms takes place through inter-related processes known as nitrogen cycle.
The atmosphere contains about 78 percent nitrogen gas. It is an inert gas and the organisms cannot use it directly. Therefore, nitrogen is first changed to soluble nitrogen compounds such as nitrates (NO₃) which the plants can absorb from the soil. Conversion of atmospheric nitrogen to soluble nitrogen compounds is called nitrogen fixation.
Source of Nitrates
There are three sources of nitrates for living organisms
i. Nitrogen Fixation by Thunderstorm
ii. Nitrogen Fixation by Living Organisms
iii. Decomposition of Dead Organic Material
i. Nitrogen Fixation By Thunderstorm
The first source of nitrates is thunderstorm and lightening. Nitrogen can combine with atmospheric oxygen at very high energy level. The lightening during rains provides this energy which is used to combine atmospheric nitrogen and oxygen to form nitrogen oxides. These oxides dissolve in water to form nitric acid which falls down into the soil and react with mineral ions there to forms nitrates.
ii. Nitrogen Fixation by Living Organisms
The second source of nitrates are bacteria and cynobacteria (blue, green, algae) which can covert free atmospheric nitrogen into soluble nitrates and nitrates. There are two types
a. Symbiotic Nitrogen Fixation
Symbiotic bacteria, such as Rhizobium, live in the nodulated roots of legumes such as bean, pea etc. These bacteria have the ability to absorb free atmospheric nitrogen and convert it to organic compounds such as nitrates which enter the soil.

b. Non-Symbiotic Nitrogen Fixation
It takes place by the activity of free living soil bacteria like clostridium and Azobaeter and blue green algae like Nostoc etc. They absorb free nitrogen from air and convert it to nitrates of the soil.
Nitrates from Decomposition
The third source of nitrogen for organisms is soil nitrates from decomposition of dead organisms and excretory wastes. This process occurs in two steps
i. Ammonification
ii. Nitrification
i. Ammonification
The dead animals, plants and excretory waste is decomposed by the saprophytic bacteria and fungi to form simple compounds like amino acids, water and carbondioxide. The amino acids are then changed to ammonia and ammonium ions. This process is called ammonification.
ii. Nitrification
Ammonia and ammonium, formed as a result of ammonification is converted to nitrites and nitrates by a process known as nitrification. It takes place by the activity of two groups of bacteria together called the nitrifying bacteria. Nitrite bacteria, such as Nitrosomonas, first convert ammonia to nitrites. The second group the nitrate bacteria such as Nitrobacter, then convert nitrites into nitrates.
The nitrates from all the above sources are taken up from the soil by the plants and are used for formation of plant proteins. These proteins are then taken up by animals as food and converted to animals protein. The dead plants, animals, the excretory waste is decomposed and the nitrates are recycled as described a above.

Biogeochemical Cycles

Biogeochemical Cycles
(Cycles in Ecosystem)
There is a constant recycling of essential nutrients within the biosphere from environment to organism and back to environment. Since the path of recycling involves living organism (bio), soil (geo) and chemical reaction these cycles or circular paths are called biogeochemical cycles. The elements which are recycled include Carbon, Hydrogen, Oxygen, Nitrogen, Phosphate and many others. Carbon, Hydrogen and Oxygen are closely associated with each other. Therefore, they form a single carbo, hydrogen, oxygen cycle, but the nitrogen and phosphorus form independent cycles.

Interdependence of Organism

Interdependence of Organism
(Interactions in an ecosystem)
The organisms in an ecosystem not only interact with their physical environment but also with each other. These interactions may be beneficial or harmful.
Positive Interactions
In this type of interaction organisms form associations for mutual benefit or for benefit of one species without harming the other. Mutualism and commensalisms belong to this type.
Negative Interactions
In this type of interaction one organism gets the benefit and the other is harmed. Parasitism, predation and grazing are considered to be harmful interactions.
The interactions between the organisms lead to close associations between them. There are two basic types of associations:
Intraspecific Association
Associations between members of the same species. These associations are found in social animals for social organizations or for formation of social groups.
Interspecific Associations
Associations between different species. In these associations, members of one species feed on members of another species and may also use them as habitat. Following are the types of interspecific associations
1. Parasitism
Parasitism is an association between two living organisms in which one lives in or on the body of the other and derives nourishment from it. In other words, one gets the benefit and the other is harmed. The organism getting the benefit is called the parasite and the one which is harmed is called the host. There are following types of parasites
Ectoparasites
They live on the surface of the body of the host. Examples, leech, lice, etc.
Endoparasites
They live in the tissues or cavities of the host. Examples Plasmodium, Tnenia, Entamoeba.
2. Symbiosis
Generally the symbiosis is defined as an association between two living organisms of different species for mutual benefit or in which one partner gets the benefit and the other is neither benefited nor harmed. In other words none of the two partners is harmed. There are two varieties of symbiosis
a. Commensalism
b. Mutualism
a. Commensalism
It is an association between two living organism of different species in which one is benefited and the other is not affected, that is it is neither benefited nor harmed. Following are some examples
Epiphytes are plants that grow on trunks and branches of trees for support only but can synthesis their own food. In htis way the epiphyte is benefited without harming the tree. Epiphytes are common in tropical rain forests. Orchids and mosses are common exampels.
Small Fishes live in the cloaca of sea cucumber for protection only. No harm is done to sea cucumber.
b. Mutualism
It is an association between two species of living organisms for mutual benefit. The two partners are dependent on each other for their survival and growth. The organisms in this association may be two animals, two plants or a plant and an animal. Example of mutualism are
Nitrogen Fixing Bacteria
They live in the nodules of leguminous plants. The bacteria fix atmospheric nitrogen for the plant and in return the plant provides shelter for the bacteria.
Lichens
It is an association between green algae and fungus. The algae cells are protected by the fungus and derive moisture from it, while the fungus obtains oxygen and carbohydrates from the algae.
Sea Anemone and Hermit Crab
The sea anemone attaches itself to shell of hermit crab. The anemone gets free transport while the crab is camouflaged by the anemone and is protected from its predators.
3. Predation
In this relationship one population called predators feeds on the members of another population known as prey. Normally predators are larger than the prey and quickly capture and kill the prey. Predators are usually animals bet a few like pitcher plant and other carnivorous plants are plant predators. Mostly the predators are secondary or tertiary consumers of an ecosystem.
The prey-predators interaction keeps the population of both the prey and the predator in equilibrium. Any species of a prey without its natural predator, disturbs the balance of the ecosystem due to explosive increase of their population. The degree of harm caused by this interaction, depends upon the relative population of the prey and predators. For example Ospreys (a kind of hawk) fed on fishes of the ocean. The supply of fishes is so large that this interaction does not cause much harm. However any disturbance in the relative number of prey and predator may result in extinction of any one or both.
4. Grazing
Grazing is a feeding association between herbivores and plants. Natural herbivores include rabbits, deer, zebras, wild horses, elephants etc. Man has also introduced cattle and sheep in the grass land both direct and indirect effects on the ecosystem. Moderate grazing is not harmful, but the over grazing greatly surfaces the growth capacity of grasses and other plants. Consequently all the producers are eliminated and green pastures are turned into barren lands. The indirect effect of overgrazing is hardening of the soil and soil erosion. The plant roots and under ground stem of grasses bind the soil together. Their destruction by grazing makes the soil loose which erodes easily. Too many animals trampling the soil makes it hard and poorly aerated. This may lead to conversion grass lands into deserts. Grasses have many adaptation to save them from effects of grazing.

Approaches to Ecology

Approaches to Ecology
Autecology (Population Approach)
It is the ecological study related to a single species (population). For example if you study effects of pollution on a single mango tree or all the mango trees in an area, it will be autecology.
Synecology (Community Approach)
It is the ecological study related to different species (community). In synecology you have to consider together the individual and the community. The type of ecological study has development the concept of succession and climax that is the progressive replacement of one community by more stable community in a particular area.
Ecosystem Approach
This approach takes into consideration the whole ecosystem as a unit. Ecosystem approach studies the biotic and the abiotic components their interaction, the flow of energy and the cycling of materials between the living and the non living and the non living components. This approach may take into consideration a small ecosystem such as a pond or a large ecosystem such as a desert.
Habitat Approach
Habitat is a particular place or area with a specific set of physical conditions in which a community of living organisms live. These organisms possess special adaptations which help them to survive in that habitat. The flora and launa of different habitat are different because of different physical conditions. Since defining habitat is relatively easy, some scientists use this approach for ecological studies. Some ecologists divide the ecology on the basis of type of habitat in which the organisms live. Habitat is divided in two main types; aquatic and terrestrial. Aquatic habitat is of three types; fresh water habitat, marine habitat and estuarine habitat. Estuaries are places where rivers and streams meet the sea. Terrestrial ecology can be divided into forests, desert, grass land habitat.
Evolutionary Approach
The distribution of organisms is result of long term evolutionary changes and their interaction with environment. Evolutionary approach tells us about changes since life originated. This knowledge comes from study of fossil record. Various evolutionary theories are also part of this approach.
Historical Approach
It deals with study of various stages and periods of time through which humans have passed during their evolution. One aspect of this approach is to study the development of tools in human history. The first period is called stone age or Neolithic period (1500 to 3000 B.C), during which humans used tools made up of stones.