Genetic Code

Genetic Code
Definition

The sequence of nitrogenous bases that specify the amino acids and the positions of the starting and stopping of chain of the translation is called Genetic Code.
Type of Genetic Codes
The nitrogen base and amino acids from different codes by their combined functions. The types of codes are as follows.
1. Single Code System
2. Double Code System
3. Triple Code System
1. Single Code System
When one nitrogen base works for one amino acid, then only four types of genetic codes are formed. There are 20 basic amino acid not synthesized by only four codes.
2. Double Code System
When two nitrogen bases work for one amino acid, it is called double code system. In this system 16 possible codes may be formed.
3. Triple Code System

• There must be at least three base sequence to code for 20 amino acids.
• Sine the total no of possibilities of variations is 64 (4 x 4 x 4 = 64). They can code for all the amino acids and also code for the start and stop sequences.
• They above hypothesis was found to be correct by Francis Crick and coworkers in 1961.
• Other scientists took one step forward and found the specific codes for the specific amino acids by adding artificial messenger RNA to the bacteria and getting the particular amino acids e.g. RNA composed entirely of Uraeil (UUU……) directed the mixture of synthesize a protein composed solely of phenylalanine. Therefore the triplet UUU specify phenylalanine amino acids. These mRNA triplets are called Codons.
• The research showed that codon AUG codes for start and three codons UAG codes for start and three codons, UAG. UAA and UGA code for the stop signal.
• It was further found out that the amino acids may specifically coded by more than one but specific codons, so there were more than one combinations possible for a single amino acids e.g. six different codons, all codes for arginine amino acids.

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.
Mutation

• 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
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.

Vernalization
Definition
Promotion o flowering by a cold treatment give to the imbeded seeds or young plant is called Vernalization.
OR
The phenomenon of cold treatment which shortens the vegetative period and hastens flowering is known as Vernalization.
Chourd (1960) defined vernalization as
The acceleration of the ability to flower by a chilling cold treatment.
Stimulation of Hormone
The process of vernalization does not induce flowering but prepares the plant for flowering. It stimulates the production of vernalin hormone which induce vernalization.
Variation in Cold Treatment
The duration of cold temperature (chilling) treatment may vary in different plants i.e. wheat requires cold treatment at seed stage while some plant need cold treatment when they become at least 10 days old.
Advantages of Vernalization

• Out of season flowers can be produced by vernalization.
• It can used to increase the yield of crops.
• Vernalization is an important method to protect plant from drought conditions