Chromosomes as Carrier of Genes

Chromosomes as Carrier of Genes
Genes are small bodies found in the chromosome.
Chromosome 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 chromosome and gene behave as hereditary units but the genes can not be considered outside the chromosome.
• At the time of 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.

Chromosomal Theory of Heredity
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
1. Hereditary Materials
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 contain little cytoplasm, therefore the hereditary material must reside within the nuclei of the gametes.
2. Segregation of Chromosomes
Chromosomes segregated during meiosis in a manner similar to that exhibited by the elements of Mendel’s model.
3. Number of Chromosome
Gametes have one copy of each pair of homologous chromosomes, diploid individuals have two copies.
4. Independent Assortment
During meiosis each pair of homologous chromosomes orients on the metaphase plate independent of any other pair.
Objection
The objection on chromosomal theory of hereditary is that when there is independent assortment of chromosomes in meiosis, the number of factors (genes) is more than the number of chromosomes. This is considered as a fatal objection about Sutton’s theory.
Evidence
The material which transmits the parental characters into the coming generation is called Hereditary Material.
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 steptococcus pneumoniae, which occurs in two distinct different forms.
R-Type
Rough surfaced, non-capsulated bacteria, without the capability of producing pneumonia.
i.e. non-virulent
S-Type
Smooth surfaced, capsulated bacteria, with the capability of producing pneumonia i.e. virulent.
Steps of Experiment

• He observed that when the injected R-type bacteria in the mice, there was no ill effect.
• When he injected the S-type, they proved to be fatal.
• 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.
• He also observed that, when the injected the living R-type with heat-killed S-type, there was a high morality 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.
A Very, 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.
Hershey and Chase’s Experiment
Introduction
In 1952, Hershey and chase performed experiment to proof that DNA is a hereditary material.
Experience at Material
Hershey and chase labeled protein coat and DNA of Bacteriophage separately. Protein coat labeled with radioactive sulphur and DNA with radioactive phosphorus. These two viruses use to attack bacterial cells.
Steps Experiment

• Hershey and chase observed that if cultures of bacteriophage are labeled with radioactive phosphorus [P32 labeling DNA] or with sulphur [S35 for labeling protein coat].
• Bacteriophage is ruptured, the DNA is released and treated with deoxyribsonucleas, the DNA breaks up into fragments in the solution.
• The empty protein coats of the ruptured membrane appear as coats all the P32 or S35 were made to inject bacteria and multiply by the help of special technique, all the S35 labeled protein were removed.
• The new phage formed contained only P32 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.