Properties of Genetic Material (DNA versus RNA) – Class 12 | Chapter – 6 | Biology Short Notes Series PDF
Properties of Genetic Material: DNA is the main hereditary material, but some contain RNA too, e.g. retrovirus. RNA mostly plays the role of a messenger in higher organisms. Though both are nucleic acids, why is there a difference in functions? Scientists elucidate that, this is due to the difference between the chemical structure of DNA and RNA. For a molecule to be considered as genetic material, it must accomplish certain properties.
Properties of Genetic Material
It became established that DNA is the genetic material from the Hershey-Chase experiment.
In some viruses, RNA was also reported as genetic material, e.g. Tobacco mosaic viruses, QB bacteriophage, etc.
According to the above mentioned rules, both the nucleic acids (DNA and RNA) have the ability to direct duplications.
Stability can be explained in DNA as the two strands being complementary if separated by heating come together in appropriate conditions.
The 2′ — OH group present at every nucleotide in RNA is a reactive group and makes RNA labile and easily degradable, hence it is reactive.
DNA is chemically less reactive and structurally more stable when compared to RNA. Thymine also confers additional stability to DNA. So, among the two nucleic acids, the DNA is a predominant genetic material.
Both RNA and DNA are able to mutate. Viruses having RNA genome and having shorter life span mutate and evolve faster.
DNA is dependent on RNA for protein synthesis, while RNA can directly code for it. The protein synthesising machinery has evolved around RNA. This concluded that the DNA being more stable is suitable for storage of genetic information, while for the transmission of genetic information, RNA is suitable.
Characteristics of a Genetic Material
It should be able to replicate.
It should be chemically and structurally stable.
It should provide scope for slow changes (mutation) that are required for evolution.
It should be able to express itself in the form of ‘Mendelian characters’.
RNA as a Genetic Material
RNA was first genetic material.
Essential life processes (such as metabolism, translation, splicing, etc.) evolved around RNA.
RNA acts as genetic material as well as catalyst.
RNA being a catalyst was reactive and hence unstable. Therefore, DNA has evolved from RNA with chemical modifications that make it more stable.
DNA being double stranded and having complementary strand resist changes by evolving a process of repair.
DNA as a Genetic Material
Alfred Hershey and Martha Chase (1952) gave unequivocal proof that DNA is the genetic material.
In their experiments, bacteriophages (viruses that infect bacteria) were used.
They grew some viruses on a medium that contained radioactive phosphorus and some others on sulphur containing radioactive medium.
Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein because DNA contains phosphorus but protein does not. In the same way, viruses grown on radioactive sulphur contained radioactive protein, but not radioactive DNA because DNA does not contain sulphur.
Radioactive phages were allowed to attach to E. coli bacteria. As the infection proceeded, viral coats were removed from the bacteria by agitating them in a blender. The virus particles were separated from the bacteria by spinning them in a centrifuge.
Bacteria which were infected with viruses that had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to the bacteria.
Bacteria that were infected with viruses that had radioactive proteins were not radioactive. This indicated that the proteins did not enter the bacteria from viruses. It proved that DNA is a genetic material that is passed from virus to bacteria.