Criteria for Genetic Material
DNA was found to be the prominent genetic material in most organisms.
Exceptions were some viruses where RNA was the genetic material.
What made DNA eligible to be the genetic material, and not other molecules like proteins, carbohydrates etc.
Important criteria to be fulfilled to be a genetic material are:
- Capable of replicating itself
- Chemically & structurally stable
- Provide scope for mutation which can lead to evolution
- Capable of expressing itself in the form of 'Mendelian Characters’
Most of the other molecules like proteins, carbohydrates, lipids failed to fulfill the above mentioned criteria.
However, RNA could also fulfill the criteria; still DNA was a preferred genetic material over RNA because of the following reasons:
- DNA is structurally more stable than RNA
- DNA is chemically more stable than RNA
- DNA has double-stranded structure which provides better ability to rectify errors during replication
- DNA can’t code directly for protein synthesis & thus depends on RNA
DNA was thus used for storage of genetic information due to its structural & chemical stability. RNA, on the other hand was used for expression of genetic information as it could directly code for proteins.
RNA World
The question that was bothering everyone was which molecule (DNA or RNA) started to exist in the world before. Walter Gilbert in 1986 hypothesized that there was once an RNA world. According to this hypothesis, all that we see today has descended from an RNA world.
RNA world was a kind of hypothetical world where RNA performed all the activities which are today performed by DNA & proteins. DNA later evolved from RNA with chemical modifications which made it more stable.
There are many scientists who do not agree to this hypothesis. Research is still going on to validate the RNA world hypothesis.
