Properties of Genetic Material (DNA versus RNA)

The debate over whether proteins or DNA carried genetic information was settled by the Hershey-Chase experiment. DNA became established as the genetic material for most organisms. However, it was later discovered that in some viruses — such as Tobacco Mosaic virus and QB bacteriophage — RNA serves as the genetic material.

This raises a natural question: why is DNA the predominant genetic material, while RNA is used for dynamic roles like messenger and adapter? The answer lies in the chemical differences between the two nucleic acids.

Chemical differences between DNA and RNA

• DNA has a deoxyribose sugar (lacks an oxygen atom at the 2' position); RNA has a ribose sugar (has a 2'-OH group).
• DNA uses the base thymine; RNA uses uracil in place of thymine.

Criteria for a molecule to act as genetic material

A molecule that serves as genetic material must satisfy four requirements:

1. Replication — It must be able to generate its own replica.
2. Stability — It must be chemically and structurally stable.
3. Scope for slow changes (mutation) — It must allow gradual changes that drive evolution.
4. Expression — It must be able to express itself in the form of Mendelian characters.

How DNA and RNA meet these criteria

Replication: Both DNA and RNA can direct their own duplication, thanks to the rule of base pairing and complementarity. Proteins fail at the very first criterion — they cannot replicate themselves.

Stability: The genetic material must remain stable across different stages of the life cycle, age, or changes in physiology. Griffith's transforming principle already hinted at this stability: heating killed the bacteria but did not destroy the genetic material's properties. This is now understood because the two complementary strands of DNA, if separated by heating, can come back together under suitable conditions.

RNA, on the other hand, has a 2'-OH group on every nucleotide. This group is reactive, making RNA labile and easily degradable. RNA is also known to be catalytic, hence reactive. DNA, lacking this group, is chemically less reactive and structurally more stable. The presence of thymine instead of uracil also adds to DNA's stability (the detailed repair mechanisms that rely on this difference are studied in higher classes).

Mutation: Both DNA and RNA can mutate. Because RNA is unstable, it mutates at a faster rate. Viruses with RNA genomes, having shorter life spans, mutate and evolve more quickly. …