Which one of the following statements about the genetic material is correct?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → In some viruses RNA is the genetic material, while in most organisms it is DNA.
The Hershey-Chase experiment proved that DNA is the genetic material, and DNA plays this role in most organisms. Later it became clear that in some viruses, such as Tobacco Mosaic Virus and QB bacteriophage, the genetic material is RNA.
Option (1) In Tobacco Mosaic Virus and QB bacteriophage, DNA is the genetic material. — Incorrect. Tobacco Mosaic Virus and QB bacteriophage are NCERT's own examples of viruses with RNA as the genetic material, not DNA.
Option (2) In some viruses RNA is the genetic material, while in most organisms it is DNA. — Correct. RNA is the genetic material in some viruses, while DNA is the genetic material in most organisms. This matches NCERT.
Option (3) In bacteriophage lambda and Escherichia coli, RNA is the genetic material. — Incorrect. Bacteriophage lambda and E. coli have DNA as their genetic material; NCERT gives their DNA length in base pairs (lambda 48502 bp). Neither uses RNA.
Option (4) In Streptococcus pneumoniae, RNA was found to be the transforming principle. — Incorrect. Avery, MacLeod and McCarty found that the transforming principle in Streptococcus pneumoniae was DNA. RNA-digesting enzymes (RNases) did not affect transformation, so it was not RNA.
NCERT Extract:
Properties of Genetic Material (DNA versus RNA) – “From the foregoing discussion, it is clear that the debate between proteins versus DNA as the genetic material was unequivocally resolved from Hershey-Chase experiment. It became an established fact that it is DNA that acts as genetic material. However, it subsequently became clear that in some viruses, RNA is the genetic material (for example, Tobacco Mosaic viruses, QB bacteriophage, etc.).”
Biochemical Characterisation of Transforming Principle – “… They discovered that DNA alone from S bacteria caused R bacteria to become transformed. They also discovered that protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not affect transformation, so the transforming substance was not a protein or RNA.”