TRANSFORMING PRINCIPLE
Read the following passage carefully and answer the given questions.
When Streptococcus pneumoniae bacteria are grown on a culture plate, some produce smooth shiny colonies while others produce rough colonies. This is because the S strain bacteria have a mucous coat, while R strain does not. Mice infected with the S strain die from pneumonia infection but mice infected with the R strain do not develop pneumonia. Griffith concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria. Some ‘transforming principle’, transferred from the heat-killed S strain, had enabled the R strain to synthesise a smooth polysaccharide coat and become virulent. This must be due to the transfer of the genetic material. However, the biochemical nature of genetic material was not defined from his experiments.
DNA is the genetic material in all organisms, is confirmed by the work of:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Alfred Hershey and Martha Chase
Hershey and Chase worked with bacteriophages, viruses that infect bacteria. They grew one lot of viruses with radioactive phosphorus, which marks DNA, and another lot with radioactive sulfur, which marks protein. Only the radioactive DNA passed into the bacteria. This was the unequivocal proof that DNA is the genetic material.
(1) Friedrich Meischer — Incorrect. He first found an acidic substance in the nucleus and named it nuclein. He did not show that it carries heredity.
(2) Matthew Meselson and Franklin Stahl — Incorrect. They showed that DNA replicates in a semi-conservative way.
(3) Erwin Chargaff — Incorrect. He found that in DNA the amount of A equals T and the amount of G equals C. He did not test which molecule is the genetic material.
(4) Alfred Hershey and Martha Chase — Correct. Their phage experiment (1952) settled that DNA is the genetic material.
Note 1: In the original NTA paper, Option (3) was ‘Oswald Avery, Colin MacLeod and Maclyn McCarty’. The NTA Answer Key accepts both Option (3) and Option (4). Option (3) has been replaced here so that there is no confusion and only one option is correct. Avery, MacLeod and McCarty showed that DNA is the transforming principle in bacteria, while Hershey and Chase gave the unequivocal proof.
Note 2: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
The Genetic Material Is DNA – …
