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.
When smooth walled bacteria were heated, the DNA became non functional due to:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Denaturation of DNA
Note 1: The given answer is as per the official NTA Answer Key. However, the question says that heating made the DNA non-functional, which does not match Griffith’s experiment in the passage, as explained below.
In Griffith’s experiment the heat-killed S bacteria still passed on the transforming principle. So the DNA did not lose its function. What heat does to DNA is denaturation: the two complementary strands separate. They come together again when suitable conditions return. Of the four options, denaturation is the only change that heat causes in DNA.
(1) Denaturation of DNA — Correct, as per NTA. Heat separates the two strands of the double helix.
(2) Folding of DNA — Incorrect. Heat does not fold DNA.
(3) Addition of individual bases and sugars of DNA — Incorrect. Heat does not add parts to DNA. It can only separate or break it.
(4) Degradation of histone proteins — Incorrect. Histones are the proteins that pack the DNA of eukaryotes. The question is about the DNA of bacteria, and it does not concern proteins.
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:
Properties of Genetic Material (DNA Versus RNA) – …
