The DNA
Read the following passage carefully and answer the given questions.
DNA is a long polymer of deoxyribonucleotides. The length of DNA is usually defined as number of nucleotides present in it. A nucleotide is the combination of three different components, such as a nitrogenous base, a pentose sugar (ribose in the case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – purines and pyrimidines. When a phosphate group is linked to OH of 5′ C of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide, depending upon the type of sugar present) is formed. James Watson and Francis Crick, in 1953, proposed a very simple but famous Double Helix model for the structure of DNA based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin. One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chains. However, this proposition was also based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.
Frederick Griffith proposed -
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Transforming principle
In 1928, Frederick Griffith worked with the bacterium Streptococcus pneumoniae. When heat-killed S (killer) bacteria were mixed with live R (harmless) bacteria and injected into mice, the mice died. Live S bacteria were recovered from them. So the R bacteria had been transformed by something from the dead S bacteria. Griffith called it the ‘transforming principle’, but he could not tell what it was chemically.
(1) DNA as genetic material — Incorrect. This was proved by Hershey and Chase with bacteriophages.
(2) Semiconservative mode of DNA replication — Incorrect. This was proved by Meselson and Stahl using 15N.
(3) DNA is hereditary material — Incorrect. Griffith did not identify DNA. Avery, MacLeod and McCarty later showed that the transforming principle is DNA.
(4) Transforming principle — Correct. Griffith proposed the transforming principle.
Note: 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 Search for Genetic Material – “… Griffith was able to kill bacteria by heating them. He observed that heat-killed S strain bacteria injected into mice did not kill them. When he injected a mixture of heat-killed S and live R bacteria, the mice died. Moreover, he recovered living S bacteria from the dead mice. He 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.”