Read the given passage and answer the following question:
It was only in 1953 that James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed a very simple but famous Double Helix model for the structure of DNA. 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. The base pairing confers a very unique property to the polynucleotide chains. They are said to be complementary to each other, and therefore if the sequence of bases in one strand is known then the sequence in other strand can be predicted.
Assertion (A): Adenine and Guanine are pyrimidines.
Reason (R): Purines include Adenine and Guanine, while Pyrimidines include Cytosine, Uracil, and Thymine.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A is false, but R is true.
Assertion (A): Adenine and Guanine are pyrimidines. — False. Adenine and guanine are purines, not pyrimidines. Purines have a double-ring structure.
Reason (R): Purines include Adenine and Guanine, while Pyrimidines include Cytosine, Uracil, and Thymine. — True. The nitrogenous bases are of two types. Purines are adenine and guanine. Pyrimidines are cytosine, uracil and thymine. Cytosine is found in both DNA and RNA, thymine only in DNA, and uracil takes the place of thymine in RNA.
So A is false and R is true. R in fact shows why A is wrong.
NCERT Extract:
Structure of Polynucleotide Chain – “Let us recapitulate the chemical structure of a polynucleotide chain (DNA or RNA). A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine). Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine.”