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.
The two polynucleotide chains of a DNA molecule are:
(i) Identical
(ii) Complementary to each other w.r.t. base pairs
(iii) A specific purine pairs with a specific pyrimidine
(iv) The bases in two strands are paired through hydrogen bonds (H-bonds) forming base pairs (bp).
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (ii), (iii) and (iv) only
(i) Identical — False. The two chains are not identical. Where one chain has adenine, the other has thymine, and where one has guanine, the other has cytosine. The chains are complementary, not copies of each other.
(ii) Complementary to each other w.r.t. base pairs — True. Because of base pairing (A with T, G with C), the sequence of one chain decides the sequence of the other. If one strand is known, the other can be predicted.
(iii) A specific purine pairs with a specific pyrimidine — True. Adenine (a purine) pairs only with thymine (a pyrimidine), and guanine (a purine) pairs only with cytosine (a pyrimidine). So a purine always comes opposite a pyrimidine, which keeps the distance between the two strands almost uniform.
(iv) The bases in two strands are paired through hydrogen bonds (H-bonds) forming base pairs (bp). — True. A and T are joined by two hydrogen bonds, and G and C by three hydrogen bonds.
NCERT Extract:
Structure of Polynucleotide Chain – “… 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.”
Structure of Polynucleotide Chain – “… (iii) The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix (Figure 5.2).”