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.
In a DNA molecule, the two polynucleotide chains are:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Antiparallel
Each polynucleotide chain has a direction (polarity): one end is the 5′-end and the other is the 3′-end. In the double helix the two chains run in opposite directions.
Option (1) Parallel — Incorrect. Parallel chains would run in the same direction, both 5′→3′. In DNA, if one chain runs 5′→3′, the other runs 3′→5′, so they are not parallel.
Option (2) Discontinuous — Incorrect. “Discontinuous” describes replication, not the structure of the chains. During replication, the new strand on one template is made as small pieces that are later joined by DNA ligase.
Option (3) Semiconservative — Incorrect. “Semiconservative” is the mode of DNA replication: each new DNA molecule keeps one old (parental) strand and gets one newly made strand. It does not describe how the two chains lie in the molecule.
Option (4) Antiparallel — Correct. The two chains have anti-parallel polarity. One chain runs 5′→3′ and its partner runs 3′→5′.
NCERT Extract:
Structure of Polynucleotide Chain – “… The salient features of the Double-helix structure of DNA are as follows: (i) It is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside. (ii) The two chains have anti-parallel polarity. It means, if one chain has the polarity 5′→3′, the other has 3′→5′.”