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.
A DNA molecule consists of double deoxyribonucleotide chains twisted around a common axis to form a:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Right-handed double helix.
In the double helix model of Watson and Crick, the two polynucleotide chains of DNA wind around a common axis like a spiral staircase. The direction in which they wind is the handedness of the helix.
Option (1) Only left-handed double helix. — Incorrect. The book describes the chains as coiled in a right-handed fashion, not left-handed.
Option (2) Right-handed double helix. — Correct. The two chains are coiled in a right-handed fashion. One full turn (the pitch) is 3.4 nm long and holds about 10 base pairs.
Option (3) In upper half left-handed and in lower half right-handed double helix. — Incorrect. There is no such mixed helix. The whole molecule is coiled in one direction, the right-handed one, along its full length.
NCERT Extract:
Structure of Polynucleotide Chain – “… (iv) The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm (a nanometre is one billionth of a metre, that is 10-9 m) and there are roughly 10 bp in each turn. Consequently, the distance between a bp in a helix is approximately 0.34 nm. (v) The plane of one base pair stacks over the other in double helix. This, in addition to H-bonds, confers stability of the helical structure ….”