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.
What is the distance between two successive nitrogenous base pairs in DNA molecule?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 0.34 nm/3.4 Å
One full turn of the DNA double helix (its pitch) is 3.4 nm long, and there are about 10 base pairs in each turn. So the distance from one base pair to the next is 3.4 nm ÷ 10 = 0.34 nm. Since 1 nm = 10 Å (angstrom), 0.34 nm is the same as 3.4 Å.
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.”