Which one of the following statements is not correct regarding the DNA?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The two chains are coiled in a left-handed fashion and the pitch of the helix is 3.4 nm.
Option (1) DNA as an acidic substance present in nucleus was first identified by Friedrich Meischer in 1869. — Correct regarding the DNA. Meischer found an acidic substance in the nucleus in 1869 and named it ‘Nuclein’. Its structure was worked out much later.
Option (2) The two chains are coiled in a left-handed fashion and the pitch of the helix is 3.4 nm. — Not correct regarding the DNA. The pitch of 3.4 nm is right, but the coiling is right-handed, not left-handed. One change of direction makes the whole statement wrong.
Option (3) The plane of one base pair stacks over the other in double helix and confers stability of the helical structure. — Correct regarding the DNA. The flat base pairs lie one above the other like the steps of a spiral staircase. This stacking, along with the hydrogen bonds, keeps the helix stable.
Option (4) The two chains have anti-parallel polarity, means if one chain has the polarity 5' to 3', the other has 3' to 5'. — Correct regarding the DNA. The two strands run in opposite directions. This is why each strand is read and copied in its own direction.
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.”