Given below are two statements:
Statement I: In a DNA double helix, the two chains have anti-parallel polarity.
Statement II: In a DNA double helix, the two chains are coiled in a left-handed fashion.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Statement I is correct but Statement II is incorrect.
Watson and Crick's double helix has two polynucleotide chains running side by side. NCERT lists how these chains are arranged: they run in opposite directions and they are coiled in a particular direction.
Statement I: In a DNA double helix, the two chains have anti-parallel polarity. — True. If one chain runs 5′ → 3′, the other runs 3′ → 5′.
Statement II: In a DNA double helix, the two chains are coiled in a left-handed fashion. — False. The two chains are coiled in a right-handed fashion.
So Statement I is correct and Statement II is incorrect.
NCERT Extract:
Structure of Polynucleotide Chain – “… (ii) The two chains have anti-parallel polarity. It means, if one chain has the polarity 5′ → 3′, the other has 3′ → 5′. (iii) The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). … (iv) The two chains are coiled in a right-handed fashion.”