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Question

Which of the following is not applicable to single stranded DNA?

A
Chargaff's rule
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B
Absence of uracil
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C
5' phosphoryl and 3' hydroxyl ends
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D
Heterocyclic nitrogenous bases
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Solution

The correct option is A Chargaff's rule
Chargaff's rule states that the quantity of purines equals to that of pyrimidine in a double stranded DNA. The number of adenine residues equals to that of thymine and that of cytosine residues equals to that of guanine, that is, [A] = [T] and [G] = [C]. Therefore, the ratio [A][T] = [G][C] = 1. This, however, cannot be generalised in a single stranded DNA molecule in which hydrogen bonding between complementary base pairs is not there. Possibilities of being AT rich or GC rich is there for single stranded DNA.

All other statements about single stranded DNA are correct. DNA is a polynucleotide which is composed of nucleotides (nucleoside + phosphate group). Nucleosides contain pentose sugar and nitrogenous bases (heterocyclic ring is present). The DNA strand has phosphoryl and hydroxyl group at its free 5' and 3' ends respectively.

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