DNA Polynucleotide Chain Structure — NEET Biology
📚 Practice ConceptSimilarly, at the other end of the polymer the sugar has a free OH of 3'C group which is referred to as 3'-end of the polynucleotide chain. The backbone of a polynucleotide chain is formed due to sugar and phosphates. The nitrogenous bases linked to sugar moiety project from the backbone.
Which of the following statements are NOT correct regarding the structure and stability of DNA? S1. The two polynucleotide chains of DNA run antiparallel to each other, one in 5′→3′ direction and the other in 3′→5′ direction. S2. The backbone of the DNA strand is formed by nitrogenous bases and phosphate groups. S3. Stacking of one base pair over another, along with hydrogen bonding, contributes to the stability of the DNA double helix. S4. In a double-stranded DNA molecule, adenine pairs with uracil and guanine pairs with cytosine. S5. DNA is less stable than RNA due to the presence of a reactive 2′-OH group on the sugar moiety.
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Correct answer: A — S2, S4 and S5
S1: The two polynucleotide chains in DNA indeed run antiparallel, with one having 5'→3' polarity and the other 3'→5' polarity. This statement is correct, as stated in feature (ii) of the double-helix structure of DNA. S2: The backbone of a polynucleotide chain is formed due to sugar and phosphates, not nitrogenous bases and phosphate groups. The nitrogenous bases project from the backbone. Thus, S2 is incorrect. S3: The plane of one base pair stacks over the other in the double helix, and this, in addition to H-bonds, confers stability to the helical structure. This statement is correct, as per feature (v) of the DNA structure. S4: In a double-stranded DNA molecule, Adenine pairs with Thymine, not Uracil. Uracil is found in RNA. The ratio between Adenine and Thymine (A/T) and Guanine and Cytosine (G/C) is constant and equals one (Chargaff's rule) in a double-stranded DNA. Thus, S4 is incorrect. S5: DNA is chemically less reactive and structurally more stable when compared to RNA. The presence of the 2'-OH group at every nucleotide in RNA makes RNA labile and easily degradable. DNA lacks this 2'-OH group. Therefore, this statement correctly attributes greater stability to DNA compared to RNA, partly due to the absence of the 2'-OH group. Thus, S5 is incorrect because it implies the presence of 2'-OH in DNA, which actually makes RNA unstable. Therefore, statements S2, S4, and S5 are NOT correct.
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