Nitrogenous Bases vs Nucleosides — NEET Biology

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.