Q.The number of purines in a segment of DNA molecule is 68. What will be the number of pyrimidines in this segment?

(d) phosphate group — a nucleoside is a nitrogenous base joined to a pentose sugar; adding a phosphate group to a nucleoside makes it a nucleotide.

Nucleoside = base + sugar; nucleotide = base + sugar + phosphate.

A nucleic acid's monomer, the nucleotide, is built in two stages. First, an N-glycosidic bond joins a nitrogenous base (purine or pyrimidine) to a pentose sugar (ribose in RNA, deoxyribose in DNA) — this base-sugar unit alone is called a nucleoside (e.g. adenosine, guanosine, cytidine, thymidine). A nucleotide is formed only when a phosphate group …

Correct answer: (c) 68

By Chargaff's rule, purines always equal pyrimidines in double-stranded DNA.

In a double-stranded DNA molecule, adenine (a purine) always pairs with thymine (a pyrimidine) by two hydrogen bonds, and guanine (a purine) always pairs with cytosine (a pyrimidine) by three hydrogen bonds. Consequently, the total number of purine bases (A + G) in a DNA segment is always equal to …