Q.Griffith worked on .............

(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 is further esterified onto the sugar's 5' carbon. So the single structural difference between a nucleoside and a nucleotide is the phosphate group — a nucleoside is simply 'a nucleotide without its 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 the total number of pyrimidine bases (T + C) — this is Chargaff's base-pairing rule. Since the segment contains 68 purines, it must also contain 68 pyrimidines.