Q.State Chargaff's rule. How did Watson and Crick's double-helical model of DNA structurally explain this rule?

UMP contains ribose sugar and is a ribonucleotide (RNA), not a deoxyribonucleotide.

A deoxyribonucleotide consists of a nitrogenous base + deoxyribose sugar + phosphate group, and is a building block of DNA. dAMP, dGMP, and dCMP (deoxy-Adenosine/Guanosine/Cytidine monophosphate) are all deoxyribonucleotides found in DNA.

UMP (Uridine monophosphate) contains the base uracil attached to ribose sugar (not deoxyribose) - uracil replaces thymine in RNA, and UMP is therefore a ribonucleotide, a component of RNA, not DNA.

Chargaff's rule: A = T and G = C, so total purines = total pyrimidines, i.e. A + G = T + C — the answer is (c).

Chargaff's rule (Erwin Chargaff) states that in a double-stranded DNA:

• Amount of adenine = amount of thymine ($A = T$), and
• Amount of guanine = amount of cytosine ($G = C$).

Adding these: $A + G = T + C$, i.e. the total purines (A + G) equal the total pyrimidines (T + C). Also the ratio $\dfrac{A+G}{T+C} = 1$ (not option (d), which wrongly writes $(A+T)/(G+C)=1$ — that ratio varies between species).

Checking the options: (a) G = T ✗, (b) A = C ✗, (d) $(A+T)/(G+C)=1$ ✗ (not universally true), (c) A + G = T + C ✓.