Question

According to Chargaff's rules, which base relationship is constant across all species?

• A. (A = C)
• B. (A = G)
• C. (A = T)
• D. (G = T)

Hint:

{Chargaff Rule:}
(A = T) and (G = C).
Purines equal pyrimidines in double-stranded DNA.

Answer 1

The Correct Option is C

Concept: Chargaff's rules describe the base composition relationships found in double-stranded DNA. These rules were discovered by the biochemist Erwin Chargaff and helped in understanding the structure of DNA.
Step 1:Complementary base pairing in DNA.
In double-stranded DNA, nitrogenous bases pair specifically through hydrogen bonding: [ \text{Adenine (A)} \leftrightarrow \text{Thymine (T)}, \quad \text{Guanine (G)} \leftrightarrow \text{Cytosine (C)} ]
Step 2:Chargaff's base ratio observation.
Chargaff observed that in DNA extracted from different organisms: [ A = T \quad \text{and} \quad G = C ]
Step 3:Implication of the rule.
Although the overall base composition may vary among species, the pairing rule ensures that the amount of adenine is always equal to thymine, and guanine equals cytosine. Thus, according to Chargaff's rules, the constant base relationship is: [ A = T ]