Q.45 Given below are two statements. One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: The number of Adenine bases in a DNA molecule is always equal to Thymine bases.
Reason R: As per Chargaff’s principle, in a given DNA molecule the proportion of purines and pyrimidines is
always equal.
In the light of above, choose the correct answer from the options given below:
1. Both A and R are true and R is the correct explanation of A
2. Both A and R true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true
Assertion A Analysis
Assertion A states that adenine (A) bases always equal thymine (T) bases in a DNA molecule. This is true due to Chargaff’s first rule, which confirms A = T and G = C in double-stranded DNA from complementary base pairing.
Reason R Analysis
Reason R claims Chargaff’s principle states purines (A + G) always equal pyrimidines (T + C). This is Chargaff’s second rule (A + G = T + C), which is also true as it maintains DNA helix uniformity.
Why Option 2 is Correct
Both statements are factually true, but R does not explain A. A specifically addresses A-T pairing (first rule), while R describes overall purine-pyrimidine balance (second rule)—they are related but distinct principles.
Options Breakdown
| Option | Explanation | Correct? |
|---|---|---|
| 1. Both true, R explains A | Incorrect—R covers purine-pyrimidine equality, not specifically A-T pairing. | No |
| 2. Both true, R does not explain A | Correct—A from first rule; R from second rule; no direct explanation link. | Yes |
| 3. A true, R false | Incorrect—R accurately states Chargaff’s second rule. | No |
| 4. A false, R true | Incorrect—A holds from base pairing in dsDNA. | No |
Exam Preparation Tips
Master Chargaff’s rules for molecular biology questions: A=T, G=C (base pairs); A+G=T+C (purine-pyrimidine). Practice similar assertion-reason formats to spot explanation mismatches quickly.
