A double stranded DNA fragment has 500 Adenine bases. If total number of bases in this fragment is 2500, then what will be the number of guanine bases?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 750
In double-stranded DNA, A pairs with T and G pairs with C (Chargaff’s rule), so A = T and G = C.
A = 500, so T = 500. A + T = 1000. The fragment has 2500 bases in all, so G + C = 2500 − 1000 = 1500. Since G = C, G = 750.
Option (1) 500 — Incorrect. 500 is the number of adenine (and thymine) bases.
Option (2) 750 — Correct. In double-stranded DNA, A = T and G = C. With 500 adenine there are 500 thymine, so A + T = 1000. The rest, 2500 − 1000 = 1500, is G + C, and half of that is guanine: 750.
Option (3) 1000 — Incorrect. 1000 is A + T together.
Option (4) 2000 — Incorrect. 2000 would come from taking 2500 as base pairs (5000 bases).
NCERT Extract:
The DNA – “… One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chains. However, this proposition was also based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.”