Select the correct answers to the questions from the given options. (Do not copy the questions, write the correct answer only.)
A sequence of DNA has 300 nitrogenous base pairs, of which 75 are Guanine. What is the number of Thymine in this sequence?
Topic: Chargaff’s rule — base pairing
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Answer
(d) 75.
Use CHARGAFF’S RULE: because adenine always pairs with thymine and guanine always pairs with cytosine, A = T and G = C.
Working, step by step: Guanine = 75, so Cytosine = 75 (since G = C) G + C together = 75 + 75 = 150 The rest must be A and T: 300 − 150 = 150 A = T, so Thymine = 150 ÷ 2 = 75
THE TRAP is arithmetic, not biology. Having found G + C = 150, it is easy to subtract and then forget the final halving, or to halve the wrong number. Write the four lines out; the question is worth one mark and takes fifteen seconds if you do.
Note also that A = T and G = C individually, but A does NOT equal G — the ratio of AT to GC varies from organism to organism, and that is exactly what Chargaff discovered.
75.