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GATE 2023 BT Q.55: solution and answer

GATE 2023 · BTQ.55 · 2 marks · NAT

DNA sample collected from an unidentified bacterial species (Y) contains 13% of adenine. The G+C content (in percentage) of Y is ____________.

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Answer and solution

ANSWER

The G+C content of species Y is 74%.

Official answer: 74Source: Official answer key, GATE 2023 BT (IIT Kanpur)

  1. Chargaff's rule for double-stranded DNA: the percentage of Adenine (A) equals the percentage of Thymine (T), and the percentage of Guanine (G) equals the percentage of Cytosine (C), because A pairs with T and G pairs with C across the two strands.

  2. Given %A = 13%. By Chargaff's rule, %T = 13% as well.

  3. So %A + %T = 13% + 13% = 26%.

  4. Since the four bases together account for 100% of the DNA, .

KEY IDEA

In double-stranded DNA, Chargaff's rule gives %A = %T and %G = %C, and all four bases sum to 100%, so %G + %C = 100% − 2(%A).

WATCH OUT FOR

Assuming A = G = C = T = 25% by default instead of applying Chargaff's pairing rule to the given 13% adenine value, or forgetting to double the adenine percentage before subtracting from 100%.

From the GATE 2023 BT paper. Every question of this paper · Download the full paper · All 9 Mutation & repair questions, with notes

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