Cytosine makes up 19% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?

Question

BlackTom AI · Accepted Answer

Take the question apart and apply base-pairing rules.

First, note that Cytosine (C) pairs with Guanine (G), so the percentages of C and G are equal. Given C = 19%, we deduce G = 19% as well.

Now, the total percent of GC pairs is C + G = 19% + 19% = 38%.

DNA must have A + T + G + C = 100%. Subtract the GC portion from 100% to find the sum of A and T: 100% - 38% = 62%.

Because Adenine (A) pairs with Thymine (T), A and T are in equal amounts. Therefore A = T, and A + T = 62% implies T = 62% / 2 = 31%.

Evaluating the options:
- 62%: This would be the sum of A and T, not the percentage of thymine alone; it incorrectly assigns the entire AT portion to a single base. 
- 25%: This would correspond to an unequal split that ignores A = T and the C = G constraint; it contradicts base-pairing symmetry. 
- 19%: This equals C or G, not T; it ignores the equal pairing and the AT portion. 
- 31%: This matches the calculated thymine percentage derived from A = T and the AT total of 62%. 
- It cannot be determined from the information provided.: There is enough information via Chargaff’s rules to determine T, so this statement is false.

So, by applying the equal-pairing relationships and the fact that A equals T, the thymine percentage is 31%, consistent with the AT total of 62% and the C/G total of 38%.

Bio 93 Section CDE 2025 Midterm 2 Practice Exam

Cytosine makes up 19% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?

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