The sequences for two short fragments of DNA are shown above. Which of the following is one way in which these two segments would differ?

Question

BlackTom AI · Accepted Answer

Because guanine–cytosine base pairs form three hydrogen bonds while adenine–thymine base pairs form only two, a DNA segment with more G–C pairs requires more thermal energy to separate the strands and therefore has a higher melting (denaturation) temperature.

1) Relevant concepts:
- Base pairing: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds).
- Melting temperature (Tm) of DNA increases with G–C content because each G–C pair contributes one extra hydrogen bond (and stronger base-stacking interactions) compared with an A–T pair.
- The sugar–phosphate backbone (and number of phosphate groups per nucleotide) is the same for both DNA segments of equal length, and thymine (T) in DNA does not prevent transcription into RNA (where U replaces T in the RNA product).

2) Step-by-step reasoning and counts:
- Segment 1 sequence (5' → 3'): ATATGAGTAGT (11 bases).
  Count bases: A = 4, T = 4, G = 3, C = 0. Total G–C pairs = 3; A–T pairs = 8.
  GC% = 3/11 ≈ 27.3%.
- Segment 2 sequence (5' → 3'): GCGCAGACGAC (11 bases).
  Count bases: G = 4, C = 4, A = 3, T = 0. Total G–C pairs = 8; A–T pairs = 3.
  GC% = 8/11 ≈ 72.7%.

3) Intermediate calculation shown above (GC counts and GC% for each segment).

4) Why the correct answer is correct:
- Segment 2 has much higher GC content (8 of 11) versus Segment 1 (3 of 11). Because G–C pairs form three hydrogen bonds versus two for A–T pairs, Segment 2 will require more energy (higher temperature) to denature. Thus Segment 1 would denature at a lower temperature than Segment 2.

5) Why the other options are incorrect:
- "More soluble in water because it has more phosphate groups": Both segments are the same length (same number of nucleotides) and thus have the same number of phosphate groups; solubility differences are not explained by base composition here.
- "Must be from a prokaryote because it has predominantly A–T base pairs": Base composition alone does not identify prokaryotic versus eukaryotic origin; genomes of both types vary in GC content.
- "Would not code for mRNA because both strands have T": DNA normally contains thymine; transcription produces RNA with U in place of T. The presence of T in DNA does not prevent coding for mRNA.

6) Conclusion:
Segment 1 would become denatured at a lower temperature than Segment 2 because A–T base pairs have two hydrogen bonds while G–C pairs have three. This matches the provided correct answer.

The sequences for two short fragments of DNA are shown above. Which of the following is one way in which these two segments would differ?单项选择题

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