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BIO NMDCAT Structure of DNA
nmdcat.online July 3, 2026

93. A segment of double-stranded DNA is analyzed and found to have a melting temperature (Tm​) of 85∘C, while a second fragment of identical length melts at 72∘C. The physical explanation for the higher Tm​ of the first fragment is

A. A higher density of negative charges along its outer phosphate rails
B. A higher proportion of Guanine-Cytosine (G-C) base pairs
✓
C. The presence of long non-coding intron loops within its sequence
D. Its unique organization into nucleosome core filaments

📝 Explanation

G-C pairs are bound by three hydrogen bonds, meaning G-C rich fragments require higher temperatures to denature than A-T rich sequences of the same length.

📖 Additional Information

  • A higher density of negative charges along its outer phosphate rails
  • A higher proportion of Guanine-Cytosine (G-C) base pairs
  • The presence of long non-coding intron loops within its sequence
  • Its unique organization into nucleosome core filaments

G-C pairs are bound by three hydrogen bonds, meaning G-C rich fragments require higher temperatures to denature than A-T rich sequences of the same length.

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