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nmdcat.online June 27, 2026

In a solution of DNA, the absorption of ultraviolet light at 260 nm is significantly increased if the DNA undergoes

A. Annealing to a complementary strand
B. Denaturation (melting) into single strands
✓
C. Supercoiling by gyrase enzymes
D. Packaging around histone proteins

📝 Explanation

The nitrogenous bases in double-stranded DNA are stacked and have a lower absorbance. When the double helix is denatured into two random, single-stranded coils, the bases become unstacked. This unstacking increases their absorbance of UV light at 260 nm, a phenomenon known as the hyperchromic effect.

📖 Additional Information

  • Annealing to a complementary strand
  • Denaturation (melting) into single strands
  • Supercoiling by gyrase enzymes
  • Packaging around histone proteins

The nitrogenous bases in double-stranded DNA are stacked and have a lower absorbance. When the double helix is denatured into two random, single-stranded coils, the bases become unstacked. This unstacking increases their absorbance of UV light at 260 nm, a phenomenon known as the hyperchromic effect.

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