Restriction enzymes that cut DNA to produce very small fragments are due to what characteristic of recognition sites?

• A. Recognition sites contain only AT pairs, which occur very frequently.
• B. Recognition sites contain only GC pairs, which occur rarely.
• C. Enzymes cut only long sequences.
• D. Enzymes cut at random locations.

Answer: (A)

Frequent cutting into small fragments happens when the enzyme’s recognition site appears very often in the DNA. If the site is made up only of A and T, it will occur by chance far more often than a GC-rich or longer site, so there are many more cut sites across the genome. That high frequency of cutting yields a large number of small fragments. In contrast, a GC-only site would appear less often, producing larger fragments, and enzymes do not cut at random locations—they cut specifically at their recognition sequences. So the feature that explains very small fragments is recognition sites composed only of AT pairs, which occur very frequently.