Question

Digestion of a 1.1‑kb fragment of DNA with BamHI produces two fragments of 700 bp and 400 bp. Digestion of the same 1.1‑kb fragment with XhoI produces two fragments of 300 bp and 800 bp. Digestion with both enzymes produces three fragments of 100 bp, 300 bp, and 700 bp. Which of the following statements is true about the DNA fragment?

A. The BamHI recognition site overlaps with the XhoI recognition site.
B. The BamHI recognition site is located within the XhoI recognition site.
C. The BamHI and XhoI recognition sites are adjacent to each other.
D. The BamHI and XhoI recognition sites are located at opposite ends of the DNA fragment.

Answer 1

The BamHI and XhoI recognition sites are located at opposite ends of the DNA fragment, as separate digestions yield two distinct fragments each, and a combined digestion yields three, with no evidence of overlapping or adjacent sites.

Step-by-step explanation:

When the 1.1‑kb DNA fragment is digested by BamHI, it yields two fragments of 700 bp and 400 bp. When digested by XhoI, it produces two fragments of 300 bp and 800 bp. Upon digestion with both BamHI and XhoI, three fragments result: 100 bp, 300 bp, and 700 bp. Given this information, we can deduce which statement is correct regarding the DNA fragment's recognition sites.

Statement D is correct: The BamHI and XhoI recognition sites are located at opposite ends of the DNA fragment because separate digestion with each enzyme yields two fragments, and combined digestion yields three fragments, one of which (the 300 bp fragment) is common between the two separate digestions, indicating that the XhoI site does not interfere with the BamHI site.