Question

As indicated in the gel image, lanes X and Y represent samples obtained from a circular DNA plasmid after complete digestion using restriction enzyme X or Y with different recognition sites, respectively. How many sites for X and Y are present in the plasmid (sizes of the bands in kilo base pairs (kb) is shown)?

A. 1 for X, 1 for Y.
B. 2 for X, 1 for Y.
C. 1 for X, 2 for Y.
D. 2 for X, 2 for Y.

Answer 1

Hint: Restriction enzymes, also known as restriction endonucleases, are proteins produced by bacteria. They cut DNA at specific points along the chain in a precise manner. They have the capability to identify these specific points by looking at the particular arrangements of the base pair of DNA at that location known as a restriction site.

Complete answer: After recognition, restriction enzymes cut each strand of double-helix DNA. They cut the DNA and make small fragments of 4-6 base pairs in size. The restriction enzyme breaks the DNA molecule into fragments by continuously cutting it at the restriction sites.
From the image, we can see that there are two fragments for sample X (one of 6 kb and the other of 4 kb), thus the size of the circular plasmid must be 10 kb. Sample in lane X has 2 bands; hence we can say that the enzyme has cut the plasmid DNA from two restriction sites. At one site, the enzyme has cut the DNA and made it linear from the circular, and at the other site, the enzyme has cut the DNA producing two bands of 6 and 4 kb respectively.
Similarly, for the sample in the Y lane, there will be two restriction sites. The enzyme must have cut the circular DNA at one site making it linear. Then the enzyme would have cut the linear DNA at 5kb making two equal bands of 5 kb each. Thus for both X and Y samples, there are two restriction sites present.
So, the correct answer is “Option D”.

Note:
Restriction enzymes detect the required sequence and cut the DNA from that point, thus are also known as molecular scissors. The sequences recognized by these enzymes are known as palindromes which read two strands in the opposite directions.