Q.A plasmid DNA and a linear DNA (both are of the same size) have one site for a restriction endonuclease. When cut and separated on agarose gel electrophoresis, plasmid shows one DNA band while linear DNA shows two fragments. Explain.
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Start your 14-day free trial to unlock the full solution →A circular plasmid with one restriction site yields a single linear piece after cutting, while a linear DNA with one restriction site is cut into two separate fragments, producing two bands on a gel.
Restriction endonucleases are molecular scissors that cut DNA at specific recognition sequences. When a DNA molecule has one such site, the enzyme will make a single cut. But the outcome — and what you see on an agarose gel — depends entirely on whether the starting DNA is circular or linear.
Think of a circular plasmid as a loop of string. If you snip it at one point, you don't get two pieces; you get one long, straight piece of string. That single linear molecule will travel through the gel as one band. Its size is the same as the original plasmid, so it appears at a position corresponding to that length.
Now consider a linear DNA — a straight piece of string with two ends already. If you cut it once somewhere in the middle, you get two separate pieces: one shorter, one longer. Each fragment moves independently through the gel, forming two distinct bands. Their sizes add up to the original length, but they are no longer connected.
The key difference is that a circular molecule has no free ends, so a single cut simply opens the circle into a line. A linear molecule already has ends, so a cut creates two separate molecules. …
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