GATE 2022 BT – Question 64
A circular plasmid has three different but unique restriction sites for enzymes ‘a’, ‘b’ and ‘c.’ When enzymes ‘a’ and ‘b’ are used together, two fragments of equal size are generated. Enzyme ‘c’ creates fragments of equal size only from one of the fragments generated by those cleaved by ‘a’ and ‘b’. The plasmid is treated with a mixture of ‘a’, ‘b’ and ‘c’ and analysed by agarose gel electrophoresis. The number of bands observed in the gel is __________.
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Correct answer: 2
Explanation
Let the plasmid length be $L$ (a circle). Three unique sites, $a$, $b$ and $c$, each cut it once.
**Step 1: enzymes $a$ and $b$ together.** Cutting a circle at two places gives two fragments, and they are equal, so each has length $L/2$.
**Step 2: enzyme $c$.** It cuts only one of those two fragments, and into two equal pieces, so that fragment becomes $L/4+L/4$.
**Step 3: all three enzymes.** The fragments are
$$\frac L2,\quad\frac L4,\quad\frac L4 .$$
**Step 4: bands on the gel.** Agarose gel electrophoresis separates by size. The two fragments of size $L/4$ run together as one band, and the $L/2$ fragment gives another:
$$\text{number of bands}=\mathbf{2}.$$