Question

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 _________.

Hint:

When multiple restriction enzymes are used on a plasmid, the number of bands in gel electrophoresis corresponds to the number of distinct fragments generated. Count the fragments carefully based on how each enzyme cleaves the plasmid.

Answer 1

Correct Answer: 2

This problem involves the use of three restriction enzymes ('a', 'b', and 'c') on a circular plasmid. Let’s break down the sequence of reactions: 1. Enzymes 'a' and 'b':
When enzymes 'a' and 'b' are used together, they cleave the plasmid at their respective unique sites, resulting in two fragments of equal size. Since the plasmid is circular, this creates two linear fragments of equal size.
2. Enzyme 'c':
Enzyme 'c' cleaves only one of the fragments produced by the combination of enzymes 'a' and 'b'. Since the fragments from 'a' and 'b' are of equal size, enzyme 'c' will cleave one of these fragments into two equal fragments.
Thus, after enzyme 'c' acts, we now have three distinct fragments:
- Two from enzyme 'a' and 'b' acting together (of equal size),
- One more generated by enzyme 'c', cleaving one of the previous fragments.
3. Analysis by Agarose Gel Electrophoresis:
In agarose gel electrophoresis, each fragment will appear as a separate band. Therefore, with three fragments in total, there will be 3 bands observed in the gel.
Thus, the number of bands observed in the gel is: \[ \boxed{3} \]