Question

A circular plasmid of $10,000$ base pairs (bp) is digested with two restriction enzymes, A and B, to produce a $3000$ bp and a $2000$bp bands when visualized on an agarose gel. When digested with one enzyme at a time, only one band is visible at $5000$ bp. If the first site for enzyme A($A1$) is present at the ${100}^{th}$ base, the order in which the remaining sites ($A2,B1$ and $B2$) are present is?

• A. $3100,5100,8100$
• B. $8100,3100,5100$
• C. $5100,3100,8100$
• D. $8100,5100,3100$

Answer 1

Answer: (A)

$3100,5100,8100$

When the 10,000 bp plasmid is digested with one restriction enzyme 5000 bp band is obtained. Hence the plasmid has two restriction sites for this enzyme that are located 5000 bp away from each other. The 5000 bp fragments on digestion by the second enzyme results in 2000 and 3000 bp fragments. So the second enzyme has 2 restriction sites. There are 2 variations for the position of the restriction site that is given in the table. Thus, the correct answer is option A.