Question

You have made a mouse genomic DNA library by cutting mouse DNA with BamHI, ligating the mouse DNA fragments into the vector pBR322 at the BamHI site and then transforming the plasmids into E. coli. The pBR322 plasmid is 4.3 kb long and has a single site for the restriction enzyme BamHI.

You select another bacterial colony and extract plasmid DNA. Assume that you have a 2 kb cloned fragment in this plasmid. If you digest the plasmid with BamHI, what restriction fragment patern will you see after gel electrophoresis?

a) 4.3 kb

b) 2.3 kb and 2.0 kb

c) 2.0 kb

d) 4.3 kb and 2.0 kb

e) 6.3 kb

Answer 1

Answer

There are enzymes which recognize specific sites on the DNA sequence and cleaves the DNA fragment at that site. These enzymes are called restriction enzymes. The fragments obtained are restriction fragments.

These restriction enzymes cut the plasmid at specific restriction site. Here, plasmid pBR322 of 4.3kb is cleaved by the enzyme Bam H1at single site. It produces a linear fragment of 4.3kb.

Another plasmid is cloned with 2kb fragment and this plasmid is digested with Bam H1. As the restriction site is the same for cloned fgragment, it produces fragment of length 2kb itself, but it will be linear fragment.

These fragments are separated using gel electrophoresis.

The pattern of bands observed in gel electrophoresis will be of same size, 4.3kb and 2kb respectively.