A plasmid is cut with the restriction enzyme BamH1 giving fragments of 3000 and 1000 bp as identified by gel electrophoresis and ethidium bromide staining. In a seperate restriction digest the enzyme EcoR1 gives fragments of 1000 and 1500 bp that are apparent on the agarose gel. What is the most likely size of the plasmid in bp?

a. 4000

b. 5000

c. 6000

d. 11 000

e. None of the above

The correct answer is option a, that is, 4000 bp.

Explanation:

In the mentioned plasmid, after digestion with BamH1, we got two fragments of length 1000 bp and 3000 bp respectively. This suggests that there are two restriction sites of BamH1 in the plasmid. Thus, the total length of the plasmid should be 4000 bp.

Again, the digestion of plasmid is done with EcoR1 and one attains two fragments of length 1000 and 1500 bp. Therefore, from initial digestion, it is clear that the plasmid is of length 4000 bp. This signifies that EcoR1 is generating one band of 1000 bp and two bands of 1500 bp, that is, the total comes out as 4000 bp. This also tells that plasmid exhibit three EcoR1 restriction sites.