Question

An 9 kb circular plasmid is cut with the EcoRI restriction enzyme, and the reaction products are run on a DNA gel and stained with ethidium bromide. Bands of 1, 3 and 5 kb are seen. How many EcoRI sites are in the plasmid? Choose the one answer that is most correct. Group of answer choices: At least 2, At least 3, At least 5, None, At least 1, At least 4

Answer 1

Answer:

At least 3

Explanation:

A 9 kb circular plasmid is cut with the EcoRI restriction enzyme, and the reaction products are run on a DNA gel and stained with ethidium bromide. Bands of 1, 3 and 5 kb are seen. So, total 3 fragments are produced. Each restriction enzyme produces two fragments after cutting the DNA but as it is a circular DNA, after first cut by the restriction enzyme at first restriction site it produces a gap and convert from circular form to linear form and then after second cut by the restriction enzyme at second restriction site the linear DNA divided into two fragments and any of the fragment again cut at third restriction site by the restriction enzyme to divide into two fragments. So, to produce total 3 fragments at least 3 restriction sites are required. So, at least 3 EcoRI sites are in the plasmid. Thus, it is the correct option.