Assuming a circular piece of DNA was used as starting material how many restriction sites were there in lane three?

Answer 1

It takes one restriction enzyme to cut a linear piece of DNA (straight line) into two pieces. For a circular piece of DNA (plasmid), it would take two of these restriction enzymes - just think of how you would split a circle into two pieces; cutting one section will only straighten out the DNA, not split it (try cutting a rubber band for a visual).

The answer to this question is completely dependent on the number of bands (representing different DNA fragment sizes) the lane produced. Since the first cut on a circular piece of DNA breaks the circle, then each consecutive cut will proceed as a linear band of DNA would.

For example, if lane three produced four bands, then it took a total of three restriction enzymes (each enzyme reacting to one restriction site). Below is an example of the linear piece of DNA from lane three, with each "|" representing a restriction site. Notice how there are three restriction sites, but four fragments ("----") are produced.

---- | ---- | ---- | ----

As explained earlier, if a circular piece of DNA were cut to produce a linear piece like the one above, it would take one restriction site. From there, the enzymes proceed like the ones in the above example. Therefore, a circular piece of DNA that produced four bands would have used four restriction enzymes, whereas a linear piece of DNA that produced four bands would have used three restriction enzymes.

Answer 2

Without knowing the specifics of the restriction enzyme used, it is impossible to determine the number of restriction sites in lane three. It would depend on the specific sequence recognized by the enzyme, which would determine the number and location of restriction sites on the circular DNA.