Question

How many linear DNA fragments will be produced when a circular plasmid is digested with a restriction enzyme having 3 sites?

• A. 4
• B. 5
• C. 3
• D. 2

Answer 1

The correct option is A 4

Restriction enzymes are DNA-cutting enzymes. Each enzyme recognizes one or a few target sequences and cuts DNA at or near those sequences. Many restriction enzymes make staggered cuts, producing ends with single-stranded DNA overhangs. However, some produce blunt ends.

Restriction enzymes are found in bacteria (and other prokaryotes). They recognize and bind to specific sequences of DNA, called restriction sites. Each restriction enzyme recognizes just one or a few restriction sites. When it finds its target sequence, a restriction enzyme will make a double-stranded cut in the DNA molecule.
In DNA cloning, researchers make many copies of a piece of DNA, such as a gene. In many cases, cloning involves inserting the gene into a piece of circular DNA called a plasmid, which can be copied in bacteria.
If the plasmid has 3 recognition sequences for a given restriction endonuclease, then 4 linear DNA fragments are obtained because, if the DNA is linear then the number of fragments obtained is (N+1) whereas if the DNA is circular then the number of fragments obtained will be N for N recognition sequences for the given restriction endonuclease in a plasmid.
So the correct option is '4'.