A linear DNA fragment and a plasmid has three restriction sites for EcoRI

how many fragments will be produced from linear DNA and plasmidrespectively. explain..?

If a linear DNA fragment and a plasmid, each has three recognition sites for EcoRI, then total of four fragments are produced from linear DNA fragment and  three from a circular plasmid. Restriction sites are the sites present on DNA molecule where restriction endonucleases like EcoRI cut the DNA.

If three sites for making cuts are present in an entire length of linear DNA fragment, then obviously four fragments are produced.

But in case of circular molecule of plasmid, three fragments will be produced when three restriction or cutting sites are present on that Plasmid (circular DNA molecule). When cut with restriction enzyme at first cutting site, it becomes linear but does not get fragmented. Then with the next two cutting sites that linear fragment will produce three fragments. This can also be simply visualised by cutting circular rubber band at three locations.

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