what is/are the reason(s) to do restriction enzyme digest?

Answer 1

A couple of reasons:

- restriction enzyme digests of genomic DNA are useful for producing random but consistent patterns of fragments. Since the restriction site requires a specific sequence, if that sequence is long, it will be very rare, and if it is short, it will be more common. You can choose a restriction enzyme based on the amount of DNA you are typing, and the pattern of fragments that results can be compared between two genomes to determine how close they are. In forensics, this is considered a smoking gun if a match is found.

- since the restriction site is palindromic, a cut produces "sticky" overhangs which can be used to religate the broken DNA, or more useful, ligate the broken DNA to another fragment, combining them. This produces recombinant DNA, a strand with DNA from two different sources.

- you can use restriction enzymes to destroy DNA. If DNA is interfering with a chemical or biologic reaction, digesting it into small enough fragments will essentially remove it and keep it from gumming up your other reactions.

Answer 2

If you wish to subclone and express a cDNA to look for protein function you will need to digest both the cloning vector and the cDNA to produce ligatable ends.
If you wish to analyse DNA fragments double digestions allow restrictions patterns to be matched between overlapping clones.

Answer 3

so that u does not grow fat