Why are certain enzymes called restriction endonucleases ? How do these enzymes produce sticky ends?
Answers
Restriction enzymes are DNA-cutting enzymesfound in bacteria (and harvested from them for use). Because they cut within the molecule, they are often called restriction endonucleases.
In order to be able to sequence DNA, it is first necessary to cut it into smaller fragments.
Many DNA-digesting enzymes (like those in your pancreatic fluid) can do this, but most of them are no use for sequence work because they cut each molecule randomly.
This produces a heterogeneous collection of fragments of varying sizes. What is needed is a way to cleave the DNA molecule at a few precisely-located sites so that a small set of homogeneous fragments are produced.
The tools for this are the restriction endonucleases. The rarer the site it recognizes, the smaller the number of pieces produced by a given restriction endonuclease.
many restriction enzymes cut in an offset fashion.
The ends of the cut have an overhanging piece of single-stranded DNA.
These are called "sticky ends"because they are able to form base pairswith any DNA molecule that contains the complementary sticky end.
Any other source of DNA treated with the same enzyme will produce such molecules.
Mixed together, these molecules can join with each other by the base pairing between their sticky ends.
The union can be made permanent by another enzyme, a DNA ligase, that forms covalent bonds along the backbone of each strand.