What is a restriction site?

Consider the word "racecar" (read it backwards). A palindrome is a sequence that is the same both forward and backward.

An endonuclease known as a restriction enzyme can identify a specific palindromic DNA sequence and cuts a section through it.

EcoR1, which was isolated and refined from Escherichia coli (E. Coli) by Frederic Sanger in the 1970s, was the first to be found.

Its recognition sequence is CTTAAG. Until you realize that DNA is double-stranded, this doesn't appear to be a palindrome:

When EcoR1 encounters this sequence in double-stranded DNA (dDNA), it will cut both strands' A and G residues, resulting in "sticky ends":

Ligases are the enzymes that are responsible for joining these sticky ends ("glued") to other DNA strands that have been opened with the same restriction enzyme.

By the way, that is how cloning originated.

Numerous other restriction enzymes, such as SAu3A (Staphylococcus Aureus) and HinD3 (from Haemophilus Influenzae serotype 3), were found after EcoR1.

While some have 4-base recognition sequences and 6-positions recognition palindromic sites, the latter will identify more occurrences of their cutting site and thus produce more cuts in a given strand.

A restriction site, also known as a recognition site, is a specific sequence of nucleotides in a DNA molecule recognized by a restriction enzyme. This sequence is typically a palindrome, meaning it reads the same forward and backward on both DNA strands. When a restriction enzyme encounters its specific recognition site, it cuts the DNA at or near the site, producing fragments with sticky ends or blunt ends, depending on the type of restriction enzyme. These fragments can then be used in various molecular biology techniques, such as DNA cloning and genetic engineering.