Restriction enzymes cut DNA molecules at a particular point by recognising a specific sequence . They are of two kinds –
Exonucleases – remove nucleotides from the ends of the DNA.
Endonucleases – make cuts at specific positions within the DNA. It recognises a specific palindromic nucleotide sequences in the DNA, cuts the strand of DNA a little away from the centre of the palindrome sites, but between the same two bases on the opposite strands. Leaving single stranded portions with sticky ends.
When cut by the same restriction enzyme, the resultant DNA fragments have the same kind of ‘sticky-ends’ and, these can be joined together (end-to-end) using DNA ligases.