DNA ligase is used to seal the nicks that remain in recombinant DNA molecule. Infact DNA ligase joins together the neighbouring nucleotides flanking a discontinuity in a DNA strand by forming a phosphodiester bond. A recombinant DNA molecule is a vector into which the desired DNA fragment has been inserted to enable its cloning in an appropriate host.
Endonucleases are the enzymes that produce internal cuts called cleavage in DNA molecules. A class of endonucleases cleaves DNA only within or near those sites, which has specific base sequences, such endonucleases are known as restriction endonucleases.
Exonucleases are enzymes that remove one or more nucleotide from the free ends.
DNA polymerase is the enzyme, which cause polymerization of nucleotides during DNA replication.