Exonuclease activity means cleavage of nucleotides only at the end while endonuclease breaks DNA strand at an internal position. DNA polymerase I has 3' - 5' exonuclease activity which removes any nucleotide which mispair during elongation of growing strand. A small segment of DNA polymerase I also shows 5' - 3' exonuclease activity which removes DNA segment which comes as an obstruction in way of growing DNA strand. Polymerase catalyses the elongation of a polymeric molecule. Endonucleases are very specific and cut DNA at very specific nucleotide sequences. These are called restriction enzymes. Kinase is an enzyme that can transfer a phosphate group.