One, a 3 to 5 exonuclease, is intimately involved in the accuracy of replication. Nucleases are enzymes that catalyze the breakdown of DNA or RNA into smaller fragments and/or nucleotides. An exonuclease catalyzes cleavage of nucleotides from the end of a DNA or RNA polymer.he enzyme primase catalyzes the synthesis of the primers from which DNA polymerases can begin synthesis (Figure 5.21). Primers are short oligonucleotides, ranging from 6 to 60 nucleotides long. They can be made of ribonucleotides or a mixture of deoxyribonucleotides and ribonucleotides.