An exonuclease is an enzyme that removes nucleotide units from the ends of a DNA or RNA molecule. This process is essential for various cellular functions, including DNA repair, replication, and degradation of RNA. Exonucleases can work in both directions, meaning they can remove nucleotides from either the 5' or 3' end of the strand. These enzymes play a crucial role in maintaining the integrity of genetic material. For example, during DNA replication, exonucleases help correct errors by removing incorrectly paired nucleotides, ensuring that the genetic information is accurately passed on. They are also involved in the degradation of RNA molecules, which is important for regulating gene expression.