There is a recursion here that is worth noting: like all proteins, DNA polymerase, the enzyme that enables DNA to replicate, is itself the product of a gene.4 Built into every genome, then, are the codes for proteins that will allow that genome to reproduce. This additional layer of complexity—that DNA encodes a protein that allows DNA to replicate—is important because it provides a critical node for regulation. DNA replication can be turned on and turned off by other signals and regulators, such as the age or the nutritional status of a cell, thus allowing cells to make DNA copies only when
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