This chapter describes the protein machines that replicate and repair the cell's DNA. DNA replication is "semiconservative" because each daughter DNA double helix is composed of one conserved strand and one newly synthesized strand. Homologous recombination can also be used to repair many other types of DNA damage, making it perhaps the most handy DNA repair mechanism available to the cell: all that is needed is an intact homologous chromosome to use as a partner— a situation that occurs transiently each time a chromosome is duplicated. Homologous recombination can flawlessly repair double-strand breaks using an undamaged homologous double helix as a template. Damage to one of the two DNA strands, caused by unavoidable chemical reactions, is repaired by a variety of DNA repair enzymes that recognize damaged DNA and excise a short stretch of the damaged strand.