DNA turnover and mutation in resting cells.

There is growing evidence that mutations can arise in non-dividing cells (both bacterial and mammalian) in the absence of chromosomal replication. The processes that are involved are still largely unknown but may include two separate mechanisms. In the first, DNA lesions resulting from the action of endogenous mutagens may give rise to RNA transcripts with miscoded bases. If these confer the ability to initiate DNA replication, the DNA lesions may have an opportunity to miscode during replication and thus could give rise to apparently 'adaptive' mutations. A second mechanism is suggested by recent work in starved bacteria, showing that there is much more turnover of chromosomal DNA than has been previously thought. This could permit polymerase errors to lead to mutations in non-dividing cells. Such cryptic DNA synthesis, which may essentially replace existing DNA rather than duplicating it, could, in principle, act as an additional source of variability on which selection may act, initially in the absence of cell division. In mammals such processes would undoubtedly have implications for germ cell mutagenesis and carcinogenesis.