Recognition and silencing of repeated DNA.

Mechanisms for repetition of DNA pose both opportunities and challenges to a functional genome: opportunities for increasing gene expression by amplification of useful sequences, and challenges of controlling amplification by unwanted sequences such as transposons and viruses. Experiments in numerous organisms have suggested the likely existence of a general mechanism for recognition of repeated character in DNA. This review focuses (a) on the nature of these recognition mechanisms, and (b) on types of chromatin modification and gene silencing that are used to control repeated DNA.