Rectification of tandemly repetitious DNA. I. Influences on rate

To account for observed patterns of uniformity and heterogeneity in tandemly repetitious DNA, earlier workers have proposed three intracellular mechanisms: the rolling circle model, gene conversion between unequally aligned sister chromatids, and unequal sister chromatid exchange (SCE). All three are forms of rectification, which is the random replacement of a specific number, β of repeating units by replicates of a group (α in number) of repeating units that are present elsewhere in the segment. For all three forms of rectification: (1) the average level of uniformity, C, in the segment increases with each rectification event by an amount given by $\text{ΔC=}\text{β(α+β)}\text{αn(n?1)}\text{(1?C)}$, in which n is the number of repeating units per segment. In gene conversion and unequal SCE, α = β (2) ΔC is affected by periodicity and clustering. (3) Mutation rate, β and the frequency of rectification events can vary with n so as to offset largely the presence of n(n?1) in the denominator. This allows rectification to operate even with very large numbers of small repeating units.