Figure 3.

Alternative mechanisms to incorporate transforming DNA into the chromosome. (A) Assimilation of a single strand of DNA to form heteroduplex DNA over most of its length. As shown in successive vertical drawings, heteroduplex DNA can then be mismatch corrected to yield a wild-type transformant. If mismatch correction is blocked, a sectored colony should be produced, with markers at opposite ends of the transforming fragment assimilated into one strand. Prior steps of forming and removing a displacement loop (D-loop) involving the complementary resident strand are not shown. (B) Assimilation of a double-stranded fragment of DNA by two crossover events confined to the ends of the recombining fragment. In this case, the central part of the region is replaced with two strands of donor DNA and mismatch correction plays no role in the process. (C) A hybrid model in which there are longer regions of heteroduplex DNA associated with two crossing-over events. In this case the markers at opposite ends of the transforming DNA will form heteroduplex on two different DNA strands of the recipient.