Figure 1. Conjugal transfer of DNA mediated by bacterial sex plasmids. (a) Transfer of extrachromosomal sex plasmid DNA such as F⁺ or F¢ DNA. An F¢ is illustrated with green F DNA and black DNA from the bacterial chromosome. Dashed lines represent newly synthesized DNA. Each line of the plasmid DNA represents a single DNA strand (polynucleotide chain). (b) Transfer of bacterial DNA of an Hfr strain, followed by homologous recombination of the transferred DNA into the recipient cell's chromosome. The usual folded and packaged (nucleoid) structure of the bacterial chromosome shown in (a) is not drawn in (b) for clarity in illustrating the recombination of Hfr DNA with the recipient chromosome (b). No real difference in chromosome structures is implied.

Figure 2. General hypotheses for possible mechanisms of DNA recombination, as formulated by Meselson and Weigle. In this figure only, double-stranded DNA is represented by helices. Newly synthesized DNA is symbolized by broken helices. (a) Break-join models create recombined DNA from the parental DNA material. (b) Copy-choice models form recombinant DNA without any material contribution of parental DNA, by switching the template for DNA replication from one molecule to another. To have no material contribution of parental DNA, the replication would have to be conservative (not the standard semiconservative segregation of DNA strands). (c) Break-copy models use part of one parental DNA molecule to initiate DNA replication using another molecule as a template, joining the two by heteroduplex DNA.

Figure 3. A model for a molecular mechanism of recombination of linear DNA in Escherichia coli, illustrated for conjugation. The model is synthesized (with modifications) from three sources (Rosenberg and Hastings, 1991; Harris et al., 1996; Razavy et al., 1996). Step-by-step description of this model appears in the text (Mechanisms Promoting Recombination). Each strand of duplex DNA is represented as a line. In single strands, arrowed ends represent 3¢-ends; nonarrowed ends represent 5¢-ends. Dashed lines represent newly synthesized DNA. The black DNA molecule represents linear DNA that will recombine with the (red) circular bacterial chromosome. SSB, single-strand binding protein.

Figure 4. Mismatch repair promotes the fidelity of DNA replication and recombination. (a) Mismatch repair proteins recognize incorrectly paired bases (shown) and 1–4 base insertion or deletion single-strand loops (not shown) that result from DNA polymerase errors. Step by step discussion of this process appears in the text (Mechanisms Aborting Recombination). (b) Abortion of recombination between imperfectly identical DNA sequences by mismatch repair proteins. Mismatch repair proteins bind to mispairs (shown) and 1–4 base insertion/deletion loops (not shown) that result from heteroduplex DNA formation between homeologous DNAs during recombination, and impede recombination. The mechanism(s) by which mismatch repair proteins inhibit recombination after they have bound the mispairs in heteroduplex DNA is not yet understood (three downward arrows).