During the process of HR between two DNA duplexes, intermediate structures are formed that can be resolved in one of two ways. Either a crossover results - after which both products contain portions of the original duplexes, or a gene conversion results in which a limited amount of sequence information is carried from one duplex to the other, but not vice versa. In DSB repair, the gene conversion pathway is preferred over the crossover pathway; so as to minimise the chance of deleterious rearrangements. In meiotic recombination, both pathways are in operation.From: http://images.google.co.il/imgres?imgurl=http://www.sanger.ac.uk/Teams/Team29/gfx/fig2.gif&imgrefurl=http://www.sanger.ac.uk/Teams/Team29/&h=340&w=402&sz=16&tbnid=Qbu_hsk2h0KxSM:&tbnh=105&tbnw=124&prev=/images%3Fq%3Dgene%2Bconversion%26um%3D1&start=1&sa=X&oi=images&ct=image&cd=1

• The double-strand break model provides an opportunity for gene conversion to take

place during the recombination process. It initiates with a double-strand cut that

breaks one of the partners in the recombination into two pieces ( Figure 14.33 ). After

the double-stranded cut, one strand in each half of the molecule is trimmed back by a

5 →3 exonuclease, so each end now has a 3 overhang of approximately 500 ′ ′ ′

nucleotides. One of these invades the homologous DNA molecule in a manner similar

to that envisaged by the Meselson-Radding scheme, setting up a Holliday junction that

can migrate along the heteroduplex if the invading strand is extended by a DNA

polymerase. To complete the heteroduplex, the other broken strand (the one not

involved in the Holliday junction) is also extended. Note that both DNA syntheses

involve extension of strands from the partner that suffered the double-stranded cut,

using as templates the equivalent regions of the uncut partner. This is the basis of the

gene conversion because it means that the polynucleotide segments removed from the

cut partner by the exonuclease have been replaced with copies of the DNA from the

uncut partner.

• The resulting heteroduplex has a pair of Holliday structures that can be resolved in a

number of ways, some resulting in gene conversion and others giving a standard

reciprocal strand exchange. An example leading to gene conversion is shown in Figure

14.33 . (from - Genome)