Hybrid DNA models involving double strand breaks

In this model, a double strand break instead of a single strand break is assumed. The double strand break caused by an endonuclease is enlarged, perhaps due to exonuclease activity, which nibbles away one strand on either side of the break generating 3' single stranded termini. One of the two broken single strands invades the other duplex, so that the duplex molecules become connected by a stretch of heteroduplex DNA (models are also available where both broken single strands of a duplex invade the other duplex; see Fig. 13.4. The formation of heteroduplex DNA generates a D-loop, which is extended by repair synthesis, to cover the entire length of the gap. The extended single strand anneals with complementary sequence at the end, so that heteroduplex DNA segments are found on either side of the gap. The single stranded D-loop becomes double stranded, once again by repair synthesis. Branch migration converts this structure into a molecule with two recombinant joints,which are resolved by cutting as shown in the preceding section. This leads to two possibilities : (i) non-crossovers with heteroduplex DNA. (ii) crossing over with heteroduplex DNA. This is illustrated in Figure 13.3.

The major difference between the models involving single and double strand breaks is that in the model with double strand breaks, the region between heteroduplex segments has the sequence of donor DNA in both molecules (this is why initiating DNA molecule is called recipient). Further, in double strand break model, there is initial loss of information, which is retrieved by resynthesis of DNA. In single strand break model, no such loss of information is involved. This model is consistent with events in yeast, where initiation of recombination is associated with receiving genetic information and where double-strand breaks are known to be involved in recombination like events.