## 4 Discussion
The extensive RecBCD-dependent degradation of DNA in _recA_ mutant cells [55] argues for less efficient replication repair for both of the above reasons. Firstly, the time taken to degrade extensive sections of the chromosome is measured in minutes even with the high speed and processivity of RecBCD-catalysed dsDNA end degradation [56]. Secondly, this extensive degradation in effect means that replisome reloading must occur far upstream of the initially blocked fork, possibly at _oriC_[95]. However, the absence of significant extension of chromosome duplication time in _recA_ or _recB_ mutant cells (Figure 3) indicates that fork repair in the absence of either activity does not impact significantly on the mean duplication time during a single cell cycle. The reduced viability [85] and chromosomal degradation seen in _recA_ mutant cells [55] (and Figure 1A (iii)) might therefore reflect the loss of replisome function when considering multiple cell cycles rather than just one. These occasional repair events may be too infrequent to have a measurable impact on mean chromosome duplication time during one cell cycle (Figure 3) but each event might take significant time and result in accumulation of cells with different numbers of chromosome equivalents over the course of multiple cell cycles. Given enough time, though, these non-wild type repair events can resolve the majority of replicative problems, evinced by the similar chromosome profiles of wild type and _recA_ mutant cells in stationary phase (Figure 2). Regarding reduced viability of _recA_ mutant cells, such viability measurements involve comparing the number of colony-forming units with the total number of cells as determined by microscopy [85]. This measure of viability therefore indicates the relative frequency with which a population of cells generates non-viable cells over the course of an extended period of time and cannot be compared to a measure of chromosome duplication time during a single cell cycle as presented in Figure 3. Infrequent engagement of RecA and RecBCD during chromosome duplication might have  
Figure 8: Summary of factors with potential influence on the probability of replisomes completing chromosome duplication and the time needed to do so. an undetectable impact on the mean time taken to replicate a chromosome during a single cell cycle. However, absence of RecA- and RecBCD-dependent processing of replication intermediates could result in aberrant events in their absence which, over multiple cell cycles, gives rise to cells that can no longer divide. The inefficiency of non-wild type fork repair mechanisms might also relate to our finding that _Arep_\(\Delta\)_uvrD_\(r\)_ropB*35_recF\({}^{-}\) cells require both RecA and RecBCD for survival (Figures 4 and 7). _rep recA_ double mutant cells are viable, but _rep recB_ double mutants are not, indicating that under some circumstances RecBCD-catalysed degradation of dsDNA ends in the absence of RecA can underpin genome duplication [58; 71]. However, our data indicate that when both Rep and UvrD are absent then RecBCD-dependent DNA degradation is not sufficient to sustain viability unless it is coupled to loading of RecA. UvrD can act as an accessory helicase and compensate partially for loss of Rep in \(\Delta\)_rep_uvrD\({}^{+}\) cells [22; 30]. This partial compensation may explain why DNA degradation without RecA loading can maintain viability in _Arep_uvrD\({}^{+}\) cells but not in \(\Delta\)_rep_\(\Delta\)_uvrD_\(r\)_ropB*35_recF\({}^{-}\) cells: dsDNA end degradation alone provides an inefficient means of reinitiating DNA replication if replisomes pause and lose function at an elevated rate, as in cells lacking both Rep and UvrD. It should also be borne in mind that, whilst transcription is a major source of replicative problems in unstressed cells [6; 22; 30; 78] (Figure 1), recombination enzymes have the ability to deal with replicative barriers other than protein-DNA complexes, unlike accessory replicative helicases [1; 7].