double-stranded DNA breaks are repaired by DNA ligase in concert with a multi-subunit complex con-
taining DNA protein kinaseand two so-called Ku proteins (Ku70 and Ku80).^56 The protein complex brings
the broken ends together, a few bases are removed at the ends and the break is repaired (Figure 6.34).
6.8 DNA Recombination
6.8.1 Homologous DNA Recombination
One of the seminal discoveries of genetics was the observation that the different genes on the parental
chromosome pairs are shuffled before donation to the offspring. How do these ‘beads on a string’ become
assorted? The answer is by homologous recombination. A diploid cell in the germinal lineage (the lineage
will give rise to haploid germ cells or gametes) contains two copies of a given chromosome. Segments from
one chromosome can recombine with corresponding segments of the other (Figure 6.35). As we shall see below
this recombination is ultimately dependent on the DNA sequence homology between the two chromosomes.
What advantages does homologous recombination provide? It enables the host organism to sort
alleles (differing copies of the same gene) into novel groups. If a copy of a particular gene in a fruit fly has
randomly acquired a mutation that yields a more efficient enzyme, then Darwinian natural selection can oper-
ate on that gene, provided it is not shackled to all the other genes on the chromosome. Thus, favourable and
unfavourable alleles can be shuffled randomly and then the many combinations in the population can be
tested by natural selection. Another advantage which recombination provides is the ability to repair a dam-
aged gene in an otherwise favourable chromosome. If no ability to assort different alleles existed then a
single unfavourable mutation in a chromosome would consign the whole of it to oblivion.
6.8.1.1 The Mechanism of Homologous Recombination. Homologous recombination is linked to
DNA replication but does not occur during replication. Rather, it takes place between intact double
helices. Genetic and DNA sequence analysis has shown that recombination is accurate to a single base
238 Chapter 6
Figure 6.33 Major pathways for DNA repair. Circled * indicates damage to a base or ribose. Longer vertical lines
indicate base pairs and shorter lines show unpaired bases. Thick horizontal lines indicate the ribose
backbone