In cases where DNA is severely damaged, a cell will engage in a phenomenon called the SOS response in an effort to salvage a functioning set of genetic information. This response, also called error-prone repair, represents a last-ditch response to salvage a chromosomal information system. In addition, recombinational repair systems act to allow one copy of the replicating DNA at a replication fork to supply information to the other daughter chromosome. Recombinational repair is a way of using one copy of the cell's information to ensure that the overall information store remains intact.
The biochemical process of recombination occurs by breaking and rejoining DNA strands. The key reaction is strand displacement initiated at a nick in the chromosome. Then a protein called RecA (which stands for recombination; rec- bacteria are unable to recombine their DNA information and therefore are abnormally sensitive to UV radiation) binds to a single-stranded DNA fragment and catalyzer its exchange with the same sequence of the duplex. RecA protein is a strand displacement protein. See Figure 1 .
RecA preferentially binds to single-stranded DNA in a cooperative fashion; this cooperativity means that RecA will cover an entire single-stranded DNA molecule rather than bind to several molecules partially. Rec A then aligns homologous segments (those with complementary information) to form base pairs. The key reaction of RecA-coated DNA is the movement of the single-stranded regions of the DNA to form a joint molecule—a process called strand displacement. This reaction involves ATP hydrolysis.
In homologous recombination, two double helices align and are nicked. Then RecA catalyzes the invasion of each double helix by one strand of the other. This forms a crossed structure called a Holliday junction. If the Holliday structure were simply broken at the point where it was formed, no genetic recombination could occur because the two original DNA molecules would simply reform. Instead, the junction migrates by displacement of one strand of DNA. Finally, the displaced Holliday junction is broken and rejoined, or resolved. The exact type of recombination between the two strands depends on which of the strands is broken and rejoined. Note that each recombination event involves two breaking and rejoining events: one to initiate strand displacement and one to resolve the Holliday junction. See Figure 2 .
If the two DNAs have the same sequence, they can form a Holliday junction, but no detectable genetic recombination takes place because no information change has occurred. If the two DNAs are very different, no recombination will take place because formation of a Holliday junction requires homologous information. If the two DNAs of the Holliday junction are similar to each other but not identical (that is, they contain mismatches), then repair enzymes, which remove the base and/or nucleotide from one of the mismatched strands, will repair the DNA. The fact that some enzymes participate both in repair and in recombination accounts for the fact that many recombination-deficient mutant bacteria are also highly sensitive to ultraviolet light.
The rare human genetic disease xeroderma pigmentosum is due to a deficiency in one of the many components of the DNA repair system. Exposure to ultraviolet light causes skin tumors. Individuals with this disease are so sensitive to ultraviolet light that they must avoid even household fluorescent lamps.
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