Recombination Machinery

Recombination does not just happen spontaneously on its own. Looking at the model of recombination, it is apparent that at the very least, an endonuclease is required (to produce the nicks that initiate the process, and separate the heteroduplex), as is DNA ligase. In fact, there is a specific set of proteins that functions to promote recombination. In bacteria, these are known as the rec (short for recombination) proteins. Rec A protein is a strand exchange promoter, while rec B, C, and D form a complex that nicks and unwinds the DNA. Other proteins involved in the process are less well characterized.

Gene Conversion

Even though recombination requires extensive homology between the two DNA molecules involved, the sequences do not have to be absolutely identical; for example, a point mutation would not interfere with the process. It is possible, therefore, that recombination could produce a DNA molecule in which there was a base mismatch. For example, if a segment of DNA with a wild-type allele was recombined with a segment of DNA from a mutant allele, the site of the mutation would be a mismatch.

Mismatch or excision repair may try to fix this situation, but since both strands will be methylated (after all, it would not be immediately post-replication) the sequence would be randomly converted to either the wild type or mutant form. Therefore, it would be possible for a mutant allele to be changed into wild-type allele. This is gene conversion.

• Mutations can occur spontaneously as errors of replication, or they can be induced by mutagens in the environment.
• If mutations occur in somatic cells, they affect that individual only; if they occur in germline cells, they affect future generations.
• Mutations can affect physical form, nutritional requirements, behavior, gene regulation, or survival.
• There are two basic types of mutations: base substitutions and frameshift mutations. Each has a different effect on translation of the genetic code.
• UV exposure can lead to DNA lesions called thymine dimers. These lesions can be repaired by a variety of mechanisms, including photoreactivation, excision repair, recombination repair, and SOS repair. The mechanism used depends inpart on the particular situation.
• Cells have mechanisms, such as proofreading and mismatch repair, that limit the mutation rate.
• Cells contain a mechanism by which segments of homologous DNA molecules can be exchanged. This forms the basis for crossing over during meiosis.
• There are proteins in cells that function to promote recombination.
• Recombination can lead to gene conversion; that is, conversion of a mutant allele to a wild-type allele (or vice versa).