In eukaryotic cell nuclei, DNA is wrapped around and firmly associated with histone proteins, forming chromatin. When DNA is damaged, the chromatin structure needs to be loosened to allow repair enzymes to gain access to the damage. This requires modifying the histone proteins. These modifications, called epigenetic alterations, do not alter the base-pair sequence. Repair-associated epigenetic alterations are usually transient, removed when no longer needed for repair. However, some remain after repair. In the human brain, long-lasting novel epigenetic alterations appear to account for the persistence of addictions to such substances as alcohol, nicotine and cocaine. Certain neurodegenerative diseases are caused by inherited mutations in genes necessary for DNA repair. Deficient DNA repair in these diseases is associated with extensive epigenetic alterations that likely have a role in the disease phenotype. Persistent epigenetic alterations due to DNA repair processes, both histone modifications and methylations of DNA, can also have positive consequences. Stimulation of brain activity (e.g. learning and memory formation) is often accompanied by the generation of DNA damage in neuronal DNA, followed by repair associated with persistent epigenetic alterations. In particular, recent research has shown the need for non-homologous end joining and base excision repair in memory formation.
Part of the book: DNA Repair
Two principal ideas have been proposed to explain the primary adaptive function of the sexual process of meiosis: (1) meiosis, and particularly meiotic recombination, is a process for repairing DNA and (2) meiosis, by means of meiotic recombination, is a process for generating beneficial genetic variation among progeny. We review the sexual processes of a number of well-studied microbial eukaryotes: Saccharomyces cerevisiae, Saccharomyces paradoxus, Schizosaccharomyces pombe, Candida albicans, Ustilago maydis, Paramecium tetraurelia, Volvox carteri, Trypanosoma brucei, Neurospora crassa, and Amoebozoa. We indicate aspects of the sexual processes of these microbial eukaryotes, where they have been established, that support the idea that meiosis is primarily a process for repairing DNA. In addition, we review the likely origin of meiotic sex among the microbial eukaryotes. A prokaryotic archaeon is the likely ancestor of eukaryotes. Extant archaea are capable of a sexual process involving syngamy and recombinational repair of genome damage, suggesting that the precursor of eukaryotic meiotic sex may already have been present in the archaeal ancestor of eukaryotes. We believe that attainment of an understanding of the adaptive function of meiotic sex in microbial eukaryotes is of considerable importance since it will likely apply to meiotic sex in eukaryotes generally.
Part of the book: Parasitology and Microbiology Research