DNA repair processes arose early in evolution. During evolution, DNA base excision repair apparently acquired additional roles in demethylation of cytosines in DNA. Demethylation is central to two mammalian fundamental processes. Embryonic reprogramming and neuronal memory require rapid gene expression alterations depending in part on demethylations. The active demethylation reactions in both processes primarily depend, first, on the family of 5-methylcytosine oxidases sharing the acronym ten-eleven translocation (TET methylcytosine dioxygenases) and, second, on DNA base excision repair enzymes. In mice, within 6 h of fertilization, the paternal chromosomes are close to 100% actively demethylated through TET and repair activity. (Methylation of maternal DNA is blocked during subsequent cycles of replication, so methyl groups on maternal DNA, passively, becomes highly diluted over the next 4 days.) Rats subjected to one instance of contextual fear conditioning create an especially strong long-term memory. At 24 h after training, 9.2% of the genes in the rat genomes of hippocampus neurons are differentially methylated, including over 500 genes with demethylation. The emergence of embryonic development in evolution depended on preexisting DNA methylation/demethylation pathways to modify gene expression. The further emergence of memory likely evolved from the earlier set of methylation/demethylation capabilities associated with embryonic development.
Part of the book: DNA Methylation Mechanism
The early history of life on Earth likely included a stage in which life existed as self-replicating protocells with single-stranded RNA (ssRNA) genomes. In this RNA world, genome damage from a variety of sources (spontaneous hydrolysis, UV, etc.) would have been a problem for survival. Selection pressure for dealing with genome damage would have led to adaptive strategies for mitigating the damage. In today’s world, RNA viruses with ssRNA genomes are common, and these viruses similarly need to cope with genome damage. Thus ssRNA viruses can serve as models for understanding the early evolution of genome repair. As the ssRNA protocells in the early RNA world evolved, the RNA genome likely gave rise, through a series of evolutionary stages, to the double-stranded DNA (dsDNA) genome. In ssRNA to dsDNA evolution, genome repair processes also likely evolved to accommodate this transition. Some of the basic features of ssRNA genome repair appear to have been retained in descendants with dsDNA genomes. In particular, a type of strand-switching recombination occurs when ssRNA replication is blocked by a damage in the template strand. Elements of this process appear to have a central role in recombinational repair processes during meiosis and mitosis of descendant dsDNA organisms.
Part of the book: DNA