Stem cells replication control

Figure 32-3 Hypothetical way of controlling stem cell replication by methylation or other marking system. Methyltransferases Ej and E2 methylate the cytosine in a 5 -CpG-3 or other palindromic sequence. In freely replicating cells these two enzymes keep the CG sequences methylated on both DNA strands. In stem cells another enzyme, perhaps a third methylase (E3), marks a location outside the palindromic DNA on one strand ( ). Replication leaves the...

Comment. One could speculate that the unicellular and the early multicellular life forms existed in a highly stressed life style, and replicated rapidly without rest, or senescence. In contrast, in the mature multicellular hosts, the overwhelming number of the somatic cell populations encode just one particular task, divide seldom if ever, and keep the rest of their genomes silenced (under the control of silenced promoters). Stem cells are in the minority, divide asymmetrically, but seldom, otherwise they rest Haploid germ cells exert little activity until after become zygotes (the united sperm and the ovum). Thereafter, they dictate the ontogenesis of the individual. How their intron counts reflect to their life style ... 

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