ABSTRACT
There are several types of epigenetic inheritance systems which play a role that has been identied as cell memory. We have described a few of them for the readers understanding.
10.3.1.1 DNA methylation The phenotype of a cell or individual is affected by particular genes that are transcribed and have heritable transcription states that can give rise to epigenetic effects. There are numerous regulation layers of gene expression and one way is through the remodeling of chromatin. Chromatin is basically the complex of DNA and the histone proteins with which it allies. Histone proteins are little spheres in shape that DNA wraps around. In case DNA which is enveloped around the histones changes, gene expression can also change as well. Furthermore, chromatin remodeling is accomplished through two main mechanisms as briey explained: (a) The rst way is posttranslational modication of the amino acids that make up histone proteins and histone proteins consist of long chains of amino acids. If the amino acids that are in the chain are modied, the shape of the histone sphere may be changed. During replication, DNA is not completely unwound. It is possible, then, that the modied histones may be carried into each new copy of the DNA. Once there, these histones may act as templates, initiating the surrounding new histones to be shaped in a new manner. By altering the shape of the histones around it, these modied histones would ensure that a differentiated cell would stay differentiated, and not convert back into being a stem cell. (b) The second way is the addition of methyl groups to the DNA, mostly at CpG sites, to convert cytosine to 5-methylcytosine, 5-methylcytosine does much like a regular cytosine, pairing up with a guanine. Nevertheless, some areas of the genome are methylated more heavily than others, and highly methylated areas tend to be less transcriptionally active, through a mechanism not fully understood. Moreover, methylation of cytosines can also persist from the germ line of one of the parents into the zygote, marking the chromosome as being inherited from this parent.
