Genomic imprinting differs from that of ordinary Mendelian rules of inheritance, as in the first aspect the gene is expressed by one allele from one parent and the other is silenced by imprinting; where in Mendelian the gene is expressed by two alleles coming from both parents. The imprinted genes are maintained in somatic cells, where in germline cells it is erased and re-established during development.
Genomic imprinting is based on DNA methylation. The patterns of 5'-CpG-3' methylation dictate gene expression. Apparently, in most cases, CpG methylation is associated with gene silencing. Each individual has a pattern of DNA methylation which is established according to his/her parents' DNA methylation patterns and the methylation acquired from the environment.
Importantly, the DNA copies which are prepared in germline cells, are specifically methylated to reflect the parental methylation. This methylation appears to be produced by the maintenance DNA methyl-transferase Dnmt1.
After fertilization, the two DNA copies - maternal and paternal suffer a wave of demethylation, however, it seems that the imprinted genes (the ones known to be DNA methylated) are not affected by it. Therefore, they maintain the parental (both maternal and paternal) methylation patterns.
There are only a few genes discovered to be imprinted so far, probably about 80. A good example of such genes are H19 and Igf2 genes. You will see here that Igf2 gene imprinting is quite peculiar because expression of it is actually activated by methylation. A good paper on methylation (especially the environmental influence on methylation patterns) is the one of Waterland and Jirtle 2003.
Genomic imprinting is based on DNA methylation. The patterns of 5'-CpG-3' methylation dictate gene expression. Apparently, in most cases, CpG methylation is associated with gene silencing. Each individual has a pattern of DNA methylation which is established according to his/her parents' DNA methylation patterns and the methylation acquired from the environment.
Importantly, the DNA copies which are prepared in germline cells, are specifically methylated to reflect the parental methylation. This methylation appears to be produced by the maintenance DNA methyl-transferase Dnmt1.
After fertilization, the two DNA copies - maternal and paternal suffer a wave of demethylation, however, it seems that the imprinted genes (the ones known to be DNA methylated) are not affected by it. Therefore, they maintain the parental (both maternal and paternal) methylation patterns.
There are only a few genes discovered to be imprinted so far, probably about 80. A good example of such genes are H19 and Igf2 genes. You will see here that Igf2 gene imprinting is quite peculiar because expression of it is actually activated by methylation. A good paper on methylation (especially the environmental influence on methylation patterns) is the one of Waterland and Jirtle 2003.
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