Abstract
Usually, most of the genes are biallelically expressed but imprinted gene exhibit monoallelic expression based on their parental origin. Genomic imprinting exhibit differences in control between flowering plants and mammals, for instance, imprinted gene are specifically activated by demethylation, rather than targeted for silencing in plants and imprinted gene expression in plant which occur in endosperm. It also displays sexual dimorphism like differential timing in imprint establishment and RNA based silencing mechanism in paternally repressed imprinted gene. Within imprinted regions, the unusual occurrence and distribution of various types of repetitive elements may act as genomic imprinting signatures. Imprinting regulation probably at many loci involves insulator protein dependent and higher-order chromatin interaction, and/or non-coding RNAs mediated mechanisms. However, placenta-specific imprinting involves repressive histone modifications and non-coding RNAs. The higher-order chromatin interaction involves differentially methylated domains (DMDs) exhibiting sex-specific methylation that act as scaffold for imprinting, regulate allelic-specific imprinted gene expression. The paternally methylated differentially methylated regions (DMRs) contain less CpGs than the maternally methylated DMRs. The non-coding RNAs mediated mechanisms include C/D RNA and microRNA, which are invovled in RNA-guided post-transcriptional RNA modifications and RNA-mediated gene silencing, respectively. The maintenance and reprogramming of imprinting are not significantly affected by reduced expression of Dicer1 and the evolution of imprinting might be related to acquisition of DNMT3L (de novo methyltransferase 3L) by a common ancestor of eutherians and marsupials. The common feature among diverse imprinting control elements and evolutionary significance of imprinting need to be identified. Key words: Genomic imprinting, differentially methylated regions (DMRs), non-coding RNA, imprinting evolution.
Highlights
Genomic imprinting is a germline-specific epigenetic modification of the genome that results in parent-oforigin-specific expression of a small subset of genes in offspring
DNA methylation is probably the more stable epigenetic mark in the embryo, which may be appropriate in this case for the imprint to survive in adulthood, while histone modifications may serve purpose in the short-lived placenta that is not required after birth
Dnmt1o could be attracted to the hemimethylated differentially methylated domains (DMDs) due to the arrangement of CpG dinucleotides, the imperfect repetitiveness, and a specific chromatin structure induced by the hemimethylated DMD
Summary
Most of the genes are biallelically expressed but imprinted gene exhibit monoallelic expression based on their parental origin. Genomic imprinting exhibit differences in control between flowering plants and mammals, for instance, imprinted gene are activated by demethylation, rather than targeted for silencing in plants and imprinted gene expression in plant which occur in endosperm. It displays sexual dimorphism like differential timing in imprint establishment and RNA based silencing mechanism in paternally repressed imprinted gene. Placentaspecific imprinting involves repressive histone modifications and non-coding RNAs. The higher-order chromatin interaction involves differentially methylated domains (DMDs) exhibiting sex-specific methylation that act as scaffold for imprinting, regulate allelic-specific imprinted gene expression.
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