Genomic+imprinting

=Genomic imprinting=

**Genomic imprinting** (or just **imprinting**) is the epigenetic phenomenon by which certain genes are or are not expressed according to the parental chromosome on which they lie. Some genes are expressed only on the maternal chromosome and others only on the paternal chromosome, with the unexpressed allele being masked by DNA methylation and chromatin remodelling to make it transcriptionally inactive. Appropriate imprinting is essential for normal development: various disorders that can arise from abnormal imprinting include Prader-Willi Syndrome and Angelman Syndrome, both involving imprinting errors on chromosome 15.

Imprinting is a dynamic process. It must be possible to erase and re-establish the imprint through each generation. The nature of the imprint must therefore be epigenetic (modifications to the structure of the DNA rather than the sequence). In germline cells the imprint is erased, and then re-established according to the sex of the individual; i.e. in the developing sperm, a paternal imprint is established, whereas in developing oocytes, a maternal imprint is established. This process of erasure and reprogramming. is necessary such that the current imprinting status is relevant to the sex of the individual. In both plants and mammals there are two major mechanisms that are involved in establishing the imprint; these are DNA methylation and histone modifications.