Life Cycle of an Imprint

Life cycle of an imprint is a dynamic process, which involves erasure, establishment, maintenance and implementation of the imprint markings.


Erasure and Establishment :


Each gamete carries sex specific imprint markings that are required for normal development. Upon fertilization, the paternal pronucleus is rapidly and actively demethylated within the zygote prior to first cellular division. In contrast, the maternal genome becomes demethylated in a passive manner during subsequent divisions, presumably due to lack of Dnmt – 1.

Erasure of the paternal methylation profile by the oocyte is a potentially powerful mechanism by which maternal factors modify chromatin structure to regulate the paternal genome. Demethylation at this stage is proposed to be required for the activation of genes necessary for the early embryonic growth. However imprint methylation marks present on both the paternal and maternal genomes are maintained despite this global demethylation event.


Another reprogramming event occurs later, within the primordial germ cells of the developing fetus. After the demethylation in the primordial germ cells, parental specific methylation is re-established during gametogenesis. This occurs in sperm post-natally within diploid genocytes prior to meiosis and within oocytes arrested at the diplotene stage of meiosis.


In the oocyte, the Dnmt 3 family of methyltransferases is required to set maternal specific methylation patterns for imprinted genes. Dnmt 3a and Dnmt 3b play an important role in establishing methylation patterns in oocytes.