Genomic imprinting: Epigenetic Regulation
Genomic imprinting is an epigenetic process that involves DNA methylation and histone methylation which doesn’t alter the genetic sequence but causes transcriptional silencing. Imprinting means marking a gene in the germline (sperm or ova) of the parents and which is maintained through mitotic cell divisions in the somatic cells of an organism. In simple term the imprinted gene cannot be expressed (it is turned off).
Mendel’s law that two parents make equal contribution to the character and genomic structure of the offspring doesn’t apply on mechanism of imprinting. Imprinting applies on few special genes, alleles inherited from the father are expressed differently than the alleles inherited from the mother. It is called differential or preferential expression of genes.
If an allele is imprinted, it’s been turned off, it cannot express itself, it is silenced. Imprinting is nothing but Transcriptional silencing of few genes from one parent.
What is the function or need of genomic imprinting?
Autosomal genes in the mammalian genome are inherited in a silent state from one of the two parents, and in a fully active form from the other, thereby rendering the organism functionally hemizygous for imprinted genes.
To date 19 imprinted genes have been identified; 5 are expressed from the maternal chromosome while the rest are expressed from the paternal chromosome.
function of imprinting is a subject of some debate; the process is thought to have an important role in regulating the rate of fetal growth.
Mechanism of genomic imprinting
In genes that undergo genomic imprinting, the parent of origin is often marked, or “stamped,” on the gene during the formation of egg and sperm cells. This stamping process, called methylation, is a chemical reaction that attaches small molecules called methyl groups to certain segments of DNA.
In imprinting it doesn’t matter what the identity of that allele is or what sequence of that allele is, all that matters is which parent it came from.
When it happens
At the time of gametogenesis (in both male and female mammals). When the germline develops, all the existing inherited methylation is erased. Then new methylation is created-
A gene that is evolved to be paternally imprinted has sequence tag which says: If you are male, methylate this gene (turn it off) before you make gametes (sperms).
A gene that is evolved to be maternally imprinted has sequence tag which says: If you are female, methylate this gene (turn it off) before you make gametes (ova).
The imprinted alleles behave normally at time of meiosis, only their expression is affected in progeny. The rule of dominance is meaningless because imprinted alleles are functionally hemizygous.
Imprinting has evolved in a manner where expression of different alleles is in balance and there is no disease and disability. We all as mammals have paternal and maternal imprinted genes.
But it may cause problems when deviation from normal happens.
...
...
What is normal –
You, me and most mammals on the planet have imprinted genes with normal phenotypes. The imprinted genes in us are silent (they don’t express) but the other allele from other parent is expressed and so things are pretty much normal.
Problem arises when there is-
- Defective allele from non-imprinted parent (the one who was supposed to express and save the chaos)
- Deletion of allele from non-imprinted parent
- Both alleles inherited from one parent which are imprinted (uniparental disomy)
...
...
...
Paternal imprinting
Maternal imprinting
The Child express the only functional allele which he gets from the father (sperm), and the allele from the mother (ova) is imprinted/silenced/transcriptionally silenced and doesn’t get to express.
Diseases associated with it-
- Prader-willi-Syndrome
- Angelman Syndrome
- Beckwith-Wiedemann syndrome
- Albright's hereditary osteodystrophy
...
...
Revision for today https://creativemeddoses.com/topics-list/therapeutic-hypothermia-neuroprotective-treatment/
Further reading https://www.sciencedirect.com/topics/medicine-and-dentistry/genomic-imprinting
Buy books here 👇👇
Buy fun review books here (these are kindle eBook’s you can download kindle on any digital device and login with Amazon accounts to read them). Have fun and please leave review.
Subscribe to the site by clicking here 🙂 https://creativemeddoses.com/subscribe/