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[[File:Cell_memory_modules.jpg|thumb|PcG proteins repress transcription in salivary glands. A shows an active transcription. B shows transcription after addition of a promoter. C shows transcription of a mutant protein. D shows transcription become depressed.]]
Cellular memory modules have the same general process of genes undergoing [[Transcription (biology)|transcription]], these genes being transferred to an unfamiliar environment, and then these genes reverting to their original characteristics preserved through transcriptional memory. Cellular memory modules preserve repressed and active [[chromatin]] states in the [[Polycomb-group proteins|Polycomb group (PcG)]] and [[Trithorax-group proteins|trithorax group (trxG)]] proteins by using Polycomb- and trithorax [[Response element|response elements]], which are just DNA sequences <ref name=":0">{{Cite journal |last=Déjardin |first=Jérôme |last2=Cavalli |first2=Giacomo |date=2004-02-25 |title=Chromatin inheritance upon Zeste-mediated Brahma recruitment at a minimal cellular memory module |url=http://emboj.embopress.org/cgi/doi/10.1038/sj.emboj.7600108 |journal=The EMBO Journal |volume=23 |issue=4 |pages=857–868 |doi=10.1038/sj.emboj.7600108 |issn=0261-4189 |pmc=PMC381013 |pmid=14963490}}</ref>. Transcription resets and alters [[Epigenetic marker|epigenetic marks]] on chromosomal memory elements that are regulated by PcG and trxG proteins <ref>{{Cite journal |last=Rank |first=Gerhard |last2=Prestel |first2=Matthias |last3=Paro |first3=Renato |date=2002-11-01 |title=Transcription through Intergenic Chromosomal Memory Elements of the Drosophila Bithorax Complex Correlates with an Epigenetic Switch |url=https://doi.org/10.1128/MCB.22.22.8026-8034.2002 |journal=Molecular and Cellular Biology |volume=22 |issue=22 |pages=8026–8034 |doi=10.1128/MCB.22.22.8026-8034.2002 |pmid=12391168}}</ref>. PcG genes maintain silent expression states during the development of [[Hox gene|Hox genes]] while trxG proteins maintain Hox gene expression patterns. PcG proteins bind to Polycomb response elements (PREs) to repress the target gene and silence their transcription<ref>{{Cite journal |last=Bantignies |first=Frédéric |last2=Cavalli |first2=Giacomo |date=2006-06-01 |title=Cellular memory and dynamic regulation of polycomb group proteins |url=https://www.sciencedirect.com/science/article/pii/S0955067406000536 |journal=Current Opinion in Cell Biology |series=Nucleus and gene expression |language=en |volume=18 |issue=3 |pages=275–283 |doi=10.1016/j.ceb.2006.04.003 |issn=0955-0674}}</ref> by excluding [[transcriptional activators]] and making the gene unable to undergo RNA synthesis<ref>{{Cite journal |last=Orlando |first=Valerio |date=2003-03-07 |title=Polycomb, Epigenomes, and Control of Cell Identity |url=https://www.cell.com/cell/abstract/S0092-8674(03)00157-0 |journal=Cell |language=English |volume=112 |issue=5 |pages=599–606 |doi=10.1016/S0092-8674(03)00157-0 |issn=0092-8674 |pmid=12628181}}</ref>. While the basis of the mechanism among cellular memory modules is the same, what initiates the mechanism and the specific proteins carrying it out differ based on the ___location of the cellular memory module within the gene. Some of these specific mechanisms and gene locations have been analyzed from experiments and outlined below.
=== Ab-Fab Mechanism ===
This experiment was able to identify a minimal cellular memory module of 219 bp originating from the ''Drosophila Fab-7'' region which regulates the ''Abdominal-B'' gene. Recruitment of trxG proteins allows for binding to the [[DNA binding site|DNA binding sites]] on the Zeste protein, overriding Zeste’s need for the Brahma (BRM) protein, and initiating the inheritance of [[Active chromatin sequence|active chromatin]]. Researchers then took this Zeste protein and mutated its binding sites which increased its role in PcG-dependent silencing. Preserved DNA sequence Ab-''Fab'' recruits BRM and trxG proteins, activating [[Embryo|embryogenesis]] and weakening the bind of PcG to Zeste protein. The effects of Ab-''Fab'' allowed the Zeste protein to return to active chromatin states following its mutation. These response elements were determined to be cellular memory modules as there is DNA overlap and both elements express the memory of both silent and active chromatin by using cell division<ref name=":0" />.
==References==
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