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'''Cellular memory modules''' are a form of [[Transgenerational epigenetic inheritance|epigenetic inheritance]] that allow cells to maintain their original identity after a series of [[Cell division|cell divisions]] and developmental processes. Cellular memory modules implement these preserved characteristics into transferred environments through [[transcriptional memory]].<ref>{{Citation |last=Paro |first=Renato |title=Cellular Memory |date=2021 |url=https://doi.org/10.1007/978-3-030-68670-3_3 |work=Introduction to Epigenetics |pages=49–66 |editor-last=Paro |editor-first=Renato |access-date=2023-04-20 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-030-68670-3_3 |isbn=978-3-030-68670-3 |last2=Grossniklaus |first2=Ueli |last3=Santoro |first3=Raffaella |last4=Wutz |first4=Anton |editor2-last=Grossniklaus |editor2-first=Ueli |editor3-last=Santoro |editor3-first=Raffaella |editor4-last=Wutz |editor4-first=Anton|doi-access=free |url-access=subscription }}</ref> Cellular memory modules are primarily found in ''[[Drosophila]]''.
== History ==
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== Locations and mechanisms: experiment overviews ==
[[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]]s, which are just DNA sequences.<ref name="Déjardin-2004">{{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=381013 |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|pmc=134728 }}</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|url-access=subscription }}</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|doi-access=free }}</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 ===
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