A quick post based on past posts to answer a question a few people asked me… Your DNA packs up tight w/help of histones because we have so much DNA it’d never fit in our cells if it weren’t all coiled up (if you stretched it out it would be ~2m (~6.5ft) long) To help it coil & stay coiled ➿ it wraps around “hair rollers” (proteins called histones) to form nucleosomes, which are like beads on string ➰  

YouTube link in case embed isn’t working: https://youtu.be/MIwYdFY-BZ4 

This saves space (& prevents things you don’t want read from being read), but when you *do* want a region read &/or transcribed, that region must be “opened up.”  It’s kinda like when you open a mobile version of a website & it has all the different sections collapsed to save room ▶️ & you have to click on them to expand them if you want to actually read them 🔽  

Much of epigenetics involves special proteins adding modifications to the DNA or its “curlers” that help the sections expand if you want to read them 🔽 & collapse if you don’t want them read 🔼. These modifications are often put on histones by post-translationally modifying lysine amino acids in the tails of the histone proteins, as we talked about here: http://bit.ly/lysineanalysis ; YouTube: https://youtu.be/Uib8B4xZXKk    

Histone proteins have “tails” containing lots of lysines.  Lysine can get modified in various ways, including methylation (1, 2, or all 3 of the amino group’s H’s can be replaced by methyl (-CH₃)) & acetylation (H replaced by “acetyl” (carbonyl attached to a methyl)).  These modifications contribute to a “histone code” that tells a cell when to express certain genes. It’s not quite this simple, but acetylation helps “loosen up” regions, whereas methylation tightens things up. This can make regions more or less accessible for transcription (making messenger RNA (mRNA) copies of a gene getting made which are later read by the ribosomes which make proteins based on their instructions). 

How do you know what’s worth reading when? Other proteins are able to read “section headers” – recognize specific DNA sequence motifs (like words) that are present in front of functionally-related genes so that, instead of reading gene-specific headers, it’s more like reading “key words” or indexing terms, so a DNA-binding protein can search for 1 search term & get “hits” on multiple regions or genes which it can act on “simultaneously” – this allows for coordinated activation or deactivation of related genes   

There are also a bunch of other levels of gene regulation and you can learn more about them here: https://bit.ly/expression_measurement; YouTube: https://youtu.be/aTbJAnF4nqI