# Chromatin --- **Chromatin** is a threadlike complex of [[DNA]] and [[protein]] that is found in [[eukaryote|eukaryotic]] cells. Its main function is to pack and concense the long DNA molecules into discrete units. The function of chromatin is to reinforce the structure of DNA during [[cell cycle|cell division]], prevene damage, and regulate [[gene expression]]. In order for the gene to be [[transcription|transcribed]], chromatin has to be [[chromatin remodeling|decondensed]]. ![[chromatin.png]] ## Structure ### Histones Chromatin is formed when the [[DNA]] double helix wraps around a protein called a "histone" at certain intervals, forming a "beads-on-a-string"-like structure. Histones are positively charged and DNA is negativly charged, so they can really squeeze in tight toghether. ### Nucleosomes The "beads" in the beads-on-a-string are called **nucleosomes**. The acetyl groups on the histones help condense the nucleosomes into chromatin. Each nucleosome is wrapped twice around a group of eight histones, and consists of about 200 base pairs of DNA. ### 30-Nanometer Fibre This structure is then wound even denser, into a **30-nanometer fibre** (which is, as the name sugests, 30-nanometers wide). This can *further* coil into a DNA supercoil. ### Supercoil A DNA **supercoil** is when the 30-nanometer fibre further wraps or coils itself. (i think) ### Chromosomes Before [[cell cycle|cell division]], this scaffolding *further* condenses into the very tightly packed bundles of [[chromosome|chromosomes]]. ## Chromatin During Transcription During [[transcription]], the chromatin has to be selectivly unwound so that the [[RNA polymerase]] can get at the segment it wants to transcribe. The enzymes largely responsible for the [[gene regulation#Transcriptional Control|transcription regulation]] arae "histone acetyl transferase" and "deacetylase". ![[condensed chromatin.png]] ___