# Chromatin: how DNA is packed and marked

> DNA is wound on spools of histone proteins. Tags on the spools, added and removed by protein complexes, help decide which genes can be reached.

Source: https://khushisingh.in/explore/03-chromatin/
Author: Khushi Singh (khushisingh.in). Licence: CC BY 4.0 for text and original illustrations, unless a credit says otherwise.
Evidence labels used on this site: established, strong evidence, emerging, hypothesis, debated. Where a page has Simple, Curious and Expert versions, all three follow.

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[Biology made simple](https://khushisingh.in/explore/) · How genes work · 3 of 8

About 2 minutes

In plants, animals and fungi, DNA is wrapped around spools of **histone** proteins; one spool with its DNA is a **nucleosome** (Luger et al. 1997). Small tags on histone tails are added by **writer** enzymes and removed by **erasers**. Acetyl tags tend to open the DNA for reading; complexes such as Polycomb add methyl tags that keep genes silent (Allis & Jenuwein 2016). (evidence: established)

Because these marks sit on top of the DNA sequence and can be copied when cells divide, they are called **epigenetic**. A plant uses them to remember winter: Polycomb marks build up on a flowering gene during the cold, so the plant flowers only in spring (Angel et al. 2011). (evidence: strong evidence)

*[Here the web page has an interactive activity; open the source URL to use it.]*

*[Here the web page has an interactive 3D model; open the source URL to use it.]*

**In my research:** [Histone marks in the early embryo →](https://khushisingh.in/research/gut-to-germline/)

## Sources

1. Luger K, Mäder AW, Richmond RK, Sargent DF, Richmond TJ (1997) Crystal structure of the nucleosome core particle at 2.8 Å resolution. Nature 389:251–260. [doi:10.1038/38444](https://doi.org/10.1038/38444)
2. Allis CD, Jenuwein T (2016) The molecular hallmarks of epigenetic control. Nature Reviews Genetics 17:487–500. [doi:10.1038/nrg.2016.59](https://doi.org/10.1038/nrg.2016.59)
3. Angel A, Song J, Dean C, Howard M (2011) A Polycomb-based switch underlying quantitative epigenetic memory. Nature 476:105–108. [doi:10.1038/nature10241](https://doi.org/10.1038/nature10241)