> For the complete documentation index, see [llms.txt](https://edu.abi.am/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://edu.abi.am/molecular-biology/information-flow-in-the-cell/chromatin-and-chromosomes.md).

# Chromatin and Chromosomes

DNA carrying genetic information is packaged in structures called **chromosomes**. Bacteria typically have a single chromosome which contains one circular DNA, while eukaryotic genomes are divided into many chromosomes. A eukaryotic chromosome contains a linear DNA molecule associated with numerous proteins classified as **histones** and non-histone proteins. This complex of DNA and proteins is referred to as **chromatin**.

Packaging DNA into chromatin fibres achieves a remarkably high level of compaction. For instance, approximately 205 cm of DNA present in every human cell is enclosed within a nucleus about 10 micrometres in diameter.

<figure><img src="/files/75l7Sz3SdlcGmSEKo2fN" alt="" width="375"><figcaption><p>Electron microscopy of chromatin. Nucleosomes remind beads on a string<br>Image source:<br>Oak Ridge National Laboratory - ORNL History, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=90681196</p></figcaption></figure>

The primary level of chromatin compaction involves the formation of **nucleosomes**, which are protein complexes composed of histones. When observed under an electron microscope, chromatin gently extracted from the nuclei resembles beads on a string. Each "bead" represents a nucleosome, around which DNA is wrapped, while the "string" connecting them is linker DNA.

<figure><img src="/files/VO0Z7WTFlzrgl0vT5dEN" alt="" width="450"><figcaption><p>Chromatin structure<br>Image source:<br>Darryl Leja, NHGRI - https://medlineplus.gov/genetics/condition/adnp-syndrome/#causes, Public Domain, https://commons.wikimedia.org/w/index.php?curid=130950339</p></figcaption></figure>

A nucleosome consists of a protein octamer comprising eight histones, each of which has a tail extending from the nucleosome core. These tails undergo various chemical modifications that can alter chromatin structure and regulate gene expression.