# 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="https://3514673221-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdDE3NSiCXcu5YQDQmgdU%2Fuploads%2F8gXcOvQDVMJkZIAAK7GH%2FFig_65_beads_on_a_string.jpg?alt=media&#x26;token=a34fc967-e4b8-4e21-8cd6-2a59b08f7df1" 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="https://3514673221-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdDE3NSiCXcu5YQDQmgdU%2Fuploads%2Fq9qZfx5hwZvhDbIFNavK%2FFig_66_chromatin.jpg?alt=media&#x26;token=e27276ab-86c6-48ac-9cb4-858166b37a3d" 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.