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.

Electron microscopy of chromatin. Nucleosomes remind beads on a string Image source: Oak Ridge National Laboratory - ORNL History, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=90681196

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.

Chromatin structure Image source: Darryl Leja, NHGRI - https://medlineplus.gov/genetics/condition/adnp-syndrome/#causes, Public Domain, https://commons.wikimedia.org/w/index.php?curid=130950339

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.