Chromatin

Chromatin is a complex of DNA and protein responsible for condensing and packaging chromosomal DNA. Chromatin is found in both bacterial and eukaryotic cells. Eukaryotic chromatin consists primarily of DNA associated with histone proteins and numerous other chromatin-binding factors that contribute to genome organization and regulation. Chromatin packages long DNA molecules into compact structures while controlling access to genetic information for processes such as transcription, DNA replication, and DNA repair. During cell division, chromatin facilitates proper segregation of chromosomes in anaphase; the characteristic shapes of chromosomes visible during this stage are the result of DNA being coiled into highly condensed chromatin. Chromatin organization is often described at several structural levels. At the most basic level, DNA wrapped around histone octamers forms nucleosomes connected by stretches of linker DNA, producing a structure sometimes referred to as a “beads-on-a-string” fiber approximately 10–11 nm in diameter. Nucleosome arrays can interact with each other and with linker histones to form higher-order chromatin structures. 30-nm chromatin fiber has been observed in vitro, although its presence and prevalence in living cells remain debated. At larger genomic scales, chromatin is organized into loops and domains that contribute to the three-dimensional architecture of the genome. Chromosomes are further partitioned into compartments associated with active (euchromatin) or inactive (heterochromatin) chromatin states, and individual chromosomes occupy distinct spatial regions within the nucleus known as chromosome territories. Many organisms exhibit variations in chromatin organization. For example, spermatozoa and avian red blood cells have more tightly packed chromatin than most eukaryotic cells. In contrast, some protozoa such as trypanosomatid do not DNA condense their chromatin into visible chromosomes at all. Bacteria organize their DNA differently, forming a chromatin or nucleoid structure organized by nucleoid-associated proteins including H-NS and StpA. Some archaeal species encode histone proteins, and package DNA into nucleosome-like assemblies of...