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The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4.
Chromatin contains the vast majority of the DNA in an organism, but a small amount inherited maternally can be found in the mitochondria. It is present in most cells, with a few exceptions, for example, red blood cells. Histones are responsible for the first and most basic unit of chromosome organization, the nucleosome.
Chromatin is a complex of DNA and protein found in eukaryotic cells. [1] The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in reinforcing the DNA during cell division , preventing DNA damage , and regulating gene expression ...
The organization of chromosomes into distinct regions within the nucleus was first proposed in 1885 by Carl Rabl.Later in 1909, with the help of the microscopy technology at the time, Theodor Boveri coined the termed chromosome territories after observing that chromosomes occupy individually distinct nuclear regions. [6]
The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter. [1] [page needed] A prokaryotic cell has three regions:
Thus, the entire chromosome, i.e. chromatin in eukaryotes consists of such nucleoproteins. [2] [13] In eukaryotic cells, DNA is associated with about an equal mass of histone proteins in a highly condensed nucleoprotein complex called chromatin. [14]
Within chromosomes, DNA is held in complexes with structural proteins. These proteins organize the DNA into a compact structure called chromatin. In eukaryotes, this structure involves DNA binding to a complex of small basic proteins called histones. In prokaryotes, multiple types of proteins are involved.
The nucleosome is the basic unit of DNA condensation and consists of a DNA double helix bound to an octamer of core histones (2 dimers of H2A and H2B, and an H3/H4 tetramer). About 147 base pairs of DNA coil around 1 octamer, and ~20 base pairs are sequestered by the addition of the linker histone (H1), and various length of "linker" DNA (~0 ...