Chromatin

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A complex of DNA, histones and nonhistone chromosomal proteins, which condense to make up the chromosomes found in eukaryotic nuclei[1]. Approximately 146 DNA base pairs are wound into a left-handed superhelix[2] around 8 histones protein subunits with around 20 base pairs wrapped around the linker histone, known as H1[3][4]. This forms a 'beads on a string' structure. Each of these repeating units is called a nucleosome.

Contents

Formation of Chromatin Fibres

Before the formation of chromatin fibres, nuclesomes are lined up with clusters of histones packed together and connected by linker DNA. Each nucleosome is made up of an octamer of histone protein subunits. These subunits are H2A, H2B, H3 and H4. A tetramer of H3 and H4 joins together with two H2A-H2B dimers. Then 146 base pairs of DNA is wrapped twice around the subunits to create a stable single nucleosome.

The 10 nm (in diameter) chain of nucleosomes is further condensed to form a thicker structure which is 30 nm in diameter. The formation of the chromatin fibre is due to the change in salt concentration of the nuclear solution present in the cell. The packaging of the fibre is seen to be in a zigzag structure by the interlocking of fibres present within it[5].

Folding and Formation of a Chromosome

Heterochromatin and euchromatins are being labelled to distinguish the way in how the chromatins are folded. Heterochromatin is tightly packed while euchromatins are not as compact. Both of these contain DNA, heterochromatin having its DNA in an inactive state while euchromatin has its DNA in an active state. The condensation and compacting of euchromatin in cell division, makes the chromatin to be fully active and with that, give rise to the clear formation, picture and structure of chromosome[6].

Chromatin is formed to strengthen the DNA and decrease it's volume in the cell, thus preventing damage.

Composition of Chromatin

Chromatins are composed primarily of small basic proteins called histones. There are two types of histones of which chromatin is made up of. These are core histones and linker histones. Core histones are highly conserved and made of globular domain which has 3 alpha helices and loops that forms "histone folds" - they are similar in yeast and humans. Core histones bind DNA by forming repeating units called nucleosomes. When two different histones want to interact with each other it is known as a "Histone Handshake".

References

  1. Alberts, B. et al., 2007. Molecular Biology of the Cell. 5th ed. s.l.:Garland Science. p.G:7
  2. Berg, Jeremy M., et al. 2011. Biochemistry. 7th Edition. Palgrave MacMillan. p.976
  3. Hardin, J., Bertoni, G. and Kleinsmith, L.J (2012). Becker's World of the Cell. 8th ed. California: Pearson. 533.
  4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4454312/
  5. Hardin, J., Bertoni, G. and Kleinsmith, L.J (2006). Becker's World of the Cell. 8th ed. California: Pearson. 533.
  6. Hardin, J., Bertoni, G. and; Kleinsmith, L.J (2006). Becker's World of the Cell. 8th ed. California: Pearson. 533.
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