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Chromatin Definition and Examples

Chromatin is found in eukaryotic cells. It is composed of proteins and a complex of RNA -DNA. Its main function is to bring the elongated DNAs into a compact form and keep them packaged so that it does not get entangled. It performs a crucial role in strengthening the cell division process. It helps to prevent DNA damage and controls the expression of genes and replication of DNA. Chromatin plays a vital role in the separation of chromosomes in the process of mitosis and meiosis. Histones are the main proteins present in the chromatin which are attached to the DNA and acts like anchors.

chromatin

Three main stages in the organization of chromatin:

  • Euchromatin Here the DNA envelops around the histone proteins which forms nucleosomes that make a structure that looks like beads are embedded on a string.
  • Heterochromatin Histones surround a 30 nm fiber which contains an array of nucleosomes in a solid form.
  • Higher-level in supercoiling of DNA of the 30nm fiber that develops a metaphase chromosome.

Many organisms do not maintain this organizational way of the system. For example, avian red blood cells have more densely arranged chromatin than other eukaryotic cells, and trypanosomatid protozoa do not tightly pack their chromatin into chromosomes that can be seen at all. Trichromatic cells have an entirely different set of structures for arranging their DNA (the prokaryotic chromosome that is equal is called a genophore and is positioned inside the nucleoid region).

chromatin

The chromatin network’s structure depends on the specific stage of the cell cycle. When the interphase occurs, the structure of chromatin is loose and allows entrance to RNA and DNA polymerase that not only transcribes but also replicates the DNA. The normal structure of chromatin that occurs during interphase is dependent on the particular genes that are contained in the DNA. Places of DNA that contain genes are transcribed into loosely packed and closely related with RNA polymerases in its structure recognized as euchromatin while those regions that contain genes that are not active ("turned off") are usually more densely packed and correlate with organizational proteins in heterochromatin. Moderation of the organizational proteins in chromatin through methylation and acetylation changes the local chromatin structure and hence, changes the gene expression. The form of chromatin networks is vaguely recognized and leaves an area of research in molecular biology.

Dynamic structure

Chromatin goes through different changes in its structures when a cell cycle occurs. Histones are proteins that form the fundamental order of chromatin that can be adjusted via different post- translational alterations to change the packaging of chromatin (histone modification). Most alterations occur on the tail portion of histones. This results in terms of chromatin having the permissibility to enter and packaging depends both on the modified amino acid and the kinds of modifications.

DNA structure

DNA is divided into three different parts based upon their structure distinction, B-, A-, and Z- DNA. A- and B-DNA are similar to each other, constructing right-handed helices, while Z-DNA is a left-handed helix along with a backbone of phosphate. Z-DNA plays a vital role in the structure of chromatin and transcription due to the characteristics of the junction between B- and Z-DNA. At the point of B- and Z-DNA, one pair of bases is turned over from normal bonding. This portrays a double role in the recognition of sites by various proteins.

p2

Nucleosome

The central repetitive portion of chromatin is called the nucleosome which is interconnected by a different portion of linker DNA which is a shorter arrangement of DNA than pure DNA in solution. Apart from the core histones, a linker histone known as H1 is present that contacts the doorway and allows entry of the strand of DNA on the nucleosome. The core particle of the nucleosome, along with histone( H1), gets called a chromatosome. Nucleosomes along with approximately 20 to 60 base pairs of linker DNA can construct non-anatomical conditions which are almost 10 nm beads embedded on a string fiber.

The nucleosomes attach to the DNA non-significantly, as needed by their function in normal DNA packaging. Large strands of DNA sequences are preferred to aid nucleosome placing. It is mainly due to the several physical features of different sequences of DNA: For example, adenine (A), and thymine (T) are more compressed into the inner minor grooves.

p3

Cell cycle-dependent structural organization

  • 1. Interphase: The chromatin structure during the interphase of mitosis allows access to transcription and repairing of DNA while packing DNA into the nucleus.
  • 2. Metaphase: The structure of chromatin during metaphase is different from that during interphase. It contains a loop of 30 nm fiber that offers support to the proteins.

Chromatin

 

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