What is chromatin? Definition, structure… 14th June 2022 – Tags: chromatin, DNA
Chromatin is a mass of genetic fabric composed of DNA and proteins that condenses to shape chromosomes all through eukaryotic mobileular department. Chromatin is placed withinside the nucleus of our cells.
The primary function of this composed of DNA and proteins is to compress DNA into a compact unit that is smaller and can fit inside the nucleus. Chromatin consists of complexes of small proteins called histones and DNA.
Histones assist organise DNA into systems known as nucleosomes, offering a base on which DNA may be wound. A nucleosome includes a DNA series of approximately a hundred and fifty base pairs that wraps round a hard and fast of 8 histones known as an octamer.
What is chromatin?
Chromatin is a highly organised complex of DNA and proteins and is one of the main components of the cell nucleus. Histone proteins help organise DNA into structural units called nucleosomes, which are then assembled into a compact structure (chromatin) and finally into very large, high-level structures (chromosomes). The localised accessibility of chromatin is largely regulated by post-translational modifications of histone and DNA proteins, which have dramatic effects on the regulation of chromatin structure, binding of chromatin-modifying complexes and regulators of transcription.
Is the complex of genomic DNA and associated proteins in the nucleus. This better DNA shape lets in cells to package deal their DNA, affords a scaffold for mobileular department and controls gene expression. The shape certain via way of means of a dynamic repertoire of proteins, alternates among condensed heterochromatin and prolonged euchromatin.
Histone proteins and DNA have the same mass in eukaryotic chromatin (although there are also cells with non-histone proteins). The nucleosome is the structural unit of chromatin, which in turn consists of DNA and proteins (histones or non-histones). This structure is repeated throughout the genetic material of an organism. The structure of the this composed of DNA and proteins packing in the higher-order structure is shown below.
Function of chromatin
Initially, chromatin was regarded as the substance that gives the cell nucleus its colour. Later it was discovered that it is not only a colouring substance, but is one of the most important regulators of DNA expression. The structure of chromatin also plays an important role in DNA replication. The assembly of DNA into chromatin and the nucleosome results in a hermetically sealed structure that is not accessible to the enzymes responsible for DNA transcription, replication and repair.
The packaging of the DNA structure is transcriptionally repressive and allows only a basal level of gene expression. If nucleosomal structures are open or disrupted, DNA can be replicated and transcribed more easily.
During the transcription process, the structure of this composed of DNA and proteins is modified by certain repressors and activators that interact with RNA to regulate gene activity. The activators alter the structure of the nucleosome, stimulating the assembly of RNA polymerase. During replication, a similar regulation of chromatin structure occurs, allowing the replication machinery to be in place at the origin of replication.
Chromatin also plays a role in regulating gene expression.Through the technique of positional variegation, genes can come to be transcriptionally inactive through setting them near the silent chromatin heterochromatin. The distance between silent heterochromatin chromatin and genes can be up to 1000 kilobase pairs. This phenomenon is called epigenetics because it produces variations in phenotype.
Scientists have proposed that the highly condensed nature of heterochromatin prevents DNA transcription. However, it is still not entirely clear how neighbouring non-heterochromatic regions are affected. The researchers found that proteins can diffuse into neighbouring regions to produce a similar repressive effect. The researchers also proposed that compartments of the nucleus not accessible to transcription factors may harbour heterochromatin. Therefore, chromatin in the nucleus may not be directly accessible to transcription factors.
The structure of chromatin affects DNA replication. For example, euchromatin and other active areas of the genome replicate earlier. Similarly, in heterochromatin and the surrounding silent zone, the replication process is slow. Other crucial features of this composed of DNA and proteins are defined below.
The most important function of chromatin is to pack long strands of DNA into a much smaller space. The linear length of DNA is very important in relation to its location. In order to fit together without becoming tangled or damaged, DNA must be compacted by some method. The compaction of DNA in the nucleus is called condensation. The degree of condensation of DNA within a body is called the packing ratio. The packing ratio of DNA is about 7000. Because of this high packing ratio, DNA is not embedded directly in the chromatin structure. Instead, there are different hierarchies of organisation.
The initial packaging is achieved by wrapping the DNA around the nucleosome. This packing is the equal for heterochromatin and euchromatin. The second level of packaging is achieved by wrapping the beads in a 30 nm fibre that is also found in mitotic chromosomes and interphase chromatin. This wrapping increases the packing ratio from 6 to 40. The third stage of compaction is achieved by winding the fibre into rings, domains and scaffolds. This final winding increases the packing ratio to 10,000 in the mitotic chromosomes and to 1,000 in the interphase chromatin.
Chromatin immunoprecipitation (ChIP) is a powerful technique to map the in vivo distribution of proteins associated with chromosomal DNA. These proteins can be histone subunits and post-translational modifications or other chromatin-associated proteins such as transcription factors, chromatin regulators, etc. In addition, ChIP can be used to identify regions of the genome associated with these proteins or, conversely, to identify proteins associated with a particular region of the genome. ChIP methodology often involves protein-DNA and protein-protein cross-linking, fragmentation of the cross-linked chromatin and subsequent immunoprecipitation of the chromatin with an antibody specific for a target protein. Isolated DNA fragments in complex with the target protein can be identified by various methods, including PCR, microarrays and DNA sequencing.
Chromosomes are further compressed during metaphase. During cell division in eukaryotic cells, the DNA must be divided equally into two daughter cells. During this phase, the DNA is highly compressed and as soon as the cell finishes dividing, the chromosome retracts. If one compares the length of metaphase chromosomes with that of linear DNA, the packing ratio can be as high as 10,000:1. This high level of packing is achieved by the phosphorylation of histone H1.