Chromosomes are made up of DNA and histone protein, which is known as chromatin. It can be found in two different structures:
- Euchromatin, which is less condensed and ready to be transcribed.
- Heterochromatin, which is in condensed form and cannot be transcribed.
Our DNA is extraordinarily lengthy. It is not found in its natural state but in a compressed and coiled form known as chromatin. Chromatin appears on chromosomes like beads under an electron microscope. These beads are called nucleosomes, and each one is made up of DNA wrapped around histone protein.
Structure of Chromatin
The structure of chromatin is not stable. During cell division, they go through a variety of structural modifications. It changes several times during the cell cycle. Inside the nucleus of eukaryotic cells, chromatin is a complex structure made up of DNA, RNA and protein. The chromatin fibre is made by folding the nucleosome further. Chromosomes are made up of coiled and condensed chromatin fibres.
A lot of factors influence the shape of chromatin. The overall structure is largely determined by the stages that occur in the cell cycle. During metaphase, when the DNA is replicated and divided into two cells, the structure of chromosomes can be seen in a light microscope as they change shape.
There are three stages of the structure in chromatin:
- Nucleosomes are generated when DNA is wrapped around histone proteins.
- The nucleosome is made up of a 30-nm fibre made up of several histones.
- The 30-nm fibre’s DNA is packaged at a higher level in the metaphase chromosome.
Effect of Chromatin in Gene Expression
The chromatin structure is not stable. It changes several times during the cell cycle. Although the chromosome is responsible for the regulation of gene expression by redistribution of genes, the redistribution of genes generally does not have an influence on gene regulation.
Some factors are responsible for gene expression:
- Post-translational modification (PTM)
- Nucleosome remodels that are ATP-dependent
- Histone chaperone
- Histone variant
The change in the structure of chromatin occurs by the limitations of its condensation. If the chromosome is in a condensed form, that is if it is a primary nucleosome, the DNA will be less available for transcription.
The loose structure of chromatin makes transcription easier. As a result, the combined effects of ATP dependent nucleosome remodel, post-translational modifications, and histone chaperons alter the structure and dynamics of the nucleosome.
Dynamic changes in chromatin structure can either stimulate or repress gene expression by facilitating interactions among enhancers and the other cis-regulatory components and their target genes.
Functions of Chromatin
The basic function of chromatin is the packaging of DNA. The length of DNA inside the nucleus is very long. The compacting of DNA involves several steps:
- The first step includes the wrapping of DNA around the nucleosome.
- The second step is the winding of beads.
- The third step gives the final package, which is a ratio of about 1000.
Chromatin also functions in the process of transcription. It is important that the nature of chromatin becomes lost to complete the process of transcription because the condensed form of chromatin stops the access to read protein.
Since the DNA is packed in chromatin, all processes of DNA are controlled by chromatin. The chromatin changes its structure because of the dynamic structure of protein and DNA. The relaxed form of chromatin gives the excess for repaired proteins to attach to DNA to repair it.
Conclusion
Chromatin is a complicated structure composed of macromolecules such as protein DNA and RNA. It is present in the nucleus of the eukaryotic cells. Due to the great length of DNA, it cannot be in its natural form inside the nucleus; it has to be compacted. Thus, chromatin is the compacted form of DNA. Chromatin has many functions related to DNA, such as gene regulation and DNA repair. The structure of chromatin is highly complicated.