As mentioned above, chromatin is composed of DNA and histones that are packaged into thin, stringy fibers. The chromatin undergoes further condensation to form the chromosome.
Chromosomes are single-stranded groupings of condensed chromatin. During the cell division processes of mitosis and meiosis, chromosomes replicate to ensure that each new daughter cell receives the correct number of chromosomes. A duplicated chromosome is double-stranded and has the familiar X shape. The two strands are identical and connected at a central region called the centromere. A chromatid is either of the two strands of a replicated chromosome.
Chromatids connected by a centromere are called sister chromatids. At the end of cell division, sister chromatids separate and become daughter chromosomes in the newly formed daughter cells. This is the most fundamental function of chromatin: compactification of long DNA strands. The length of DNA in the nucleus is far greater than the size of the compartment in which it is stored.
To fit into this compartment the DNA has to be condensed in some manner. Packing ratio is used to describe the degree to which DNA is condensed. To achieve the overall packing ratio, DNA is not packaged directly into structure of chromatin. Instead, it contains several hierarchies of organization. The first level of packing is achieved by the winding of DNA around the nucleosome, which gives a packing ratio of about 6.
This structure is invariant in both the euchromatin and heterochromatin of all chromosomes. Collecting all this material into a microscopic cell nucleus is an extraordinary feat of packaging. For DNA to function when necessary, it can't be haphazardly crammed into the nucleus or simply wound up like a ball of string. Consequently, during interphase, DNA is combined with proteins and organized into a precise, compact structure, a dense string-like fiber called chromatin, which condenses even further into chromosomes during cell division.
Each DNA strand wraps around groups of small protein molecules called histones , forming a series of bead-like structures, called nucleosomes , connected by the DNA strand as illustrated in Figure 1. Under the microscope, uncondensed chromatin has a "beads on a string" appearance. The string of nucleosomes, already compacted by a factor of six, is then coiled into an even denser structure known as a solenoid that compacts the DNA by a factor of The solenoid structure then coils to form a hollow tube.
This complex compression and structuring of DNA serves several functions. The overall negative charge of the DNA is neutralized by the positive charge of the histone molecules, the DNA takes up much less space, and inactive DNA can be folded into inaccessible locations until it is needed.
There are two basic types of chromatin. Euchromatin is the genetically active type of chromatin involved in transcribing RNA to produce proteins used in cell function and growth. The predominant type of chromatin found in cells during interphase, euchromatin is more diffuse than the other kind of chromatin, which is termed heterochromatin.
The additional compression of heterochromatin is thought to involve various proteins in addition to the histones, and the DNA it contains is thought to be genetically inactive. Heterochromatin tends to be most concentrated along chromosomes at certain regions of the structures, such as the centromeres and telomeres.
Genes typically located in euchromatin can be experimentally silenced not expressed by relocating them to a heterochromatin position. Throughout the life of a cell, chromatin fibers take on different forms inside the nucleus.
Chromatin is a substance within a chromosome consisting of DNA and protein. The DNA carries the cell's genetic instructions. The major proteins in chromatin are histones, which help package the DNA in a compact form that fits in the cell nucleus. Changes in chromatin structure are associated with DNA replication and gene expression. Chromatin is the material that makes up a chromosome that consists of DNA and protein.
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