Chromatin is the combination of DNA and proteins that make up the contents of the cell nucleus. The main functions of chromatin are to package DNA into a smaller volume suitable for cells, to strengthen DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene expression and DNA replication.
The main protein components of chromatin are histones, which condense DNA. Chromatin is only found in eukaryotic cells.
Prokaryotic cells have a very different DNA structure known asgene pool(chromosomes without chromatin).
The structure of chromatin depends on several factors.
The overall structure depends on the phase of the cell cycle, and during interphase the chromatin structure loosens, allowing RNA and DNA polymerases to transcribe and transcribe.copy DNA。
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The local structure of interphase chromatin depends on the genes present in the DNA: actively transcribed ("en") DNA-coding genes are, and have been found to be, more loosely packed.RNA polymerase(known aseuchromatin), while DNA encoding inactive (“off”) genes is associated with structural proteins and is more densely packed (heterocromatina）。
Epigenetic chemical modifications of chromatin structural proteins also alter local chromatin structure, particularly histone chemical modifications through methylation and acetylation. As cells prepare to divide, that is, enter mitosis or meiosis, the chromatin packs more densely to facilitate the separation of chromosomes at later stages.
As a result, at this stage of the cell cycle, the individual chromosomes of many cells become visible under the light microscope.
In general, the organization of chromatin is divided into three levels:
DNA encapsulates histones and forms nucleosomes; a "pearls on a string" structure (euchromatin).
Various histones are packed into 30 nm fibers composed of nucleosome assemblies (heterochromatin) in their most compact form.
The 30 nm fibers undergo a greater degree of DNA packaging in metaphase chromosomes (during mitosis and meiosis).
However, there are many cells that do not follow this organization. For example, bird sperm and red blood cells have more compact chromatin than most eukaryotic cells.trypanosomes-protozoaIt does not condense its chromatin into chromosomes visible for mitosis.
The optimized structure of chromatin during interphase allows DNA repair and transcription factors to easily access the DNA while condensing it in the nucleus. The structure varies depending on how the DNA needs to be obtained. Genes that require regular access by RNA polymerase require the more flexible structure offered by euchromatin.
The structure of chromatin undergoes several changes. Histones are the building blocks of chromatin that alter DNA packaging through various post-translational modifications. Acetylation relaxes chromatin, facilitating replication and transcription.
When certain residues become methylated, they hold DNA tightly together and restrict access to various enzymes. A recent study revealed the presence of a bivalent structure in chromatin: methylated lysine residues at positions 4 and 27 of histone 3.
It is believed that this could be relevant to development; Lysine 27 methylation is higher in embryonic cells than in differentiated cells, whereas lysine 4 methylation positively regulates transcription through the recruitment of nucleosome remodeling enzymes and histone acetylases.
Polycomb proteins play a role in gene regulation by modulating chromatin structure.
Most of the DNA in cells is the normal DNA structure. However, in nature DNA can form three structures: DNA-A, DNA-B and DNA-Z. The A and B chromosomes are very similar and form a right helix, while Z DNA is a more unusual left helix with a zigzag phosphate backbone. Due to the nature of the connection between B-DNA and Z-DNA, Z-DNA is believed to play a specific role in chromatin structure and transcription.
At the junction of B-DNA and Z-DNA, a base pair slips out of normal bonding. They serve dual roles as recognition sites for many proteins and as torsion stress sinks via RNA polymerase or nucleosome binding.
Nucleosomes and "Pearls"
The basic repeating elements of chromatin are nucleosomes, linked together by the splicing of DNA segments, a much shorter arrangement than pure DNA in solution. In addition to the core histones, there is the linker histone H1, which contacts the exit/entry of the DNA strands in the nucleosome. Together with histone H1, the nucleosomal core particle is called a chromosome. Nucleosomes have about 20 to 60 base pairs of DNA attached to them and can form "bead-like" fibers about 10 nm in length under non-physiological conditions.
Nucleosomes bind to DNA non-specifically as required by their role in the overall packaging of DNA. However, large DNA sequence preferences exist to control the positioning of nucleosomes. This is mainly due to the different physical properties of different DNA sequences: adenosine and thymine, for example, pack more easily into the internal minor groove. This means that nucleosomes can bind preferentially at one position every 10 base pairs (the helical repeats of DNA), where the DNA rotates to maximize the number of A and T bases in the internal minor groove.
30 nm phases
With the addition of H1, the "bead" structure coils back into a 30 nm diameter helical structure, called a 30 nm fiber or filament. The exact structure of chromatin fibers in cells is unknown and remains controversial.
This level of chromatin structure is believed to be the form of euchromatin that contains actively transcribed genes. EM studies have shown that 30 nm fibers are highly dynamic, unfolding into 10 nm fiber structures (“beads on a string”) when traversed by RNA polymerase involved in transcription.
Existing models generally assume that nucleosomes are perpendicular to the fiber axis and that linker histones line the interior. The 30 nm stable fibers are based on the regular positioning of the nucleosomes with the DNA.
Ligated DNA is relatively resistant to bending and rotation. As a result, the length of the attached DNA is critical to fiber stability and requires that the nucleosomes be separated by a length that allows them to rotate and fold into the desired orientation without undue stress on the DNA.
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From this point of view, different lengths of adapter DNA should result in different folding topologies of chromatin fibers. Recent theoretical work based on electron microscopy imaging of reconstituted fibers supports this idea.
Spatial organization of chromatin in the cell nucleus.
The arrangement of the genome in the nucleus is not random: certain regions of the genome tend to be in certain spaces. Certain regions of chromatin are enriched in the nuclear envelope, while other regions are held together by protein complexes.
However, this arrangement has only been well characterized in female mammals, where one of the two X chromosomes is condensed within Barr.
As a result, these genes are permanently turned off and women do not receive a "double dose" compared to men. There is some debate as to how much idle X is actually compressed.
Fluctuations between open and closed chromatin can lead to interruptions or bursts of transcription. Other factors may also play a role, such as the association and dissociation of transcription factor complexes with chromatin.
Unlike simple probabilistic transcription models, this phenomenon could explain the high variability in gene expression between cells in isogenic populations.
The metaphase structure of chromatin differs significantly from that of interphase. Optimized for physical strength and driveability, it forms the classic chromosome structure seen on karyotypes.
The structure of the condensed chromosome is believed to be a 30 nm fibrous ring attached to a core protein structure. However, its properties are not well defined.
The physical strength of the chromatin is critical at this stage of division to prevent DNA damage when the daughter chromosomes separate. To maximize potency, chromatin composition changes as it approaches the centromere, primarily through substitution of histone H1 analogues.
It should also be noted that while most of the chromatin is highly condensed during mitosis, there are small areas that are not as densely condensed. These regions normally correspond to the promoter regions of genes that are active in that cell type prior to entry into mitosis.
Lack of compression in these regions is called marking, an epigenetic mechanism thought to be important for passing on the "memory" of which genes were active before entering mitosis to daughter cells. Since transcription stops during mitosis, this marking mechanism is necessary to support the transmission of this memory.
Chromatin is a genetic material or a macromolecule comprising DNA, RNA, and associated proteins, which constitute chromosomes in the nucleus of a eukaryotic cell. This chromatin is located within the cell nucleus. The main functions of this genetic material include: Preventing DNA damage.What is chromatin structure and function and types? ›
There are two types of chromatin– euchromatin and heterochromatin. Heterochromatin is an area of highly coiled DNA and stains darkly whereas euchromatin is an area of less coiling and stains lightly. As a result of this chromosomes show transverse bands of lightly and darkly stained regions.What is chromatin and its function? ›
Definition. 00:00. Chromatin refers to a mixture of DNA and proteins that form the chromosomes found in the cells of humans and other higher organisms. Many of the proteins — namely, histones — package the massive amount of DNA in a genome into a highly compact form that can fit in the cell nucleus.What is the structure of the chromatin? ›
The basic unit of the chromatin is the nucleosome, which comprises 147 base pairs of DNA wrapped around an octamer of core histones (made of two molecules of each H2A, H2B, H3, and H4 histones). Each nucleosome is linked to the next by small segments of linker DNA.What is type of chromatin? ›
Chromatin exists in two forms. One form, called euchromatin, is less condensed and can be transcribed. The second form, called heterochromatin, is highly condensed and is typically not transcribed. Under the microscope in its extended form, chromatin looks like beads on a string.What is the structure of chromatin quizlet? ›
Describe the structure of chromatin. Negatively charged DNA loops twice around positively charged Histone octamer to form nucleosome "bead."What is the structure and function of chromatin in the nucleus? ›
Chromatin is located in the nucleus of our cells. The primary function of chromatin is to compress the DNA into a compact unit that will be less voluminous and can fit within the nucleus. Chromatin consists of complexes of small proteins known as histones and DNA.Where is chromatin structure function? ›
Chromatin is a genetic material or a macromolecule comprising DNA, RNA, and associated proteins, which constitute chromosomes in the nucleus of a eukaryotic cell. This chromatin is located within the cell nucleus. The main functions of this genetic material include: Preventing DNA damage.What is the structure and function of the chromosomes? ›
Chromosomes are composed of DNA and proteins packed tightly to form long chromatin fibers. Chromosomes house genes responsible for the inheritance of traits and guidance of life processes. Chromosome structure consists of a long arm region and a short arm region connected at a central region known as a centromere.What is the function of the chromatin quizlet? ›
The primary functions of chromatin are 1) to package DNA into a smaller volume to fit in the cell, 2) to strengthen the DNA to allow mitosis, 3) to prevent DNA damage, and 4) to control gene expression and DNA replication.
The basic unit of chromatin organization is the nucleosome, which comprises 147 bp of DNA wrapped around a core of histone proteins. Nucleosomes can be organized into higher order structures and the level of packaging can have profound consequences on all DNA-mediated processes including gene regulation.What is chromatin quizlet? ›
Chromatin. -Genetic material in a non-dividing cell. -the material of which the chromosomes of organisms are composed. It consists of protein, RNA, and DNA.What are the two major types of chromatin? ›
The two types of chromatin are heterochromatin and euchromatin. Chromatin is a complex form of proteins and DNA that forms the chromosomes in the nucleus. The heterochromatin form is highly dense in nature, while the euchromatin type is less dense in nature.What is the structure of chromatin in genetics? ›
INTRODUCTION: CHROMATIN STRUCTURE
The nucleosome consists of 147 base pairs of DNA wrapped 1.7 times around an octamer of histone proteins (two each of histones H2A, H2B, H3, and H4).
The two classes of chromatin- regulating proteins are 1) enzymes that modify histones through methylation, acetylation, phosphorylation, adenosine diphosphate–ribosylation, glycosylation, sumoylation, or ubiquitylation and 2) enzymes that remodel DNA-histone structure with energy from ATP hydrolysis.