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Here are your notes on Chromatin!
DNA is the main genetic constituent of cells, carrying information in a coded form from cell to cell and from organism to organism.
Within cell nucleus, DNA is not free but is complexed with proteins in a structure called chromatin.
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Chromatin consists of double-stranded DNA (two hydrogen-bonded polynucleotide strands) to which large amounts of protein and a small amount of RNA are bound.
Image Courtesy : www.personal.psu.edu/chromatinFiber.jpg
The nuclei contain a constant amount of DNA, and are practically constant in different tissues. It confirmed that DNA was the genetic material and that genetic information is not lost during differentiations of various somatic tissues.
Each species has a characteristic amount of DNA. Sometimes called the С value. All the diploid cells (somatic cells) in an organism have the same DNA content, designated 2C. Gametes are haploid and therefore have the half DNA content or 1C. Some tissues, such as liver, contain occasional polyploid cells whose nuclei have a higher DNA content (e.g., 4C or 8C).
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Eukaryotes vary greatly in DNA content but always contain much more DNA than prokaryotes. Lower eukaryotes in general have less DNA, such as nematodes have 20 times more DNA than Escherichia coli, while Drosophila has 40 times more.
Vertebrates have greater DNA content, about 700 times more than E. coli. The highest DNA content is that of the salamander Amphiuma (168 picograms DNA in a single nucleus). One picogram of DNA is equivalent to 31 cm of DNA.
DNA in metaphase chromosomes is usually compacted between 5000- and 10,000- fold, and DNA content tends to be proportional to the size of the chromosome. The largest human chromosome (1), which is 10 µm long, has been calculated to accommodate about 7.2 cm of DNA in a tightly packed form (about 7000-fold).
Chromatin is complex of DNA and histones:
In eukaryotic cells, chromatin contains DNA, RNA, basic proteins called histones, and nonhistone (more acidic) proteins. The content of RNA and nonhistone protein is variable between different chromatin preparations, but histone and DNA are always present in a fixed ratio of about one to one by weight.
Histones are only of five types, each one present in large amounts. Histones are small proteins which are basic because they have a high content (10 to 20 per cent) of the basic amino acids arginine and lysine. Being basic, histones bind tightly to DNA, which is an acid. The four main histones, H2A, H2B, H3 and H4 are very similar in different species.
These four histones are present in equimolar amounts. Histone H1 is not conserved between species and has tissue specific forms. It is present only once per 200 base pairs of DNA. It is loosely associated with chromatin and is not a component of the DNA-histone structural unit called the nucleosome core, but instead it is bound to the linker segments of DNA that join neighbouring nucleosomes.
When eukaryotic nuclei were spread on electron microscopic specimen holders, it was found that chromatin has a repeating, structure of ‘beads’ about 10 nm in diameter connected by a ‘string’ of DNA. Most if not all of the DNA is present in this form. The beads-on-a-string appearance is not the true structure of chromatin, but it is an artifact and it results in a loss of histone H1.
If H1 is removed due to milder treatments, beads are not stretched but can be observed as a 10 nm fibre, with the beads touching each other. The 10 nm fibre therefore represents the first level of organization of chromatin within cells.
The existence of a repeating unit of chromatin, called the nucleosome, was also predicted from biochemical studies. Studies of chromatin digestion by nucleases showed that DNA was cut into multiples of a unit size, which was later found to be about 200 base pairs of DNA in length, i.e., DNA was cut at intervals of 200, 400, 600, 800 (and so on) basepairs.
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Other studies showed that histones H3 and H4 tend to associate in solution, forming tetramers consisting of two each. Since the four histones are present in equimolar amounts (two of each per 200 base pairs of DNA), in 1974 R. Kornberg proposed a model for the nucleosome in which the four histones Н2А, H2B, H3 and H4 are arranged in octamers containing two of each protein, located every 200 base pairs of DNA.
The histone octamers are in close contact (as in the 10 nm fibre), and the DNA is coiled on the outside of the nucleosome (Fig. 6-8).
When nucleosomes are in close apposition in 10 nm filaments, the packing of DNA is about five-to seven-fold, i.e., five to seven times more compact than free DNA. In metaphase chromosomes further more folding of 10 nm fibre occurs.
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Chromatin fibres in interphase nuclei and mitotic chromosomes revealed under electron microscope as a thick fibre having a diameter varying from 20 to 30 nm. This fibre probably represents the structure of inactive chromatin. The DNA of a 20 to 30 nm showed about 48 fold.
Nonhistone proteins are very heterogeneous. They vary in different tissues and include RNA and DNA polymerases, among other enzymes.
Prokaryotes do not have histones, but a basic protein that might serve the same function.
