Introduction
Chromatin is DNA that is complexed with proteins, including histones and other proteins known poetically as non-histonechromosomalproteins. When the cells are diving, the chromatin is condensed or coiled into the much larger, highly visible bodies we call chromosomes. when the chromosomes” relax” they form incredibly long threads .in fact ,if a DNA molecule from the largest human chromosome were straightened out, it would be about twelve centimeters long. Diffuse DNA, it turns out, occupies most of the nucleoplasms most of the time. the DNA is bunched up as tiny ,beadlike globules, each consisting of about 200 DNA base pairs wound around clusters consisting of eight molecules of those proteins, called histones .between each of these beadlike globules, the DNA molecule continue as an open string of about fifty nucleotide pairs. Each chromosome consists of one long DNA molecule wrapped around globules of histones, and hundreds of non histone chromosomal proteins. Chromatin occurs in two forms: euchromatin and heterochromatin.
The fluid material of the nucleus is known as nucleoplasm, which is surrounded by the familiar double-layered membrane, often portrayed as continuous with the endoplasmic reticulum. The nuclear membrane is riddled with numerous pores like structures that are particularly evident in freeze-fracture preparations; one of the important features of nucleoplasm is nucleolus. One or more nucleoli are seen in stained sections of most nuclei. The nucleolus has given cytologists fits. One reason for it appears to be both granular and fibrous in electron micrographs.
Euchromatin & Heterochromatin
The euchromatin contains nearly all the functional genes, at least nearly all the genes we have been able to map and identify. Between stages of cell division, much larger, irregular blobs of chromatin can be seen in the nucleus: this is the heterochromatin can be seen in the nucleus; this is the heterochromatin, and it comes in several varieties. Constitutive heterochromatin, for example, largely consists of histones and thousands, or even millions, of tandem repeats of short and probably meaningless sequences of DNA nucleotide of DNA nucleotides. It is sometimes called centric heterochromatin because it is concentrated on either side of the centromere (spindle fiber attachment of each chromosome. we have no idea what constitutive heterochromatin does for a living).
Actually, the difference between euchromatin and heterochromatin is not as clear as we might like it to be .the term heterochromatin applies to any chromatin that is condensed at any chromatin that is condensed at any given time between cell divisions. So the same genetic material that is active euchromatin in some cells may be condensed heterochromatin in other cells .when we identify any chromatin that appears different ways at different times, we call it
Facultative heterochromatin.
Consider two prominent examples of facultative heterochromatin:
1) In female mammals, only one of the two X chromosomes is active in any one cell; the other is inactive, condensed, and therefore heterochromatic
2) In certain cells, such as the white blood cells called polymorph nuclear leukocytes, and the red blood cells of birds, the entire chromosomal component is inactive, condensed heterochromatin. Such heterochromatin appears to be inactive and of no particular use at all .as a matter of fact, you won’t find any chromatin in red blood cells of mammals. These cells have simply discarded their nuclei as they develop .in this way; mammals are one up on birds in that mammalian red blood cells are not burdened by useless, condensed chromatin.
The mass of chromatin in non-diving cells, which resembles nothing so much as a bundle of yarn subjected to the attentions of a demented kitten, is sometimes referred to as the chromatin net because of its netlike appearance under the electron microscope. Parts of the chromatin net are thin and diffuse, while other parts are thicker and more tangled, but both are considered to be composed of euchromatin .it has been shown that only the DNA in the diffuse portions of the net shows functional gene activity i.e, transcribing m-RNA at any one time. The relatively condensed portions are temporarily inactive.
One of central questions in genetics involves the control of the activity of chromosomes. Keep in mind that virtually every cell in the body has an identical set of chromosomes. So if chromosomes determine the behavior of a cell, dictating whether the cell will become muscle, blood, nerve or whatever and if all the cells have the same kinds of chromosomes, How do cells specialize? How do they become different from each other?Obviously, all that DNA in any cell simply defines that cell’s potential .The entire DNA complement can’t be active. Some parts must operate while others are shut down. It has been suggested that cells may regulate some of their activities in part through the use of agents that cause condensation and diffusion of different regions of chromatin. This would explain one way in which genes might act in a selective manner. There are certainly others. As far as regulation itself is concerned, however, it really pushes the question back one step. What regulates condensation and diffusion of chromatin? And the operative parts of the DNA complex can change with time.thus, we don’t continue to grow all ours lives. The parts of the chromosomes that are responsible for growth and development shut down at some point, others come into play and the aging process takes over. We can’t reactivate those parts of the chromosomes. The first person who is able to make old cells young again will be rich and idol of millions
Nuclear RNA
In addition to chromatin, the nucleoplasm contains several kinds of RNA, including some tRNA, early stages of rRNA, and mRNA destined for the cytoplasm. There are also large amounts of long RNA molecules of unknown function, termed heterogeneous nuclear RNA .Heterogeneous nuclear RNA includes the initial transcripts of m RNA, t RNA and r RNA, but may also include RNAs of still unknown function. Most of it never leaves the nucleus and is very rapidly metabolized there.
The nucleoplasm also contains a potpourri of small molecules, including the ones that are the building blocks of DNA and RNA, Various enzymes, a lot of ATP, and a number of inorganic ions.
Chromatids role in cell division
The two chromatids of the chromosomes are not visibly distinguishable in early prophase, and ,in the cells of many tissues, so not become distinct until metaphase or even early anaphase .in some cells, however the two chromatids become quite distinct by late prophase .each is super coiled indivisually,and by metaphase, the two chromatids are held together only by a structure known as the centromere.The centromere ,as seen under the microscope ,is a ring like structure surrounding the thickened chromatids ,the chromatids are some what constricted at the centromere .in stained preparations, the centromere appears under the light microscope as a prominent light-staining body separating the chromosome into arms.
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