Chromosomes, eukaryotic interphase

The DNA in a eukaryotic cell nucleus during the interphase between cell divisions exists as a nucleoprotein complex called chromatin. The proteins of chromatin fall into two classes histones and nonhistone chromosomal proteins. 

In the nuclei of eukaryotes (see p. 196), DNA is closely associated with proteins and RNA. These nucleoprotein complexes, with a DNA proportion of approximately one-third, are known as chromatin. It is only during cell division (see p. 394) that chromatin condenses into chromosomes that are visible under light microscopy. During interphase, most of the chromatin is loose, and in these conditions a morphological distinction can be made between tightly packed heterochromatin and the less dense euchromatin. Euchro-matin is the site of active transcription. 

The eukaryotic cell cycle (see Fig. 12-41) produces remarkable changes in the structure of chromosomes (Fig. 24-25). In nondividing eukaryotic cells (in GO) and those in interphase (Gl, S, and G2), the chromosomal material, chromatin, is amorphous and appears to be randomly dispersed in certain parts of the nucleus. In the S phase of interphase the DNA in this amorphous state replicates, each chromosome producing two sister chromosomes (called sister chromatids) that remain associated with each other after replication is complete. The chromosomes become much more condensed during prophase of mitosis, taking the form of a species-specific number of well-defined pairs of sister chromatids (Fig. 24-5). 

The effects of chromosome structure on gene regulation in eukaryotes have no clear parallel in prokaryotes. In the eukaryotic cell cycle, interphase chromosomes appear, at first viewing, to be dispersed and amorphous (see Figs 12-41, 24-25). Nevertheless, several forms of chromatin can be found along these chromosomes. About 10% of the chromatin in a typical eukaryotic cell is in a more condensed form than the rest of the chromatin. This form, heterochromatin, is transcriptionally inactive. Heterochromatin is generally associated... 

Heterochromatin is that part of the chromatin in the eukaryotic cell where the chromosomes remain condensed and transcriptionally inactive in the interphase, even when... 

The chromosomes are located in the nucleus, except during mitosis or meiosis when the nuclear membrane dissociates and the chromosomes condense into "metaphase chromosomes" and separate into the two sets destined for the "daughter" cells. Eukaryotic chromosomes are visible as well-defined structures in the light microscope when cells are in mitosis or meiosis. In interphase, these compact structures disperse throughout the nucleus and can no longer be discerned clearly by light microscopy. 

Chromatin compaction - Wrapping DNA about histone cores to form nucleosomes (see here) accomplishes part of the compaction necessary to fit the long eukaryotic DNA into the nucleus. However, much of the chromatin in the nucleus is even more highly compacted. The next stage in compaction involves folding the beaded fiber into a thicker fiber like that shown in Figure 28.12. These fibers may be further folded on themselves to make the thicker chromatin fibers visible in both metaphase chromosomes and the nuclei of nondividing (interphase) cells. 

DNA and RNA serve as the genetic material for prokaryotic and eukaryotic cells, for viruses, and for plasmids, each of which stores it in a different arrangement or location. In prokaryotes, DNA is not separated from the rest of the cellular contents. In eukaryotes, however, DNA is located in the nucleus, where it is separated from the rest of the cell by the nuclear envelope (see Fig. 10.20). Eukaryotic DNA is bound to proteins, forming a complex called chromatin. During interphase (when cells are not dividing), some of the chromatin is diffuse (euchromatin) and some is dense (heterochromatin), but no distinct structures can be observed. However, before mitosis (when cells divide), the DNA is replicated, resulting in two identical chromosomes called sister chromatids. During metaphase (a period in mitosis), these condense into discrete, visible chromosomes. 

These data indicate that chromosomes are attached to the nuclear membrane in eukaryotes. Do interphase chromosomes possess specific regions which bind with certain areas on the inner nuclear membrane Comings (1968) has suggested that there exists an orderly arrangement of chromatin in the interphase nucleus and that this order is maintained by the nonrandom attachment of chromosomes to the inner nuclear membrane. Evidence for such order includes the apparent nomandom arrangement of chromosomes in metaphase and irradiation studies which suggest that chromosomes in somatic cells occur in polarized and fixed positions (Comings, 1968). 

Chromatin The filamentous material of eukaryotic chromosomes, consisting of DNA with associated histones and other proteins. During interphase it is dispersed... 

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