Chromatids Chromatin

Gertain cells of insects, eg, Chironomus, contain giant chromosomes that have been repficated for ten cycles without separation of daughter chromatids. These copies of DNA fine up side by side in precise register and produce a banded chromosome containing regions of condensed chromatin and fighter bands of... 

Phosphorylation has been thought to be correlated to the mitotic chromatin condensation and the transcriptional regulation in interphase (Nowak and Corces, 2004). The mitotic phosphorylation, which was first identified in 1978 (Gurley et al, 1978), occurs at Ser (Wei et al, 1998), Ser (Goto et al, 1999), and Thr (Preuss et al, 2003) in histone H3. The Ser phosphorylation is catalyzed by the aurora kinase family (de la Barre et al, 2000), and is required for the initiation of chromosome condensation but not for its maintenance (dephosphorylation of mitotic chromosomes does not induce chromosome decondensation) (Van Hooser et fl/.,1998). In meiosis, Ser phosphorylation is also required for the cohesion of sister chromatids rather than the condensation (Kaszas and Cande, 2000). 

Karymov MA, Tomschik M, Leuba SH, Caiafa P, Zlatanova J (2001) DNA methylation-dependent chromatin fiber compaction in vivo and in vitro requirement for hnker histone. Faseb J 15 2631—2641 Kaszas E, Cande WZ (2000) Phosphorylation of histone H3 is correlated with changes in the maintenance of sister chromatid cohesion during meiosis in maize, rather than the condensation of the chromatin. J Cell Sci 113(Pt 18) 3217-3226... 

Figure 20.28 Diagrammatic representation of mitosis in a cell with a single pair of homologous chromosomes. In prophase, the chromatin condenses into chromosomes, each of which consists of a pair of chromatids that have been formed by replication during interphase, and the nuclear envelope disappears. In metaphase, each chromatid attaches to the spindle fibres (microtubules) at a centre point, the centromere. In anaphase, the two chromatids of each chromosome become detached from each other and move to opposite poles of the cell along the microtubules. In telophase, the chromatids have reached the poles. Two nuclear envelopes then form and enclose each new set of chromatids, now once again called chromosomes. The microtubules disappear and the chromosomes uncoil and re-form into the long chromatin threads. Finally the cell membrane is drawn inward by a band of microfilaments to form a complete constriction between the newly formed nuclei, and two new cells are formed. The process is called cytokinesis.

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). 

Figure 27-5 (A, B) Two possible models of the 30-nm chromatin fiber.55 (A) Thoma et al.85 (B) Woodcock et al.6i 87 The fully compacted structure is seen at the top of each figure. The bottom parts of the figures illustrate proposed intermediate steps in the ionic strength-induced compaction. (C) Possible organization of the DNA within a metaphase chromosome. Six nucleosomes form each turn of a solenoid in the 30-nm filament as in (A). The 30-nm filament forms 30 kb-loop domains of DNA and some of these attach at the base to the nuclear matrix that contains topoisomerase II. About ten of the loops form a helical radial array of 250-nm diameter around the core of the chromosome. Further winding of this helix into a tight coil 700 nm in diameter, as at the top in (C), forms a metaphase chromatid. From Manuelidis91.

Baetz KK, et al. The ctfl3-30/CTF13 genomic haploinsuffi-ciency modifier screen identifies the yeast chromatin remodeling complex RSC, which is required for the establishment of sister chromatid cohesion. Mol. Cell. Biol. 2004 24 1232-1244. 

Swiss mouse (liver chromatin) Chromatid binding + Bryan et al. 1974... 

The homologous chromatids (paternal and maternal) are paired. The banding pattern results from reproducible packing of DNA and protein within each amplified site along the chromosome. Dark bands are regions of more highly compacted chromatin. 

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. 

Fig. 46, Folded fiber model of the chromosome (Du Praw, 1965a, 1965b, 1966). a) Each interphase chromatin fiber consists of a DNA molecule held in the form of a secondary helix by means of its protein envelope b) replication of the fiber takes place successively from both ends toward the middle c) after replication the daughter fibers are folded into a condensed, compact chromosome. It is postulated that this folding is brought about by contractile protein molecules in the membrane d) scheme of chromatid stracture.

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