Mitosis Centromere

Rattner JB 1991 The structure of the mammalian centromere. Bioessays 13 51-56 Rieder CL, Cole R 1999 Chromatid cohesion during mitosis lessons from meiosis. J Cell Sci 112 2607-2613... 

Centromere protein A (CENP-A), one of several variants of histone H3, is phosphorylated on Ser 7 by Aurora B kinase which is equivalent to Ser 10 of histone H3 (Zeitlin et al, 2001). Recent studies demonstrate that Aurora A kinase also phosphorylates CENP-A (S7) (Kunitoku et al, 2003) (Table 1). The presence of CENP-A in centromeric nucleosomes is required for kinetochore organization and function (Choo 2001). Loss of CENP-A phosphorylation function at Ser 7 caused a mislocalisation of Aurora B, a putative partner phosphatase (PPl-yl) and inner centromere protein (INCENP). H3.3, another variant of histone H3 is phosphorylated on Ser 31 in vivo (Table 1). H3.3 (S31) is a mitosis-specific modification that is present only in late prometaphase and metaphase. Furthermore, H3.3 (S31) is excluded from centromeres. However it is enriched in distinct chromosomal areas immediately adjacent to centromeres (Hake et al, 2005). 

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.

In the fine structure of cells, microtubules make up fibers such as the spindle fibers that attach to centromeres of chromosomes to pull chromatids apart during mitosis and meiosis. Microtubules function in a number of cellular processes, including motility of cells and subcellular components. Microtubules assemble into tubulin, a substance that can change the shape of cells. 

CENTROMERE A specialized part of a chromosome that attaches to a spindle fiber in mitosis or meiosis. 

The centrioles migrate to opposite poles of the cell and the mitotic spindle is formed, apparently joining the cell membrane through the centrioles to the centromere of each chromosome. Spindle fibres consist of one type of protein, tubulin, of molecular weight 60,000. It is the organisation of these molecules to form the mitotic spindle which is blocked by the drugs colchicine, colcemide, nocodazole, vincristine and vinblastine (Fig. 10.3) with the consequence that mitosis is arrested in metaphase. 

A classic example of this strategy is offered by mitosis. In this case it is imperative that microtubules become attached to the centromeres, so that the chromosomes can be transported to opposite ends of the splindle, but centromes are extremely small and their distribution in space is virtually random. Looking for centromeres is literally like looking for a needle in a haystack, and yet the exploratory mechanism of dynamic instability always finds them, and always manages to find... 

Centromere is that site on the chromosome that is attached to the mitotic spindle during divison of the nucleus in mitosis. The centromere is also the point in the condensed chromosome where the two sister chromatids are connected. In the late prophase, the centromere has two kinetochores, one for each sister chromatid. 

Kinetochore— A disk of protein bound to the centromere to which microtubules attach during mitosis, linking each chromatid to the spindle. 

In what stage of mitosis do the centromeres split and the sister chromatids move apart ... 

The spindle fibres are attached to the chromosomes at the centromere and align them on the equatorial plate. The spindle arrangement contains microtubules composed of the protein tubulin. Colchicine is a specific spindle poison, which binds to tubulin, and inhibits its polymerization. Consequently, colchicine blocks mitosis, causing polyploidy, the unequal partition of chromosomes and metaphase arrest. 

Most satellite DNA is composed of repeats of 14-500 base pairs in tandem repeats of 20-100 kb. In situ hybridization studies with metaphase chromosomes have localized these satellite DNAs to specific chromosomal regions. In most mammals, much of this satellite DNA lies near centromeres, the discrete chromosomal regions that attach to spindle microtubules during mitosis and meiosis. Satellite DNA is also located at telomeres, the ends of chromosomes, and at specific locations within chromosome arms in some organisms. These latter sequences can be useful for identifying particular chromosomes by fluorescence in situ hybridization (FISH), as Illustrated in Figure 10-5. 

Simple-sequence DNA located at centromeres may assist in attaching chromosomes to spindle microtubules during mitosis. As yet, however, there is little clear-cut experimental evidence demonstrating any function for most simple-sequence DNA, with the exception of the short repeats at the very ends of chromosomes discussed in a later section. 

A FIGURE 10-27 Microscopic appearance of typical metaphase chromosome. Each chromosome has replicated and comprises two chromatids, each containing one of two identical DNA molecules. The centromere, where the chromatids are attached, is required for their separation late in mitosis. Special telomere sequences at the ends function in preventing chromosome shortening. 

As cells exit from mitosis and the condensed chromosomes uncoil, certain sections of the chromosomes remain dark-stalnlng. The dark-stalnlng areas, termed heterochromatin, are regions of condensed chromatin. The llght-stalning, less condensed portions of chromatin are called euchromatin. Heterochromatin appears most frequently—but not exclusively—at the centromere and telomeres of chromosomes and Is mosdy simple-sequence DNA. 

---