Centromeric attachments

Evidence indicating that, in interphase nuclei, the chromosomes are attached to the nuclear membrane comes from ultrastructural studies and from analysis of the segregation of newly synthesized DNA. Woollam et al. (1967) describe attachment of both distal and centro-meric ends of pachytene chromosomes to the nuclear membrane of mouse spermatocytes moreover, these authors suggest, on the basis of the nearness of centromeric attachments to the sex vesicle in these cells, that centromeric and distal attachment points are at opposite poles of the nucleus (cf. also, Sved, 1966). Davies and Tooze (1966) have examined mitotic chromosomes of newt erythroblasts, a cell type characterized by scarcity of endoplasmic reticulum. In interphase erythroblasts, numerous areas are found where chromatin appears to be closely associated with the nuclear membrane. At mitosis the chromosomes are observed to carry fragments of nuclear membrane, sometimes appearing as membrane-limited sheets of chromatin, continuous with the chromosomes. 

Figure 36-5. The two sister chromatids of human chromosome 12 (x 27,850). The location of the A+T-rich centromeric region connecting sister chromatids is indicated, as are two of the four telomeres residing at the very ends of the chromatids that are attached one to the other at the centromere. (Modified and reproduced, with permission, from DuPraw EJ DNA and Chromosomes. Holt, Rinehart, and Winston, 1970.)...

Centromere The clear constricted portion of the chromosome at which the chromatids are joined and by which the chromosome is attached to the spindle during cell division. [NIH]... 

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 centromere (Fig. 24-9) is a sequence of DNA that functions during cell division as an attachment point for proteins that link the chromosome to the mitotic spindle. This attachment is essential for the equal and orderly distribution of chromosome sets to daughter cells. The centromeres of Saccharomyces cere-visiae have been isolated and studied. The sequences essential to centromere function are about 130 bp long and are very rich in A=T pairs. The centromeric sequences of higher eukaryotes are much longer and, unlike those of yeast, generally contain simple-sequence DNA, which consists of thousands of tandem copies of one or a few short sequences of 5 to 10 bp, in the same orientation. The precise role of simple-sequence DNA in centromere function is not yet understood. 

Eukaryotic chromosomes have two important special-function repetitive DNA sequences centromeres, which are attachment points for the mitotic spindle, and telomeres, located at the ends of chromosomes. 

Kinetochore. A structure that attaches laterally to the centromere of a chromosome it is the site of chromosome tubule attachment. 

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. 

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... 

Three types of microtubule can readily be defined in the mitotic spindle. Polar microtubules overlap (and probably interact) between the poles and are involved in pushing the poles apart in anaphase. Astral microtubules radiate in all directions and also help separate the poles. Kinetochore microtubules attach themselves to specialized protein structures (kinetochores) located on each side of the centromere of each chromosome. These microtubules are involved in moving the chromosomes to the metaphase plate and in separating sister chromatids at anaphase. The microtubules in the spindle are very dynamic and have a half-life of only a few seconds. This appears to be especially important in the capture of chromosomes by the kinetochore microtubules. Microtubules that miss the target kinetochores are quickly lost because their dynamic instability soon leads to depolymerization. The new microtubules that form may hit the target and be partially stabilized through plus-end capping. 

Figure 19.1 During the metaphase of cell division, a chromosome becomes two sister chromatids attached at the centromere. Chromosome banding exemplified using human chromosome 17.

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. 

Chromatid is one copy of a newly replicated chromosome, still attached at the centromere to the other sister cojjy. 

Whereas the benomyl screens established the existence of the spindle checkpoint and identified some of the key components in checkpoint signaling, a fundamental question that remained unanswered was what exactly is monitored by the checkpoint. Two general models have been proposed. One is that the checkpoint monitors the attachment of spindle microtubules at the kinetochore, a stmcture that forms on each chromosome to mediate microtubule binding. Unattached kinetochores keep the checkpoint active and delay anaphase (27). A second model is that the checkpoint monitors force across the centromere, the region of the chromosome where kinetochores assemble... 

When both sister kinetochores are attached correctly, they are pulled in opposite directions by the microtubule fibers and the centromere is under tension (Fig. 3a). In this model, the absence of centromere tension would keep the checkpoint active. Small molecules that target tubulin have provided a way to test these models experimentally. Nocodazole depolymerizes microtubules, which creates unattached kinetochores (Fig. 3c), whereas taxol stabilizes microtubules but inhibits their dynamics, which decreases the centromere tension (Fig. 3b) (29). 

This finding suggests that checkpoint signaling, as determined by Mad2 localization, is sensitive to attachment but does not respond directly to centromere tension. The interpretation of these experiments is complicated, however, because tension is required for kinetochores to bind the full complement of microtubules loss of tension may activate the checkpoint indirectly (32). 

Figure 3 Manipulation of chromosome-microtubule attachments with small molecules, (a) In the absence of microtubule poisons, the attachment of both kinetochores to spindle microtubules creates tension across the centromere, (b) Taxol reduces tension across the centromere by inhibiting microtubule dynamics, (c) Nocodazole creates unattached kinetochores by depolymerizing microtubules.

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

Following replication of its DNA, each chromosome contains two, which are attached to each other by a centromere. 

---