Mitotic chromosomes attachment

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

Feedback control of anaphase onset, or mitotic checkpoint signaling, is one mechanism that contributes to ensuring accurate chromosome segregation. Delaying anaphase in response to unattached kinetochores, however, is not sufficient. Chromosomes must attach to spindle microtubules in a particular... 

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. 

Destruction of cohesin allows the spindle microtubules to pull the separated chromatids to opposite poles of the cell. Failure of spindle attachment to a single kinetochore activates the SAC (spindle assembly checkpoint), which arrests cells at metaphase until corrections are effected and equal distribution of chromosomes has been ensured. A sensory mechanism initiates the wait anaphase signal from an imattached kinetochore and triggers the accu mulation of the checkpoint components that comprise the Bub (budding uninhibited by benomyl)-Mad (mitotic arrest deficient) families of proteins. 

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. 

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. 

Rg. 17.1 The mitotic-spindle checkpoints ensure that chromosomes are aligried correctly before anaphase. If chromosomes are not attached stably to the microtubules, the checkpoint control prevents entry into anaphase, the irreversible step in cell division. ... 

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. 

Metaphase is the stage of mitosis at which chromosomes are already attached to the mitotic spindle at the equator, but have not segregated and are not yet assembled at the opposite poles of the cell. 

Loss of A PC function leads to uncontrolled activation of the Wnt// -catenin pathway and provides a proliferative advantage to the mutated cell (reviews Moon et al., 2002 Fodde, 2003). In addition, chromosomal instability is observed in cells with decreased APC function. The C-terminus of APC encompasses domains responsible for proper attachment of the mitotic spindle to the kinetochore, and it is assumed that loss of this function elicits chromosomal instability. It is remarkable that two central aspects of tumor formation, namely proliferative advantage and chromosomal instability, are observed upon deactivation of the Wnt// -catenin pathway. Malfunction of other components of the Wnt/ -catenin pathway has also been observed in tumors. The scaffold protein axin has been... 

In each half of the spindle, a single centrosome at the pole organizes three distinct sets of microtubules whose (—) ends all point toward the centrosome (Figure 20-3lb). One set, the astral microtubules, forms the aster they radiate outward from the centrosome toward the cortex of the cell, where they help position the mitotic apparatus and later help to determine the cleavage plane in cytokinesis. The other two sets of microtubules compose the spindle. The kinetochore microtubules attach to chromosomes at specialized attachment sites on the chromosomes called kinetochores. Polar microtubules do not interact with chromosomes but instead overlap with polar microtubules from the opposite pole. Two types of interactions hold the spindle halves together to form... 

The mitotic apparatus of animal cells comprises the astral microtubules forming the asters, the polar and kine-tochore microtubules forming the football-shaped spindle, the spindle poles derived from the duplicated centrosomes, and chromosomes attached to the kinetochore microtubules (see Figure 20-31). 

Cells with abnormal numbers of chromosomes form when certain cell-cycle checkpoints are nonfunctional. As discussed in Chapter 21, the unreplicated-DNA checkpoint normally prevents entry into mitosis unless all chromosomes have completely replicated their DNA the spindle-assembly checkpoint prevents entry into anaphase unless all the replicated chromosomes attach properly to the metaphase mitotic apparatus and the chromosome-segregation checkpoint prevents exit from mitosis and cytokinesis if the chromosomes segregate improperly (see Figure 21-32, steps [H- 3D. As advances are made in Identifying the proteins that detect these abnormalities and mediate cell-cycle arrest, the molecular basis for the functional defects leading to aneuploidy in tumor cells will become clearer. 

During mitosis, mitotic spindles attach to centromeres and provide the critical force needed to separate chromosomes prior to segregation into the two daughter cells. Human centromeres have also been shown to contain long stretches of AT rich sequences, including a highly conserved repetitive sequence (GGAAT)n. 

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