Spindle fiber chromosomal

The elastic stress may be external or internal. External stresses are exerted on the chromatin during the cell cycle when the mitotic spindle separates chromosome pairs. The 30-nm fiber should be both highly flexible and extensible to survive these stresses. The in vitro experiments by Cui and Bustamante demonstrated that the 30-nm fiber is indeed very soft [66]. The 30-nm fiber is also exposed to internal stresses. Attractive or repulsive forces between the nucleosomes will deform the linkers connecting the nucleosomes. For instance, electrostatic interactions, either repulsive (due to the net charge of the nucleosome core particles) or attractive (bridging via the lysine-rich core histone tails [49]) could lead to considerable structural rearrangements. 

When microtubules were visualized by electron microscopy (EM), after the improvement of methods of fixation, it was realized that they formed the structural basis of flagellar axonemes and of so-called spindle fibers, as well as occurring as individual filaments in the cytoplasm. Their designation as part of the cytoskeleton suggested that they acted mainly as fixed structural supports. Subsequent research has focused more and more on their dynamic behavior and on their role as tracks for motor proteins, which may, for example, transport chromosomes during cell division. Microtubules are found in all eukaryotic cells and are essential for many cellular functions, such as motility, morphogenesis, intracellular transport, and cell division. It is that dynamic behavior that allows microtubules to fulfill all of these functions in specific places and at appropriate times in the cell cycle. 

Micronucleus Test The micronucleus test is an in vivo test usually carried out in mice. The animals are treated in vivo, and the erythrocyte stem cells from the bone marrow are stained and examined for micronuclei. Micronuclei represent chromosome fragments or chromosomes left behind at anaphase. It is basically a test for compounds that cause chromosome breaks (clastogenic agents) and compounds that interfere with normal mitotic cell division, including compounds that affect spindle fiber function. 

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. 

G2 ends as the first of the five stages of cell division, prophase, begins. Prophase is defined as the first appearance of condensed chromosomes. This is followed by metaphase, in which spindle fibers appear leading from centromeres to opposite sides, or poles, of the cells. The sister chromosomes (i.e., the pairs formed by replication of single chromosomes in S) become aligned in one plane... 

Figure 1 Overview of mitosis, (a) (i) Chromosomes are replicated before mitosis, and sister chromosomes are held together, (ii) The spindle forms and chromosomes attach to spindle fibers, (iii) Chromosomes move to the center of the spindle at metaphase, (iv) Sister chromosomes separate at anaphase and move in opposite directions, (v) The cell divides as the cleavage furrow forms between the separated chromosomes, (vi) Two daughter cells form, each with exactly one copy of each chromosome, (b) Stmcture of colchicine, a small molecule that targets microtubules.

Inone S, Sato H. CeU motility by labile association of molecnles. The natnre of mitotic spindle fibers and their role in chromosome movement. J. Gen. Physiol. 1967 50 259-292. 

Satellite DNA is thought to play a structural role in chromosomes. Certain satellite DNA sequences are concentrated near the centromeres of chromosomes, the site where spindle fibers attach when sister chromatids are separated. Other satellite DNA sequences are located in... 

Examination of fixed samples revealed the existence of a fibrous structure, known as the mitotic spindle, which appears at each mitosis and disappears after the chromosomes have separated. One of the great challenges in the study of cell division has been to understand the organization and function of the mitotic spindle. Use of the small molecule colchicine (Fig. 2.14(b)) has contributed to our understanding of the physical properties of the spindle fibers and how they might drive chromosome movements, as well as their molecular components. 

Centromere This is the region of the chromosome where the mitotic-spindle fibers attach. Attachment occurs via the kinetochore that consists of repeated segments of DNA located on either side of the centromere. The relative position of the centromere is constant, so the relative lengths of the two arms are constant for each chromosome. This is an important feature for chromosome identification. 

Fig. 1. A living primary spermatocyte of Pardalophora apiculata (a grasshopper) as viewed in Inoue s rectified polarizing microscope. A., Metaphase kinetochores of one bivalent are indicated by arrows ("k ) and polar regions by p". Birefringent chromosomal spindle fibers run from each kinetochore toward a pole the diffuse background birefringence of interpolar fibers is identified only with difficulty In the prints but Is readily measured. B., Metaphase the opposite compensator setting. C, D. Anaphase 22 and 36 minutes, respectively, after A. XI,500.

The general dynamics of spindle formation are considered here the details of events at individual chromosomes are discussed in section beginning on page 273. Spindle fibers are rapidly formed, are readily destroyed by a variety of physical and chemical... 

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