Chromosome in mitosis

Tubulins arose very early during the course of evolution of unicellular eukaryotes and provide the machinery for the equipartitioning of chromosomes in mitosis, cell locomotion, and the maintenance of cell shape. The primordial genes that coded for tubulins likely were few in number. As metazoan evolution progressed, natural selection processes conserved multiple and mutant tubulin genes in response to the requirements for differentiated cell types (Sullivan, 1988). 

In this chapter, we build on the general principles learned in Chapter 19 about the structure and function of the ml-crofilament cytoskeleton and show how many of the same concepts also apply to microtubules. We begin the chapter by examining the structure and assembly of microtubules and then consider how microtubule assembly and microtubule motor proteins can power cell movements. The discussion of microtubules concludes with a detailed examination of the translocation of chromosomes in mitosis. Although we consider microtubules, microfilaments, and intermediate filaments Individually, the three cytoskeletal systems do not act completely Independently of one another. An important example of their Interdependence can be found in cell division when Interaction between actin mlcrofllaments and microtubules determines the plane of cleavage. 

A FIGURE 20-1 Microtubules (blue) organized around the MTOC and spindle poles ( ) establish an internal polarity to movements and structures in the interphase cell (/eft) and the mitotic cell right). Assembly and disassembly (H) cause microtubules to probe the cell cytoplasm and are harnessed at mitosis to move chromosomes. Long-distance movement of vesicles (B and El) are powered by kinesin and dynein motors. Both motors are critical in the assembly of the spindle and the separation of chromosomes in mitosis. 

Microtubules provide routes for active transport of molecules and vesicles through the cell. The transport is carried out by microtubule-dependent motor proteins, the cytoplasmid dyneins and kinesins. Cytoplasmic dyneins are closely related to the ciliary dyneins found in cilia and flagella. Different kinesins and to, some extent, dyneins are involved in different cellular processes, such as the separation of chromosomes in mitosis, as well as transport through the cytoplasm. During cytoplasmic transport, one part of the motor protein is attached to the microtubule... 

Nucleus The nucleus is separated from the cytosol by a double membrane, the nuclear envelope. The DNA is complexed with basic proteins (histones) to form chromatin fibers, the material from which chromosomes are made. A distinct RNA-rich region, the nucleolus, is the site of ribosome assembly. The nucleus is the repository of genetic information encoded in DNA and organized into chromosomes. During mitosis, the chromosomes are replicated and transmitted to the daughter cells. The genetic information of DNA is transcribed into RNA in the nucleus and passes into the cytosol where it is translated into protein by ribosomes. 

Murray AW, Szostak TW 1985 Chromosome segregation in mitosis and meiosis. Annu Rev Cell Biol 1 289-315... 

Saka Y, Sutani T, Yamashita Y et al 1994 Fission yeast cut3 and cutl4, members of a ubiquitous protein family, are required for chromosome condensation and segregation in mitosis. EMBO J 13 4938-4952... 

Stratmann R, Lehner CF 1996 Separation of sister chromatids in mitosis requires the Drosophila pimples product, a protein degraded after the metaphase/anaphase transition. Cell 84 25-35 Sumner AT 1991 Scanning electron microscopy of mammalian chromosomes from prophase to telophase. Chromosoma 100 410-418... 

The cytoskeleton is the collective name for all structural filaments in the cell. The cytoskeletal filaments are involved in establishing cell shape, and providing mechanical strength, locomotion, intracellular transport of organelles and chromosome separation in mitosis and meiosis. The cytoskeleton is made up of three kinds of protein filaments actin filaments (also called microfilaments), intermediate filaments and microtubules. 

Figure 1. Hierarchical model of chromosome structure, (a) In interphase cells, DNA is packed in a nucleus as forming nucleosome and chromatin, (b) DNA forms nucleosome structure together with core histone octamer, which is then folded up into 30nm fiber with a help of linker histone HI. This 30 nm fiber is further folded into 80 nm fiber and 300 nm loop structures in a nucleus. In mitosis, chromosome is highly condensed. Proteins which are involved in each folding step are indicated above and non-protein factors are indicated below, (c) The amino acid sequences of histone tails (H2A, H2B, H3 and H4) are shown to indicate acetylation, methylation and phosphorylation sites. (See Colour Plate 1.)...

During subsequent interphase, the DNA replicates to create identical chromatids tor each chromosome in preparation fora subsequent mitosis in the cell cycle... 

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.

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