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Eukaryotic cell cycle: cell growth, mitosis, and interphase G1: Cell prepares for chromosome replication. S: DNA replicates and new chromosomes (sister chromatids) are formed. G2: Cell prepares for mitosis and cell division. M: Mitosis http://www.cellsalive.com/ Mitosis: 1. 2. Occurs in haploid (1N) and diploid (2N) somatic cells. Continuous process with 4 cytologically distinct stages. Prophase Chromosomes shorten, thicken, and become visible by light microscopy. Centrioles move apart and mitotic spindle begins to form. Centrioles migrate to opposite sides of nucleus and nuclear envelope begins to disappear. Metaphase Nuclear envelope disappears completely. Replicated chromosomes held together at the centromere are aligned on equator of the spindle (metaphase plate). Anaphase Centromeres split and daughter chromosomes migrate to opposite poles. Cell division (cytokinesis) begins. Telophase Nuclear envelopes reform, chromosomes become extended and less visible, and cell division continues. INTERPHASE PROPHASE Centrosomes (with centriole pairs) Early mitotic spindle Centrosome Chromatin Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Fragments of nuclear envelope Centrosome Kinetochore Spindle microtubules METAPHASE ANAPHASE Cleavage furrow Metaphase plate Spindle Figure 8.6 (continued) TELOPHASE AND CYTOKINESIS Daughter chromosomes Nuclear envelope forming Nucleolus forming Fig. 1.15 Interphase and mitosis in an animal cell Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. Nuclear Envelope Breakdown (NEBD) • Nuclear envelope = a pair of membrane bilayers, which are joined at the pore complexes, an additional layer called nuclear lamina (a mesh of filament proteins) underlie the inner membrane. • Lamina may organize chromatin into functional domains, provide structure to nucleus; • breaks down in mitosis so that mitotic spindle microtubules can associate with kinetochores. • NEBD is initiated with one to three holes which expand over the entire envelope; • holes occur opposite of invaginations on the envelope. Centrosome • Centrosome contains a centriole pair linked by a matrix; • nine proximal triplets are found at the center of centrosome • Centrioles are also needed to make cilia and flagella. centriole cilia Centrioles • they may recruit microtubule nucleating material (pericentriolar material) • yeast don’t have centrioles but have spindle pole bodies; acentriolar mechanisms for spindle formations using motor proteins • centrioles duplicate from a template. Each centriole makes a copy; when cells enter S phase, centriole duplication takes place. Centrioles • Sperm cells contain a pair of centrioles; eggs have none. The sperm's centrioles are absolutely essential for forming a centrosome which will form a spindle enabling the first division of the zygote to take place. Chromosome alignment • microtubule and chromosome interactions occur at kinetochores- specialized disk shaped structures located on each side of the centromere, kinetochore captures multiple microtubules to build a kinetochore fiber. • If a chromosome binds to one kinetochore initially, it will be attracted to one pole. • Two opposing forces align the chromosomes along the metaphase plane. Microtubules attached to opposite sides of the dyad shrink or grow until they are of equal length. • Microtubules motors attached to the kinetochores move them toward the minus end of shrinking microtubules (a dynein); • toward the plus end of lengthening microtubules (a kinesin). • The chromosome arms use a different kinesin to move to the metaphase plate. Taxol • Chromosome movement in mitosis also involves polymerization and depolymerization of the microtubules. Taxol, a drug found in the bark of the Pacific yew, prevents depolymerization of the microtubules of the spindle fiber. This, in turn, stops chromosome movement, and thus prevents the completion of mitosis. Taxol is being used with some success as an anticancer drug. Centromeres • Centromeres lack common signatures among chromosomes. • Centromeric repeat lengths approximate nucleosome lenghts. • Centromeres have their own histone which replace H3. • Centromeric histone maintains centromeric structure.