12 mitosis - Objectives 1. To know how a single eukaryotic...

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Unformatted text preview: Objectives 1. To know how a single eukaryotic cell divides into two daughter cells 2. To recognize some of the remarkable features of different phases during a nuclear division 3. To understand the mechanism of chromosome translocation inside a dividing cell 1 Nuclear and Cell Division • M phase: A. Nuclear division (mitosis) B. Cytoplasmic division (cytokinesis) 2 Mitosis • • Changing appearance and behavior of chromosomes To ensure that each of the 2 daughter nuclei receives 1 copy of each duplicated chromosomes Divided into different phases: 3 • 4 A. Prophase Nucleus: 1. 2. Nucleoli disperse Chromosomes start to condense as visible discrete objects (to prevent tangling during distribution of chromosomal DNA) Each chromosome consists of 2 sister chromatids attached to each other at centromere 3. 5 Cytoplasm: 1. A small zone of granular material located adjacent to nucleus (centrosome organizing center for mitotic spindle assembly) 2. The pair of centrosomes (duplicated (duplicated during S phase) separate from each other and move toward opposite poles 3. A structure formed by microtubules stretching between the opposite poles (mitotic spindle), guiding chromosome movement 6 Centrosome Procentriole(s) The Centrosome Cycle 7 B. Prometaphase 1. Breakdown of nuclear envelope 2. Spindle microtubules enter nuclear area and make contact with chromosomes • A special type of DNA sequence at centromere of chromosome (CEN sequences) - A plate-like, 3-layered protein structure attached to CEN sequences (kinetochore) 8 (a Kinesin) 9 • 3 types of spindle microtubules: a. Attached to kinetochores (kinetochore microtubules) b. Interact with microtubules from opposite poles (polar microtubules) c. Shorter ones; form asters at each pole; interact with proteins lining plasma membrane (astral microtubules) 10 (astral microtubules) (kinetochore microtubules) (polar microtubules) 11 12 C. Metaphase • Maximally condensed chromosomes aligned at plane equidistant between the poles (metaphase plate) Individual chromosomes can be identified and classified, depending on the differences in size and shape (karyotype) • 13 D. Anaphase • The 2 sister chromatids separate and begin moving toward opposite poles: Kinetochore microtubules get shorter; chromosomes are pulled toward the poles (anaphase A) Polar microtubules lengthen; the poles move away from each other (anaphase B) 14 - 15 16 E. Telophase 1. 2. 3. 4. 5. Daughter chromosomes have arrived at the poles and revert to extended fibers Nucleoli develop Spindle disassembles Nuclear envelopes form Undergoes cytokinesis to divide into 2 daughter cells 17 Cytokinesis • Starts during late anaphase or early telophase • Larger organelles (e.g., ER and Golgi (e.g., complex) → fragmented into small vesicles early in mitosis → reassemble in daughter cells • Mechanism is different in animals and plants: 18 A. Animal cells • A belt-like bundle of actin microfilaments forms beneath plasma membrane during early anaphase (contractile ring) Interaction of actin filaments with myosin molecules The ring tightens around cytoplasm - 19 20 - The cell is divided along a plane that passes through the central region of spindle (spindle equator) • A slight indentation → deepens into a cleavage furrow → encircles the cell → opposite surfaces make contact → the cell is split into two (cleavage) 21 B. Plant cells • Divide by assembling a plasma membrane and a cell wall between 2 daughter nuclei An array of microtubules derived from polar microtubules (phragmoplast), which oriented perpendicular to the new cell wall: 1. Guide small vesicles (containing non(containing cellulose materials) from Golgi complex to and align across equatorial plate 22 2. Fusion of vesicles forming a large flattened sac (cell plate) 3. Expanding cell plate makes contact with original cell wall and thus separates the 2 daughter cells 4. Deposition of cellulose microfibrils 23 Fingerlike tubules from vesicles → fusion of the neighouring vesicles Tubular network Leading edge of the growing network contacts the parent plasma membrane → loses cytoplasmic gaps → becomes a continuous flattened partition (cell plate) 24 ...
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