Cell division.pdf

Cell division Eukaryotic Cell Cycle The eukaryotic cell cycle has 5 main phases: 1. G1 (gap phase 1) time of cell growth 2. S (synthesis) - 2 sister chromatids are produced interphase 3. G2 (gap phase 2) chromosome condense 4. M (mitosis) 5. C (cytokinesis) The length of a complete cell cycle var...

Cell division Eukaryotic Cell Cycle The eukaryotic cell cycle has 5 main phases: 1. G1 (gap phase 1) time of cell growth 2. S (synthesis) - 2 sister chromatids are produced interphase 3. G2 (gap phase 2) chromosome condense 4. M (mitosis) 5. C (cytokinesis) The length of a complete cell cycle varies greatly among cell types. 2 Interphase Following S phase, the sister chromatids appear to share a centromere. In fact, the centromere has been replicated but the 2 centromeres are held together by cohesin proteins. Proteins of the kinetochore are attached to the centromere. Microtubules attach to the kinetochore. During G2 the chromosomes undergo condensation, becoming tightly coiled. Centrioles (microtubule-organizing centers) replicate and one centriole moves to each pole. Mitosis Mitosis is divided into 5 phases: 1. prophase 2. prometaphase 3. metaphase 4. anaphase 5. telophase 4 Mitosis Prophase: -chromosomes continue to condense -centrioles move to each pole of the cell -spindle apparatus is assembled -nuclear envelope dissolves 5 Mitosis Prometaphase: -chromosomes become attached to the spindle apparatus by their kinetochores -a second set of microtubules is formed from the poles to each kinetochore -microtubules begin to pull each chromosome toward the center of the cell 6 Mitosis Metaphase: -microtubules pull the chromosomes to align them at the center of the cell -metaphase plate: imaginary plane through the center of the cell where the chromosomes align 7 Mitosis Anaphase: -removal of cohesin proteins causes the centromeres to separate -microtubules pull sister chromatids toward the poles -in anaphase A the kinetochores are pulled apart -in anaphase B the poles move apart 8 Mitosis Telophase: -spindle apparatus disassembles -nuclear envelope forms around each set of sister chromatids -chromosomes begin to uncoil -nucleolus reappears in each new nucleus 9 Cytokinesis Cytokinesis – cleavage of the cell into equal halves -in animal cells – constriction of actin filaments produces a cleavage furrow -in plant cells – plasma membrane forms a cell plate between the nuclei -in fungi and some protists – mitosis occurs within the nucleus; division of the nucleus occurs with cytokinesis 10 Overview of Meiosis Meiosis is a form of cell division that leads to the production of gametes. gametes: egg cells and sperm cells -contain half the number of chromosomes of an adult body cell Adult body cells (somatic cells) are diploid, containing 2 sets of chromosomes. Gametes are haploid, containing only 1 set of chromosomes. 11 Features of Meiosis Meiosis includes two rounds of division – meiosis I and meiosis II. During meiosis I, homologous chromosomes (homologues) become closely associated with each other. This is synapsis. Proteins between the homologues hold them in a synaptonemal complex. Crossing over: genetic recombination between non-sister chromatids -physical exchange of regions of the chromatids chiasmata: sites of crossing over The homologues are separated from each other in anaphase I. The Process of Meiosis Prophase I: -chromosomes coil tighter -nuclear envelope dissolves -homologues become closely associated in synapsis -crossing over occurs between non-sister chromatids 13 The Process of Meiosis Metaphase I: -terminal chiasmata hold homologues together following crossing over -microtubules from opposite poles attach to each homologue, not each sister chromatid -homologues are aligned at the metaphase plate side-by-side -the orientation of each pair of homologues on the spindle is random 14 The Process of Meiosis Anaphase I: -microtubules of the spindle shorten -homologues are separated from each other -sister chromatids remain attached to each other at their centromeres 15 The Process of Meiosis Telophase I: -nuclear envelopes form around each set of chromosomes -each new nucleus is now haploid -sister chromatids are no longer identical because of crossing over 16 The Process of Meiosis Meiosis II resembles a mitotic division: -prophase II: nuclear envelopes dissolve and spindle apparatus forms -metaphase II: chromosomes align on metaphase plate -anaphase II: sister chromatids are separated from each other -telophase II: nuclear envelope re-forms; cytokinesis follows Meiosis vs. Mitosis Meiosis is characterized by 4 features: 1. Synapsis and crossing over 2. Sister chromatids remain joined at their centromeres throughout meiosis I 3. Kinetochores of sister chromatids attach to the same pole in meiosis I 4. DNA replication is suppressed between meiosis I and meiosis II. 18