The Cell Cycle Cell Division Functions Reproduction Growth and development Tissue renewal repair Cell Division in Eukaryotic Cells Two Processes Involved Mitosis division of genetic information chromosomes between two new cells Cytokinesis division of cytoplasmic contents into two cells Mitotic Cell Cycle Eukaryotic cells Interphase 3 subphases G1 gap 1 Normal cell activity G2 gap 2 Preparation for division S DNA Synthesis Mitotic Phase 2 subphases Mitosis nuclear division Cytokinesis cytoplasmic division One mother cell two identical daughter cells G1 DNA unduplicated and not condensed S DNA synthesis DNA duplicated and not condensed G2 DNA duplicated not condensed Cytokinesis DNA unduplicated and condensed Mitosis Duplicated and condensed Human cell may take 24 hrs Mitotic phase is about 1 hr S phase is 10 12 hrs G1 is 4 6 while G2 is 5 6 hrs G1 is the most variable Mitosis in an animal cell G2 of interphase two centrosomes form by copy of a single one each centrosome contains two centrioles Prophase each duplicated chromosome appears as 2 identical sister chromatids joined at their centromeres The mitotic spindle begins to form its composed of chromosomes and the microtubules that extend from them Prometaphase the microtubules extending from each centrosome can invade nuclear area the chromosomes become more condensed Both chromatids of each chromosome has a specialized protein at the centrosome called a kinetochores Metaphase the centrosomes are at opposite poles of the cell The chromosomes convene at the metaphase plate equidistant from the spindle poles the chromosomes centromeres le at the plate Anaphase the shortest phase only lasts a few minutes Begins when the cohesion proteins are cleaved Allows two chromatids of each pair to part suddenly Each chromatid becomes a full blown chromosome By the end the two ends of the cell have equivalent and complete collection of chromosomes Telophase two daughter nuclei form in the cell Nucleoli reappear and chromosomes become less condensed Any remaining spindle microtubules become depolymerized Mitosis division of one nucleus into two twin nuclei is done Cytokinesis the division of the cytoplasm is usually well under way by late telophase so the daughter cells appear shortly after end of mitosis In animal cells cytokinesis had formation of a cleavage furrow which pinches the cell in two Mitosis in Plant Cells Prophase chromosomes condensing and nucleolus is disappearing The mitotic spindle is also beginning to form Prometaphase discrete chromosomes are now visible each has 2 identical sister chromatids Later in this phase the nuclear envelope will fragment Metaphase the spindle is complete and the chromosomes attached to the microtubules kinetochores are all at the metaphase plate Anaphase the chromatids of each chromosome has separated and the daughter chromosomes are moving to the end of the cells as the kinetochore microtubules shorten Telophase daughter nuclei are forming meanwhile cytokinesis has started the cell plate which will divide the cytoplasm in two is growing toward the perimeter of the parent cell Binary Fission Proakryotic Cell Division 1 Chromosome replication begins Soon after one copy of the origin moves rapidly toward the other end of the cell by a mechanism not fully understood yet 2 Replication continues One copy of the origin is now at the end of the cell Meanwhile the cell elongates 3 Replication finishes The plasma membrane grows inward and a new cell is deposited 4 Two daughter cells result Cell Cycle Regulation in Eukaryotes Controlling cellular checkpoints is crucial to growth development and maintenance Cell cycle control system cyclically operating set of molecule that both triggers and coordinates events Check point is a control point where stop and go ahead signals can regulate the cycle Variation in timing rate of cell division Chemical Signals Regulate Cell Cycle Experiment with mammalian cells Interpretation Experiment 1 Chemical signals in the S phase cell induced the G1 cell to synthesize DNA Interpretation Experiment 2 Chemical signals in the M phase cell induced the G1 cell to begin mitosis Conclusion the results of fusing a G1 with a cell from S or M phase of the cell suggests that molecules present in the cytoplasm during the S or the M phase control the progression of those phases Examples of Chemical Signals Cyclin dependent kinase Cyclins proteins that activate kinases Individually these are inactive Cdk enzyme that activates proteins by phosphorylation Cyclin Cdk complex activated kinase provides signal to cell An example is the Maturation Promoting Factor MPF MPF maturation promoting factor Triggers cell s passage past the G2 checkdpoint into M phase Concentrations of cyclin and cyclin dependent kinase complexes correlate with cell cycle transitions Anchorage dependence Cells only divide when anchored Density dependent inhibition and stop at high cell density Crowded cells stop dividing Caused by external factor Loss of Cell Cycle Controls in Cancer Cells Cancer cell not responsive to growth factors Cell produces growth factor Cell signal system blocked Abnormal cell control system Cell culture response Normal mammalian cells divide 20 50 times and then stop dividing Cancer cells are immortal and continue to divide indefinitely In vivo loss of cell cycle controls can have serious consequences A tumor grows from a single cancer cell Cancer cells invade neighboring tissue Cancer cells spread to other body parts Cancer cells may survive and establish a tumor in another body part Sexual Reproduction Meiosis duplication of chromosomes Asexual Reproduction No genetic variation clones Mitosis only Reproductive cells have the same number of chromosomes as vegetative cells Multicellular organisms budding etc Sexual Reproduction Variation genetic information from two parents Consequently the genetic make up of the offspring are different from either parent Sex cells gametes chromosome number is halved by meiosis Parents and offspring have the same number of chromosomes Chromosomes and Sexual Reproduction Somatic one from each parent Homologous Meiosis pair of sex chromosomes 46 total haploid n X Y cells are normally diploid 2n Two copies of each chromosome are present Human somatic cells contain 22 pairs of homologous chromosomes autosomes and 1 chromosomes are physically and genetically similar BUT NOT IDENTICAL produces gamete cells which possess 23 chromosomes in humans Gametes are are called
View Full Document
Unlocking...