The Cell CycleCell 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 cellsMitotic Cell Cycle - Eukaryotic cells Interphase – 3 subphases G1 (gap 1) - Normal cell activity S - DNA Synthesis G2 (gap 2) - Preparation for division Mitotic Phase – 2 subphases Mitosis (nuclear division) Cytokinesis (cytoplasmic division) One mother cell two identical daughter cellsHuman 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.G1- DNA unduplicated and not condensedS- DNA synthesis- DNA duplicated and not condensedG2- DNA duplicated not condensedCytokinesis- DNA unduplicated and condensedMitosis- Duplicated and condensedMitosis 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 theircentromeres. 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, thechromosomes become more condensed. Both chromatids of each chromosome has aspecialized 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 chromosomesare 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 Division1.) Chromosome replication begins. Soon after, one copy of the origin moves rapidly toward theother 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 divisionChemical Signals Regulate Cell CycleExperiment with mammalian cellsInterpretationExperiment 1 - Chemical signalsin the S phase cell induced theG1 cell to synthesize DNAInterpretationExperiment 2 - Chemical signalsin the M phase cell induced theG1 cell to begin mitosisConclusion: 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 (Cdk) – enzyme that activates proteins by phosphorylation Cyclins – proteins that activate kinases-Individually, these are inactive- 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 phaseConcentrations 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 indefinitelyIn 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 chromosomesChromosomes and Sexual Reproduction- Somatic cells are normally diploid (2n). Two copies of each chromosome are present, one from each parent- Human
View Full Document
Unlocking...