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Download In eukaryotes, heritable information is passed to the next generation
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In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization Cell Cycle and Mitosis 8.1 to 8.11 All the DNA in a cell constitutes the cell’s genome A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) DNA molecules in a cell are packaged into chromosomes Genetic information is stored and transmitted through DNA Fig. 12-3 20 µm Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Somatic cells (nonreproductive cells) have two sets of chromosomes - DIPLOID Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells - HAPLOID Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division Genetic Information is stored and transmitted through DNA Fig. 12-4 0.5 µm Chromosomes Chromosome arm Centromere DNA molecules Chromosome duplication (including DNA synthesis) Sister chromatids Separation of sister chromatids Centromere Sister chromatids The cell cycle is a complex set of stages that is highly regulated with checkpoints, which determine the ultimate fate of the cell Majority of the cell cycle Time when a cell’s metabolic activity is very high and the cell performs various functions 3 stages ◦ G1 – cell growth ◦ S – DNA replication (Synthesis of DNA) ◦ G2 – prepare for mitosis Interphase About 10% of the cell cycle 2 stages ◦ Mitosis – nuclear division ◦ Cytokinesis – cytoplasm division Results in two genetically identical cells M Phase (mitotic phase) The cell cycle is directed by internal controls or checkpoints. • For many cells, the G1 checkpoint seems to be the most important one • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase Cell Cycle Checkpoints Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclindependent kinases (Cdks) The activity of cyclins and Cdks fluctuates during the cell cycle MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase Cell Cycle Checkpoints Fig. 12-17 M G1 S G2 M G1 S G2 M G1 MPF activity Cyclin concentration Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle Degraded cyclin G2 Cdk checkpoint Cyclin is degraded MPF Cyclin accumulation Cdk Cyclin (b) Molecular mechanisms that help regulate the cell cycle An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture Internal and External signals provide stop-and-go signs at the checkpoints Fig. 12-18 Scalpels Petri plate Without PDGF cells fail to divide With PDGF cells proliferate Cultured fibroblasts 10 µm Fig. 12-19 Anchorage dependence Density-dependent inhibition Density-dependent inhibition (a) Normal mammalian cells 25 µm 25 µm (b) Cancer cells Mitosis is conventionally divided into five phases: ◦ Prophase ◦ Prometaphase ◦ Metaphase ◦ Anaphase ◦ Telophase Cytokinesis is well underway by late telophase Mitosis passes a complete genome from the parent cell to the daughter cell Fig. 12-6b G2 of Interphase Chromatin Centrosomes (with centriole(duplicated) pairs) Prophase Early mitotic Aster Centromere spindle Nucleolus Nuclear Plasma envelope membrane Chromosome, consisting of two sister chromatids Prometaphase Fragments Nonkinetochore of nuclear microtubules envelope Kinetochore Kinetochore microtubule Fig. 12-7 Aster Centrosome Microtubules Chromosomes Sister chromatids Metaphase plate Kinetochores Centrosome 1 µm Overlapping nonkinetochore microtubules Kinetochore microtubules 0.5 µm Fig. 12-6d Metaphase Anaphase Metaphase plate Spindle Centrosome at one spindle pole Telophase and Cytokinesis Cleavage furrow Daughter chromosomes Nuclear envelope forming Nucleolus forming Fig. 12-8b Chromosome movement Kinetochore Microtubule Motor protein Chromosome Tubulin Subunits Fig. 12-9 Cleavage furrow Contractile ring of microfilaments 100 µm Daughter cells (a) Cleavage of an animal cell (SEM) Vesicles forming cell plate Wall of parent cell Cell plate 1 µm New cell wall Daughter cells (b) Cell plate formation in a plant cell (TEM) What must occur before mitosis? ◦ DNA replication ◦ Pass G2 checkpoint What is “checked” at the ‘M’ checkpoint? ◦ Chromosome attachment to microtubule What follows mitosis? ◦ Cytokinesis What are the products of the mitosis? ◦ 2 genetically identical cells Why do we need mitosis? ◦ Growth, repair, asexual reproduction Summary of Mitosis
