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Chapter 9-10 The Reproduction of Cells Mitosis & Meiosis Cell Division “Omnis cellula e cellula” – Virchow – “all cells from cells” Cells reproduce to form genetically identical daughter cells Functions of Cell Division Reproduction: – Making more cells Growth: – Enables multicellular organisms to grow and develop from a single cell Repair: – Replacing cells that die from normal wear and tear and accidents Prokaryotic Cell Division Binary Fission: – “division in half” Steps in Binary Fission – Bacterial DNA is duplicated – Cell grows – Cell splits in two Eukaryotic Cell Division The Cell Cycle – Interphase G1, S, and G2 phases – Mitotic Phase Mitosis Cytokinesis Interphase The cell spends 90% of its time in interphase G1: – “Gap 1” – Cell growth S: – “Synthesis” – DNA is replicated G2: – “Gap 2” – Cell growth – Preparation for division Mitotic Phase 5 subphases: – Prophase – Prometaphase – Metaphase – Anaphase – Telophase Prophase Nucleoli disappear Chromatin condense into chromosomes Mitotic spindle begins to form in the cytoplasm Centrosomes move away from each other Prometaphase Nuclear envelope disappears Bundles of microtubules extend from each pole towards the equator of the cell Some microtubules attach to kinetochores Metaphase Centrosomes are at opposite poles Chromosomes line up on metaphase plate Mitotic spindle is fully formed at this point Anaphase Sister chromatids separate at the centromere Chromatids move towards the poles Cell becomes more ovoid in shape Telophase Daughter nuclei begin to form at the poles of the cell Nuclear envelopes reform Nucleoli reappear Chromotin becomes less dense chromosomes disappear Cytokinesis Division of the cytoplasm Animal Cells: – Cleavage furrow forms – Pinches the cell into two Plant Cells: – Cell plate forms – Divides the cell in 2 Animal Cells Plant Cells Controlling Cell Division Cells rely on internal and external cues to control cell division Growth factor – Substance that is required for certain cells to divide Density of cells – If cells become too dense in their container, they will stop dividing they’re competing for resources! Restriction Point Late in G1, before S phase, the cell passes the “point of no return” – Can we go ahead and divide? – If yes… cell division occurs! – If no, the cell enters G0 phase Essentially exits the cell cycle and just stops Most grown human cells are in this phase MPF “Maturation Promoting Factor” – A specific type of cyclin-dependent kinase (cdk) – Cdks are regulatory proteins Helps control the cell cycle – The “master switch” for a cell’s passage from interphase to mitosis Control of the Cycle The phases are triggered by the accumulation of control substances called cyclins. The cell division cyclins interact with molecules called kinases. Kinases are proteins that phosphorylate other chemical messengers or enzymes that trigger the cell cycle phases. Control of the Cycle The cyclin-dependent kinase, cdk, has the ability to activate either a cellular messenger for either DNA replication or mitosis. The messengers are called cyclins. M-cyclin for mitosis and the Scyclin for DNA synthesis Control of the Cycle In G1 the cdk is inactive. Control of the Cycle The S cyclin protein forms. Levels of cyclins can change, but kinases cannot. Control of the Cycle S cyclin combines with the cyclindependent kinase which is in it DNA promoting (S) form. Control of the Cycle The messenger molecule attaches to the activated cdk. The messenger is phosphorylated and then breaks away from the cdk. S begins. Control of the Cycle The messenger leaves. The S-cyclin is destroyed. The cdk (cyclindependent kinase) is again inactive. Control of the Cycle At end of S, the cdk will change to the M form (mitosis promoting form). M- cyclin will be produced. M-cyclin will attach to the M form of cdk to form a MPF (maturation promoting factor). Control of the Cycle The signal molecule can attach and then becomes phosphorylated. The signal molecule will detach, signaling mitosis. M-cyclin is destroyed. Cyclin-dependent kinase (cdk) returns to inactive form. Control of the Cycle Three checkpoints exist (G1, G2, and during metaphase of M). Checkpoints are regulated by amount of cyclins ( in different forms). The most important checkpoint is in G1 (RESTRICTION POINT). If enough cyclins are present the cell goes through the rest of the cycle. If not, the cell stops dividing and enters G0. Normal v. Cancer Cells Have densitydependent inhibition Will only divide when attached to a substrate (anchored) Must have sufficient growth factors or nutrients for division Lack densitydependent inhibition Will divide in solution or when not anchored Unaffected by presence of external growth factors or nutrients Cancer Tumor: – When cancerous cells divide, they form a mass of cancer cells within an otherwise normal tissue Benign tumor: – Cancerous cells remain in one spot Malignant tumor: – Becomes invasive enough to impair the functions of one or more organs Metastasis: – The spread of cancer cells beyond their original site