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Transcript
Binary Fission-Bacterial Cell Division -Asexual reproduction of prokaryotes -No mitosis -Circular DNA and organelles replicate, the copies migrate to opposite sides of the elongating cell, and the cell splits in two forming two identical cells Roles of Cell Division -Reproduction -Growth -Repair -Remember: In all organisms, cell division must distribute identical genetic material (an exact copy of the genome) into two daughter cells Genome -The cell’s hereditary endowment of DNA -Usually packaged into chromosomes for manageability Chromosome -Made of a DNA and histone protein complex called chromatin -During cell division, the chromatin becomes highly condensed into the chromosomes Structure: -At cell division, each chromosome has been duplicated during interphase -The duplicated chromosome consist of two sister chromatids -Centromere: The point where two sister chromatids are connected Goal of Cell Division -To create two identical daughter cells by splitting all the sister chromatids and giving one copy of each to each new cell Cell Cycle -Interphase: (90% of cycle) when the cell grows and duplicates the chromosomes -Mitotic Phase: When the chromosome are split into separate cells. This phase includes cytokinesis Interphase -G1: First Gap --The cell grows and carries out regular biochemical functions -S: Synthesis --When the DNA is replicated or synthesis (chromosomes are replicated) -G2: Second Gap --Cell completes preparations for division Mitotic Phase -Mitosis: Division of replicated chromosomes -Cytokinesis: Division of the cell’s cytoplasm Purpose -To divide the two copies of the DNA equally -To separate the sister chromatids into separate cells Mitosis Steps -Prophase -Prometaphase -Metaphase -Anaphase -Telophase Prophase -Nucleoli disappear -Chromatin condenses into chromosomes (wrap around histones) -Centrioles separate to opposite ends of the cell (grows microtubules) -Mitotic spindle begins to form Prometaphase -Nuclear envelope dissolves -Spindle fibers join with the kinetochore of the centromeres (middle of chromosomes) Kinetochore -Specialized regions of the centromeres where spindle microtubules attach Metaphase -Centrioles now at opposite ends of the cell -Chromosomes line up on the metaphase plate -Spindle apparatus fully developed Anaphase -Centromeres break and the duplicated chromosomes are pulled away from each other toward opposite ends of the cell -Cell elongates: poles move slightly further apart Telophase -Chromosomes uncoil back to chromatin -Nuclear envelope reforms -Nucleoli reappear -Spindle fibers disappear -Cytokinesis usually starts (Reverse of prophase) Cytokinesis-Animal -Cleavage furrows form -Microfilaments contrast and divides the cytoplasm into two parts Cytokinesis-Plants -Cell plate develops from Golgi vesicles -New cell wall developed around the cell plate (middle) Regulation of Cell Division -The cell cycle is regulated by a molecular control mechanism to tell cells when to divide -This system moves the cell through its stages through a series of checkpointsenzymes (Kinase) -At each checkpoint, signals tell the cell to either keep dividing or stop Checkpoints -A critical control point in the cell cycle -The major checkpoints in the cell cycle are the G1 checkpoint (growth), G2 checkpoint (DNA), and M phase checkpoint (spindles) -Cells must receive a “go-ahead” signal at each checkpoint before proceeding to the next G1 Checkpoint Most important checkpoint -If a cell gets the “go-ahead” signal at this checkpoint, it usually completes the whole cell cycle and divides -If it does not receive this signal, it enters a nondividing phase called the G o phase. (Most specialized cells are in this state) -Later, if a cell needs to divide, a “go-ahead” signal is given and the cell reactivates into the Mphase Control of the Cell Cycle -Kinases are enzymes that control the cell cycle (adds activate phosphate) -They are always in a cell but only active when they are connected to a cyclin protein so they are called cyclin-dependent Kinases (Cdk) -When cyclin is connected to a Cdk, it forms an activated protein complex called MPF (M-phase Promoting Factor) -Specific Kinases give the “go-ahead” signals at the G1 and G2 checkpoints How Mitosis is Initiated? -As a specific example, cyclin molecules combine with Cdk molecules producing enough molecules of MPF to pass the G2 checkpoint and initiate the events of mitosis -MPF is the protein complex required for a cell to progress from G2 to Mitosis --Role of MPF: to trigger a chain of protein kinase activations and starts mitosis How does a Cell Stop Mitosis? -During anaphase, MPF switches itself off by starting a process that leads to the destruction of cyclin molecules -It activates a cyclin-degrading enzyme, which lowers the amount of cyclin in the cell -Without cyclin molecules Cdk molecules become inactive bringing mitosis to a close Growth Factors -External signals that affect mitosis Ex: -PDGF -Density-dependent inhibition -Anchorage dependence PDGF -Platelet-Derived Growth Factor -Stimulates cell division to heal injuries Normal Cell Division has 2 Key Characteristics -Density dependent inhibition -Anchorage dependency Density Dependent Inhibition -The process in which crowded cells stop dividing -The number of cells in an area force competition for nutrients, space, and growth factors, so when cells are crowded, they get signals to stop dividing -When density is high- no cell division -When density is low- cells divide Anchorage Dependence -Normal cells must be attached to a substratum like the extracellular matrix of a tissue to divide -Prevents cells from dividing and floating off in the body Cancer Cells -Do not exhibit either density dependent inhibition or anchorage dependency -The control mechanisms for cell division have failed -The process that changes a normal cell to a cancerous one id transformation -A tumor is a mass of abnormal cells within otherwise normal tissue -If the abnormal cells remain at the original site, it is a benign tumor Malignant -A tumor becomes invasive enough to impair the functions of one or more organs -Individuals who have malignant tumors have cancer Metastasis -A condition where cells from a malignant tumor separate and enter the blood or lymph vessels spreading the cancer to other organs Comment -Regulation of cell division is a balance between: --Mitosis: making new cells --Apoptosis: cell suicide or death -Cancer can result if either process doesn’t work