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Transcript
Cellular Reproduction Why is there a limit on cell size? If a cell continues to grow, the surface area of the membrane might not be able to transport enough nutrients and waste. Transport of substances within the cell is also more difficult in larger cells. What is the Cell Cycle? Once a cell reaches its size limit, it has to stop growing or divide. Cells reproduce by a cycle of growing and dividing called the Cell Cycle. Stages of the cell cycle Interphase – Cell grows, carries out normal cell functions, and replicates (makes copies of) its DNA, and prepares for mitosis Mitosis – Cell division: nucleus and DNA divide Cytokinesis – Cytoplasm divides, splitting into 2 complete identical cells What happens during interphase? Cell grows and develops, duplicates its DNA, and prepares for division Stages of Interphase G1 (Gap1) – period immediately after a cell divides. Cell grows, carries out normal cell functions, and prepares to replicate DNA. (Some cells do not divide again) S (Synthesis) – DNA is copied G2 (Gap2) – Cell prepares for mitosis. Microtubules for cell division are formed, DNA is checked for mistakes Chromosomes and Chromatin Chromosomes – Condensed form of DNA, the genetic material that is passed from generation to generation of cells. Chromatin- Relaxed form of DNA during interphase, which allows for DNA to replicate How do prokaryotes divide? Binary Fission (it’s the same method as mitosis.) Chromosome structure Sister chromatid- each half of the X-shaped chromosome is an identical copy of that chromosome’s DNA. Centromere- the structure at the center where the sister chromatids are attached PROPHASE Chromatin condenses into X-shaped chromosomes (containing 2 identical sister chromatids). The nucleolus disappears. The nuclear membrane disintegrates and the The spindle apparatus begins to form. METAPHASE Sister chromatids are pulled by motor proteins along the spindle apparatus towards the middle of the cell Sister chromatids line up along the equator ANAPHASE Chromatids are pulled apart The microtubules of the spindle apparatus shorten, which pulls at the centromere of each sister chromatid, causing them to separate into 2 identical chromosomes. • The chromosomes move toward the poles of the cell (opposite sides) TELOPHASE The last stage of mitosis is telophase. Chromosomes arrive at the poles and begin to relax or decondense 2 new nuclear membranes begin to form Nucleoli reappear Spindle apparatus breaks down and parts are recycled to build the cytoskeleton Nucleus has divided, but cell division is not yet complete Cytokinesis Cytoplasm and all other cell contents divide, resulting in 2 identical cells How does a cell regulate its cycle? Proteins (cyclins and CDKs) regulate the cell cycle by controlling the start and end of various activities within the cell Quality control checkpoints monitor the cycle and can stop it if something goes wrong. Ex. Check DNA for mistakes after replication How is cancer a result of uncontrolled mitosis? Cancer is uncontrolled growth and division of cells – a failure in the regulation of the cell cycle. Cancer cells can crowd out normal cells Carcinogens are agents that are known to cause cancer Apoptosis Apoptosis is programmed cell death Examples: cells between fingers and toes during development of hands and feet cells in the leaves that fall in autumn damaged cells (like sunburned skin) Stem Cells Embryonic Stem Cells – after a sperm fertilizes an egg, the cells divide repeatedly until there about 150 cells which have not become specialized yet. Each can develop into many various types of cells. Embryonic stem cell research is controversial because of ethical concerns about the source of the cells. Adult Stem Cells – found in various tissues, might be used to repair the same kind of tissue in which they are found. They might be able to develop into different types of cells, or used in treatments of diseases. Less controversial because the cells can be obtained with the consent of their donors.