Meiosis vs. Mitosis Mitosis

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Cell Increase and Decrease Cell division increases the number of somatic (body) cells Two parts of cell division: Mitosis (division of nucleus) Cytokinesis (division of cytoplasm) Apoptosis (cell death) decreases the number of cells.0

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Cell division occurs when: Body growth Maintenance and repair Fighting infection Replacing worn/dead cells Apoptosis occurs when: Tail of tadpole disappears  frog Skin between human fingers and toes dies during development Both cell increase and apoptosis occur during normal development and growth – example of homeostasis.0

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The Cell Cycle An orderly sequence of events that occurs from the time a cell is first formed until it divides into two new cells.0http://www.cellsalive.com/cell_cycle.htm

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Most of the cell cycle is spent in interphase: G1 stage – cell growth, cell doubles its organelles (cell structures), prepares for DNA replication S stage – DNA replication occurs G2 stage – cell makes proteins needed for cell division Amount of time spent in interphase varies – average for adult mammals is 20 hours Nerve cells and muscle cells exit the cell cycle  G0 phase0

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Following interphase is the M stage, including mitosis and cytokinesis (definitions slide #1). During mitosis, two copies of DNA made during replication are separated, and become the nuclei of the two daughter cells – takes about 4 hours. The cell cycle ends when cytokinesis, the splitting of the cytoplasm, is complete.0

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Control of the cell cycle Three checkpoints: During G1 prior to the S stage – if DNA is damaged, apoptosis occurs During G2 prior to the M stage – will not proceed if DNA is damaged or not copied During the M stage prior to the end of mitosis – if chromosomes are not properly aligned 0

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Chromosome Structure In a non-dividing cell – genetic material is in the form of chromatin (DNA & protein) In a dividing cell, chromatin undergoes coiling to form chromosomes Proteins called histones package the DNA so it can fit into the nucleus (2 meters of DNA fit into nucleus that is 5 micrometers)0

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After replication, there are 2 identical sister chromatids, held together by a centromere0

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Each species has a set number of chromosomes: Humans – 46 Crayfish – 200 Corn – 20 Adder’s tongue fern – 1262 Chimpanzee - 48 Sand dollar – 52 Dog – 78 Cat - 320

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Body cells contain the diploid (2n) number of chromosomes – 2 chromosomes of each kind (1 from each parent) Sex cells (eggs and sperm) contain only 1 chromosome of each kind – haploid (n) number of chromosomes

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Mitosis – occurs in body cells – diploid cells divide to produce diploid cells – daughter cells are genetically identical to parent cellshttp://www.cellsalive.com/mitosis.htmAnimation of mitosis

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Mitosis overview

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Centriole Chromatin Nucleolus (in yellow) Nuclear membrane Spindle fibers Chromosome (replicated) Centromere Sister Chromatids (each half of replicated chromosomes) Daughter Chromosomes (once the replicated chromosome splits) Cell membrane Cleavage furrow Asters Centrosome (=aster + centriole)

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Late InterphaseCentrosomes (which contain pair of centrioles and an aster – which are short microtubules) duplicate Chromatin condenses into chromosomes

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Early ProphaseChromosomes become visible Centrosomes move to opposite ends of the cell Nuclear membrane and nucleolus disappear

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Late ProphaseSpindle fibers form Chromosomes become attached to spindle fibers – centromere attaches to spindle fibers

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Last Updated: 8th March 2018

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