MITOSIS AND MEIOSIS Chapter 10 and 11-4 WHAT IS MITOSIS? For growth and to replace old, worn out or damaged cells. Occurs in body cells therefore forms body cells. Results in 2 identical cells of the original parent cell. Maintains chromosome number Creates diploid cells (2N) has two sets of chromosomes One division of the nucleus Also known as binary fission In single-celled organisms, cell division is a form of asexual reproduction. CELL GROWTH There are limits to cell growth 1. DNA information overload: The bigger the cell, the greater the demand on the DNA. Compare a cell to a growing town. The town library has a limited number of books. As the town grows, these limited number of books are in greater demand, which limits access. A growing cell makes greater demands on its genetic library. If the cell gets too big, the DNA would not be able to serve the needs of the growing cell. 2. Exchange materials: The bigger the cell, the harder it is to move materials and wastes in and out of the cell. - Small cells maintain more efficient transport systems. - Imagine a cell shaped like a cube. As length of the sides increases, its volume increases faster than its surface area, decreasing the ratio of surface area to volume. - If a cell gets too large, the surface area of the cell is not large enough to get enough oxygen and nutrients in and waste out. CELL GROWTH WHAT IS THE SOLUTION? The Cell Cycle: Interphase + Cell division Interphase = cell growth (preparation to divide) Cell division = dividing the cell Cell Division = Mitosis + Cytokinesis Mitosis the division of the nucleus Cytokinesis the division of the cytoplasm G1 Phase: The cell grows in size and develops duplicate organelles. S Phase: DNA # is doubled. G2 Phase: gets ready for mitosis. Finishes all growth. Checkpoint 1 *Has 2 complete sets of DNA Checkpoint 2 6 CELL DIVISION 3 MAJOR STEPS 1. Interphase is the phase in which cells spend 90% of their time getting ready to divide. Has 3 parts: 1. G1 2. S 3. G2 G1 phase The first period of cell growth (size), when the cell grows its organelles. S phase The period when DNA is duplicated, replicated or copied. Chromosomes Chromosomes Why is this stage so important? G2 phase The period when a cell finishes all growth and prepares for mitosis. CELL DIVISION 3 MAJOR STEPS Chromosomes = DNA and Histones(protein) Chromosome Chromatid Duplicated Chromosome Sister Chromatids Centromere Where they join 2. Mitosis (Cell Division) 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase 3. Cytokinesis (Splitting of the cytoplasm) Results in 2 new identical daughter cells. CELL DIVISION 3 STEPS Centriole copies are made (G1) INTERPHASE Cell Organelles are being replicated. More Cytoplasm is being made. The cell is getting ready for MITOSIS (G2) DNA is replicating (S) X X X X 4. Nuclear Membrane begins breaking down 1. Centrioles start moving apart. 2. Spindle fibers form 3. Chromosomes shorten and thicken into visible chromosomes. MITOSIS PROPHASE XX XX NOTE: THE NUCLEAR MEMBRANE IS NO LONGER PRESENT NOTE: THE NUCLEAR MEMBRANE IS NO LONGER PRESENT 1.Centrioles are at opposite poles. And move toward the middle of the cell. MITOSIS METAPHASE (STEP 2) 2.Spindles attach to the chromosomes > > > > > > > > MITOSIS ANAPHASE (STEP 3) 1.Centrioles begin to pull sister chromatids to opposite sides of the cell. 1.Chromosomes continue to be pulled toward the centrioles. >> > > > > > > MITOSIS TELOPHASE (STEP 4) 2.Cell Membrane begins to separate into two cells 3.Nuclear Membranes begin to reappear. 4. Spindles disappear. 3.Nuclear Membranes begin to reappear. 4. Spindles disappear. CYTOKINESIS Cytoplasm splits between the two identical cells THE STAGES OF THE CELL CYCLE The process is VERY SIMILAR in each type of cell. There are only 2 differences: 1.Plant cells do not have centrioles 2.The cell membrane cannot pinch because of the cell wall. 3.Instead a cell plate forms between the 2 nuclei division. 18 REGULATING THE CELL CYCLE Contact inhibition: stop dividing when crowded Cyclins: Protein regulators Checkpoints Internal Regulators (timing) External Regulators (growth factors) Uncontrolled Cell Growth Cancer: can form masses called tumors Unresponsive to internal and external regulators. Deprive healthy cells from nutrients. Results in uncontrolled cell growth. CANCER CANCER Tumor: uncontrolled cell division Benign (cysts, warts) = not too harmful Malignant = Cancer = harmful Metastasis: cancer cells break off & spread Kinds Carcinomas: skin & organ linings (breast & lung CA) Sarcomas: bone & muscle Lymphomas: tissues that make blood cells Leukemia: uncontrolled production WBC Cancer Effects on Cell Cycle Healthy cells stop dividing when crowded Cancer cells keep dividing & steal blood supply cancer cell bloodstream normal cell 21 _______________________ 22 MITOSIS VS. MEIOSIS For growth and to replace old, worn out or damaged cells. Occurs in body cells therefore forms body cells. Results in 2 identical cells from the original parent cell. Maintains chromosome number Creates diploid cells (2N) One division of the nucleus To make sperm and eggs. Occurs in sex cells therefore forms sex cells or gametes. Results in 4 genetically different cells from the original parent cell. Cuts chromosome number in half Creates haploid cells (1N) Two divisions of the nucleus Spermatogenesis Oogenesis In males: These cells develop into 4 individual sperm. In oogenesis: These cells would have developed into three small polar bodies and only one large egg. THE POLAR BODIES ASSIST WITH EQUAL CHROMOSOMAL DIVISION, HOWEVER THE POLAR BODIES DIE Uneven division of cytoplasm Fertilization: Union of sperm and Egg - Zygote- cell resulting from fertilization of egg by sperm (full number of chromosomes) Re-establishes the full number (diploid) of chromosomes in an organism 2 chromosomes 4 chromosomes PURPOSE OF MEIOSIS Also known as reduction division To produce sex cells or gametes with half the number of chromosomes. Called haploid cells Sex Cells Reproductive cells produced by sex organs Sperm and ova In meiosis the cell divides twice. Meiosis involves 2 cell divisions. Meiosis I Prophase I Metaphase I Anaphase I Telophase I Cytokinesis Meiosis II Prophase II Metaphase II Anaphase II Telophase II Cytokinesis STEPS OF MEIOSIS Prophase I (Step one) - Identical steps to Mitosis prophase EXCEPT (a) Homologous (matching) chromosomes pair up, also known as tetrads (b) crossing over occurs exchange genetic material Metaphase I - Identical steps to Mitosis Metaphase EXCEPT (a) Spindle fibers move homologous pairs of chromosomes to the center of the cell STEPS OF MEIOSIS Anaphase I identical steps to Mitosis Anaphase EXCEPT (a) Spindle fibers move Homologous chromosomes apart Telophase I identical steps to Mitosis Telophase EXCEPT (a) These 2 new cells now enter Meiosis II. MEIOSIS II THE STEPS ARE IDENTICAL TO THE STEPS OF MITOSIS. Prophase II Metaphase II Anaphase II Telophase II 4 cells with half the number of original chromosomes 31