Mitosis

Key points 1. All cells within an organism have the same genes. 2. What makes cells different from each other is that different genes are turned on and turned off in different cells. 3. The DNA must be copied and then divided exactly so that each cell gets an identical copy.

Eukaryotic Cell Division Two forms Mitosis grow, replace dead or worn out cells, or to repair wounds Asexual reproduction in fungi, protists, some plants/animals Meiosis Sexual reproduction

DNA and Cell Division During cell division, the genetic material DNA, needs to be copied and divided between the two new cells DNA in cells is divided into long chains called chromosomes (“volumes” of DNA) Chromosome DNA is wrapped around proteins called histones to organize it Nucleosome: unit of DNA wrapped around histones

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Mitosis Cells divide to make more cells. While all the other organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA. Mitosis is normal cell division, which goes on throughout life in all parts of the body. Meiosis is the special cell division that creates the sperm and eggs, the gametes. Humans have 46 chromosomes, 23 from each parent. Every cell has the same 46 chromosomes Each species has a characteristic number of chromosomes: corn has 20, house flies have 10, chimpanzees have 48.

Machinery of Mitosis The chromosomes are pulled apart by the spindle, which is made of microtubules. The spindle fibers are attached to each centromere (which is part of the chromosome), and anchored on the other end to a centrosome (which is the organizing center for the spindle). There are 2 centrosomes (Centrioles), one at each end of the spindle. The chromosomes are lined up between the poles of the spindle. When the spindle fibers contract, the chromosomes are pulled to the opposing poles. The cell then divides to separate the two poles. Stages of mitosis: prophase, metaphase, anaphase, telophase.

Mitosis Mitosis is divided into 5 phases: 1. prophase 2. Prometaphase / metaphase 3. anaphase 4. Telophase / Cytokenesis

Figure: 10-04 Title: The eukaryotic cell cycle. Caption: Interphase and cell division are the two major phases of the cell cycle.

Prophase In prophase, the cell begins the process of division. 1. The chromosomes condense. The proteins attached to the DNA cause the chromosomes to go from long thin structures to short fat one, which makes them easier to pull apart. 2. The nuclear envelope disappears. The double membrane that surround the nucleus dissolves into a collection of small vesicles, freeing the chromosomes to use the whole cell for division 3. The centrosomes move to opposite poles. During interphase, the pair of centrosomes were together just outside the nucleus. In prophase they separate and move to opposite ends of the cell. 4. The spindle starts to form, growing out of the centrosomes towards the chromosomes.

Metaphase Prometaphase: -chromosomes become attached to the spindle apparatus by their kinetochores -a second set of microtubules is formed from the poles to each kinetochore -microtubules begin to pull each chromosome toward the center of the cell Metaphase: -microtubules pull the chromosomes to align them at the center of the cell -metaphase plate: imaginary plane through the center of the cell where the chromosomes align

Anaphase - the centromeres divide by the removal of cohesin proteins causes the centromeres to separate - Then the spindle fibers/ microtubules contract, and the chromatids are pulled to opposite poles.

Telophase -The chromosomes are at the poles of the spindle. - spindle apparatus disassembles -nuclear envelope forms around each set of sister chromatids -chromosomes begin to uncoil -nucleolus reappears in each new nucleus The cytoplasm is divided into 2 separate cells, the process of cytokinesis.

Cytokinesis Cytoplasmic Division Usually occurs between late anaphase and end of telophase The organelles (other than the chromosomes) get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in. Two mechanisms Cell plate formation (plants) Cleavage (animals)

Cytokinesis: Animal Cell Cleavage furrow Figure 9.9 Page 159

Summary of Mitosis Prophase: Metaphase Anaphase Telophase Chromatin condenses into chromosomes Nuclear envelope and nucleolus disappears centrioles move to opposite sides of the cell Spindle fibers form and attaches to centromeres on the chromosomes Metaphase Chromosomes lined up on equator of cell centrioles at opposite ends of cell Anaphase Centromeres divide: each chromosome becomes two chromatids Chromosomes pulled to opposite poles by the spindle fibers Telophase Chromosomes de-condense Nuclear envelope and nucleolus reappears Spindle fibers disappear Cytokinesis: the cytoplasm is divided into 2 cells

(a) Interphase (b) Late prophase (c) Metaphase (d) Anaphase Figure: 10-07 Title: The cell cycle in a plant cell. Caption: Interphase and mitosis in the African blood lily. The chromosomes are stained bluish-purple, and the spindle microtubules are stained pink to red. Compare these micrographs with the drawings of mitosis in an animal cell shown in Figure 10-8. (Each micrograph is of a different single cell that has been fixed and stained at a particular stage of mitosis. Only chromosomes and microtubules have been stained.) (d) Anaphase (e) Telophase (f) interphase http://www.youtube.com/watch?v=C6hn3sA0ip0

Cancer Cancer is a disease of uncontrolled cell division. It starts with a single cell that loses its control mechanisms due to a genetic mutation. That cell starts dividing without limit, and eventually kills the host. Normal cells are controlled by several factors. Normal cells are mortal. This means that they can divide about 50 times and then they lose the ability to divide, and eventually die. This “clock” gets re-set during the formation of the gametes. Cancer cells escape this process of mortality: they are immortal and can divide endlessly. Normal cells that suffer significant chromosome damage destroy themselves due to the action of a gene called “p53”. Cancer cells either lose the p53 gene or ignore its message and fail to kill themselves.

Cancer Progression There are many different forms of cancer, affecting different cell types and working in different ways. The normal, unmutated versions of the oncogenes provide the control mechanisms for the cell. The mutations are caused by radiation, certain chemicals (carcinogens), and various random events during DNA replication. Once a single cell starts growing uncontrollably, it forms a tumor, a small mass of cells. No further progress can occur unless the cancerous mass gets its own blood supply. A tumor with a blood supply will grow into a large mass. Eventually some of the cancer cells will break loose and move through the blood supply to other parts of the body, where they start to multiply. This process is called metastasis.

Cancer Treatment Two basic treatments: surgery to remove the tumor, and radiation or chemicals to kill actively dividing cells. It is hard to remove all the tumor cells. Tumors often lack sharp boundaries for easy removal, and metastatic tumors can be very small and anywhere in the body. Radiation and chemotherapy are aimed at killing actively dividing cells, but killing all dividing cells is lethal: you must make new blood cells, skin cells, etc. So treatment must be carefully balanced to avoid killing the patient. Chemotherapy also has the problem of natural selection within the tumor. If any of the tumor cells are resistant to the chemical, they will survive and multiply. The cancer seems to have disappeared, but it comes back a few years later in a form that is resistant to chemotherapy. Using multiple drugs can decrease the risk of relapse: it’s hard for a cell to develop resistance to several drugs at the same time.