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Biology: Cell division

The steps of mitosis and meiosis as well as a discussion about why cells need to divide.

Paul Epps

on 5 March 2013

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Transcript of Biology: Cell division

Cell Division Division Mitosis: Why does a cell divide? What are sex cells and why do we need them? Why can't a cell just get bigger? When does a cell divide? A cell divides for three reasons: 1) Growth 2) To make sex cells 3) To make a copy of itself so the species continues. There is a ratio between the volume of a cell and the surface area of the cell, if the volume of the cell becomes significantly larger than the surface area the cell can no longer efficiently bring nutrients in and rid the cell of waste. Sex cells are the cells responsible for reproduction. In humans that would be the sperm cells for males and egg cells for females. In plants there is also egg cells but pollen cells replace sperm. They each have half the normal number of chromosomes (Haploid) so that when they combine they will have the correct number for that organism (Dipliod). How does cell division help a species continue? If a single celled organism doesn't divide then the cell will eventually wear out and die. If it divides new organelles are made and the cell continues. A cell divides when it gets signals from the body, these signals are proteins that trigger the cell to begin division. Cancer is when cells divide uncontrollably. Mitosis is how cells divide for growth or for reproducing themselves (most single celled organisms) Interphase This is the normal state a in which a cell is found. It is not really a step of mitosis. The cell is not dividing--some say it is resting, but this isn't really true it is making proteins and going about everyday activities. There are 3 stages to interphase: G1, S, and G2. In G1 the cell grows. It continues to increase in size until the ratio of surface area to volume reaches a certain point, then it is time for the S phase. During the S phase all of the DNA in a strand must make a copy of itself--one for each new cell. Once the S phase is completed it is on to the G2 phase which is a second growth phase--but really most of the energy is readying the cell for mitosis. Prophase After the DNA has replicated mitosis can begin.
1. The DNA prepares for the process by coiling tightly into chromosomes.
2. The membrane that surrounds the nucleus dissolves.
3. Two small organelles called centrioles move to opposite ends of the cell.
4. Thin tubules called spindle fibers form between the centrioles, stretching across the cell. Metaphase The chromosomes now are free of the constraints of the nucleus, so they move into the cell and ultamately line up on the metaphase plate. The plate runs perpendicular to the line from centriole to centriole. Remember there are two copies of each chromosome and they line up next to each other in a line. The chromosomes seperate from their matching chromosome pair and move along a spindle fiber towards the centriole. Once the chromosomes reach the centrioles:
1. A new nuclear envelope forms around them creating a new nucleus.
2. The spindle fibers dissolve.
3. The chromosomes relax into chromatin. Anaphase Telophase Cytokinesis After mitosis the cell has two nuclei, this won't do, the cell must divide. The membrane begins to pinch in the center until the cell pinches into two cells. (In plants and algae the cell membrane doesn't pinch--a cell plate forms across the middle. The cell plate becomes a cell wall--this divides one plant cell into two.) Once that happens the cells continue the cell cycle, moving into Interphase. Meiosis: Meiosis is how sex cells are formed. It is very similar to mitosis the difference is all stages except interphase happen twice. Interphase Prophase Metaphase Anaphase Telophase Cytokinesis Prophase II Metaphase II Anaphase II Telophase II Cytokinesis II Cells undergo a round of DNA replication, forming duplicate chromosomes Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. The nuclear membrane dissolves, the centrioles move to the poles and spindle fibers form. Spindle fibers attach to the chromosomes as they line up perpendicular to the spindle fibers. The fibers pull the homologous chromosomes toward opposite ends of the cell. Nuclear membranes form. The cell seperates into two cells. The chromosomes relax into chromatin. We now have two cells with the same number of chromosomes as we started with. Now the process starts over except the DNA has not doubled. The nuclear membrane dissolves and the chromosomes thicken again. The centrioles move to the poles and the spindle fibers form. The cell membrane pinches into two cells, in plants the cell wall forms. This is the end of Telophase. The chromosomes line up perpendicular to the centrioles. The chromosomes are pulled apart at the centromere. Each half of a chromosome is called a chromatid, the chromatids split and move towards the centrioles. Once the chromatids reach the centriole the nuclear membrane forms again, the chromatids relax into chromatin, and cytokinesis starts. The cell membrane pinches into two cells, cell wall re-forms. There are now four haploid cells each with one set of chromosomes.
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