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Biology: Cell division
Transcript of Biology: Cell division
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.