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Biology-Mr.Willets
Lab-Mitosis
Cells divide for one of three reasons- 1) so that a multicellular organism can get larger (GROWTH) 2) to replace damaged cells in a multicellular organism (REPAIR) 3) to reproduce either sexually or asexually (REPRODUCTION)
Growth, repair and asexual reproduction all use a type of cell division called MITOSIS. (actually, only the division of the nucleus is called mitosis). When mitosis occurs, both new cells have a full set of chromosomes (DNA), exactly the same as the parent cell had before it divided.
Mitosis occurs in stages. During this lab, you will find, examine and draw cells that were killed in each stage of mitosis. For all diagrams, USE A PENCIL!!!
The slides you will use are made from the tip of the root of an onion plant (Allium ). The tip of a root is always growing and so many cells are dividing. Focus first using low power and then switch to high power to locate the various stages. You do not have to find them in order. If you see a good example of a later stage first, examine and draw it. Your friendly teacher will show you the stages using the TV scope.
A) INTERPHASE-
This is not a stage of mitosis but the time when a cell carries on its normal business. To prepare for mitosis, the cell grows, makes new cell parts and replicates its DNA. A cell in interphase looks like a normal cell. The nucleus is visible but the chromosomes inside are not. There may be one or two dark dots inside the nucleus. These are the nucleoli (where ribosomes are made) Find and draw an interphase cell.
B) PROPHASE-
During this phase of mitosis, the nucleus goes through many changes to prepare for division. The chromatin starts to thicken and becomes visible as chromosomes. The nuclear membrane disappears. Spindle fibers start to form. These fibers will eventually be used to pull the two sets of chromosomes apart. During prophase, the nucleus is still roundish but with chromosomes becoming visible as individual units. The spindle fibers will not be visible in these slides. Find a cell in prophase and draw it.
C) METAPHASE
By the time the cell reaches metaphase, the nucleus no longer exists and the chromosomes are all in the center of the cell (equator). The spindle fibers have attached to the chromosomes. Each chromosome is really a double chromosome (remember-replication has occurred). The two parts of the chromosome (called chromatids) are attached by a structure called a centromere. Look for a cell with the chromosomes lined up at the equator. Individual chromosomes will be hard to see. Draw this metaphase cell.
D) ANAPHASE-
During this phase, the two sets of chromosomes have separated and are being pulled apart by the spindle fibers to opposite ends of the cell (poles). Find an anaphase cell and draw it.
E) TELOPHASE-
During this phase, each set of chromosomes goes back to being chromatin, the spindle fibers disappear and each forms a normal looking nucleus. The cytoplasm then divides forming two cells.
The division of the cytoplasm is called cytokinesis. This happens differently in plants and animals. In plant cells, a cell plate forms in the middle of the cell and grows to the edges. In animal cells, the outer part of the cell membrane pinches in to form two cells. The indentation formed is called a cleavage furrow.
Find a cell in telophase. This will look like two small cells next to each other but the wall between them will not have fully formed. The chromosomes may still be partially visible in each nucleus. Draw the telophase cell.
ANIMAL CELL MITOSIS-
An example of pinching in will be shown on the TV scope. Make a drawing of it.
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