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SC BIOL 110 - Mitosis and Meiosis

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Mitosis and MeiosisIntroduction:Growth of multi-cellular organisms involves an increase in the number of cells. The same process also helps to repair cellular damage. This process is more commonly known as mitosis. Mitosis only occurs in the somatic cells of the body which include all cells of the body except for egg or sperm cells, also known as gametes. Cell division involves two major events: the distribution of identical copies of the genetic information from the parent cell to 2 daughter cells followed by cytoplasmic division. As a result of cell division, all the cells of a multicellular organism have the same genetic information. The parent cell divides by mitosis, producing two daughter cells. These 2 identical cells contain exactly the same genetic material as the parent cell. Mitosis involves a series of events which results in the equal distribution of chromosomes between the two new cells. There are 4 main phases to this process: prophase, metaphase, anaphase and telophase. You must remember that these phases are not clearly defined within the cellular division process and are better thought of pictures taken during the division process. Mitosis is a relatively short period in the overall life of a cell. Typical mitotic divison maytake anywhere from 2-4 hours to occur. The rest of the cells life is spent in interphase, a non-dividing period consisting of G1, S and G2 phases. During interphase the cell is participatin in many important activities, including growth and replication of DNA. Before a cell can divide, the DNA must be replicated, this occurs during S phase of interphase.Some specialized cells do not divide. These specialized cells enter a stage known as G0 in which they are not going through active division. They permanently remain in interpahse as they synthesize molecules, move or perform other specific activities.Below is a diagram of the typical stages of mitosis. Please take note of the chromosomes in the cell and how they divide into two new daughter cells at the end of mitosis.Interphase: This is an active metabolic stage and the cell is performing normal functions. DNA is replicated during S phase. DNA is still in the chromatin (long and stringy) form during this phase. The nucleus is still intact during this phase. Please look at the picture below to see what DNA looks like before and after it replicates.Part (a) of this figure represents DNA, in its chromosome form, before replication has occurred. Part (b) of the figure shows a replicated chromosome. The whole structure at thispoint is called a chromosome and the individual structures are known as chromatids. These chromatids are connected to each other at the centromere.Prophase: The cell is now preparing itself for division. The DNA begins to condense from chromatin into chromosomes. It is not possible to see the duplicated chromosomes until late prophase. The nuclear membrane and the nucleoli of the cell begin to disappear. You will also begin to see tiny tube like structures known as spindles forming.Metaphase: The duplicated chromosomes become connected to the now formed spindles at the centromere. The spindles pull the chromosomes tight so that they line up along the middle of the cell, metaphase/equatorial plate. Anaphase: During this phase the duplicated chromosomes are separated. The spindles that are connected to the centromeres of the chromosomes pull each individual chromatid toward an opposite pole of the cell. Once the chromatids have been separated from each other they are recognized as daughter chromosomes.Telophase: Now the separated chromosomes form two groups at opposite poles of the cell. Oncethis occurs, the chromosomes begin to uncoil back into the chromatin form. The spindle fibers begin to break down and a nuclear membrane re-forms around the chromosomes. After cell division occurs, at the end of telophase, cytokinesis occurs. This is the division of the cytoplasm.You can tell this is occurring with the formation of a cell plate in plant cells or a cleavage furrow in animal cells.ProcedurePlant CellYou will obtain a prepared slide or an onion root tip. These cells have been stained to be able to observe cellular structures. Since the root tip is the actively growing region, we are able to observe mitosis in these cells.1) Place the root tip under the microscope and locate it using low power. Once this is in focus, refocus at both medium and high power (remember, do not use the Oil lens)2) You can now scan your slide to identify the various stages of mitosis. Remember that all cells are growing and diving at different rates and therefore you will see all stages of the cell cycle present in the root.3) Using the boxes below as a guide, please draw an example in your notebook of each cycle that you observe in the root tip. Interphase Prophase Metaphase Anaphase TelophaseAnimal CellYou will obtain a prepared slide of the whitefish blastula. The blastula is in the early embryonic stages meaning that the cells are dividing rapidly. You again will be able to observe various stages of mitosis. Again, please located an example of each stage and observe the differences between mitosis in a plant and animal cell.4) What differences do you notice between the stages of mitosis in the whitefish blastula andthe onion root tip?5) Using the circles below as a guide, please draw the stages of mitosis as you observe themin the whitefish blastula. This time, you also need to include specific processes that are occurring during each stage. Interphase Prophase Metaphase Anaphase TelophaseMeiosisMeiosis is a process which only occurs in the gamete cells (egg and sperm). This process must occur so that the genetic material can be divided in half. During sexual reproduction, the organism gets half of their genetic material from their mom and half from their dad. When an egg and sperm cell join during reproduction a zygote is created. This zygote is said to have a diploid number of chromosomes. This can also be written as 2n, meaning that there are 2 complete sets of chromosomes present. Individual egg and sperm cells are considered haploid or1n, meaning that there is only 1 complete set of chromosomes present.Meiosis is similar to mitosis in that there is cellular division occurring. But, there are several differences, the biggest being that at the end of meiosis, 4 gamete cells are formed with a haploid number of chromosomes.During the first


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SC BIOL 110 - Mitosis and Meiosis

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