Meiosis is the process by which eukaryotic organisms produce egg and sperm having half the genetic information (haploid) of the other cells in the organism’s body (diploid). You will notice some similarities to mitosis but some definite differences in meiosis that result in genetic diversity in the gametes as well as the offspring formed by the eventual fusion of the egg and sperm.
Two of the most noted differences in meiosis are that 1) there are two complete divisions resulting in four daughter cells and, 2) meiosis has crossing over, an even that occurs in Prophase I that increases genetic variation in the gametes.
Crossing over occurs between homologous chromosomes. These are the pairs of chromosomes in an organisms body that carry genes for the same trait located at identical positions on the two chromosomes. Even though they carry the same gene they may, however, possess different alleles for the gene which result in different forms of the trait.
1. Draw a tetrad in the space below and label the following: centromeres, sister chromatids, homologous chromosomes.
2. What are the two elements of meiosis that add variation to our population? (Hint: one occurs in prophase I and the other in metaphase I).
3. Which of the chromosomes #2-5 could be homologous with chromosome #1? Explain your answer. I think number 5 because it is the exact same as 1.
1. First view the recording on my message board under “Supplemental Learning Materials” for this lab. You will need the “code words” from the recording for one of your conclusion questions.
Link to recording:
http://www.connectionslivelesson.com/p2fwrdno8yk/ 2. After you have viewed the recording, gather your required materials. You will need 4 forks, 4 knives, 4 spoons, 6 rubber bands, and some twine or string to use as cell membrane. If you can have two different types of flatware that would be best (for example: 2 forks, 2 knives, and 2 spoons that are different than the other set of 2 forks, 2 knives, and 2 spoons). 3. Your model cell consists of 6 chromosomes (3 homologous pairs). Use the string to form the cell membrane for your cell(s). Begin with two forks, two knives, and two spoons (one of each from each set) inside your string cell membrane as shown in the recording. Proceed through each of the steps of meiosis using your flatware “chromosomes.” Use the textbook pages 324-325 for help. Phase
Replicate the DNA of your chromosomes by adding a second fork, knife, or spoon to each existing fork, knife, and spoon. Hold each pair of “chromatids” together with a rubber band (centromere). Prophase I
Create tetrads (XX) by pairing homologous chromosomes (set them next to each other). Model crossing over as best you can given the model we are using. Metaphase I
Line up your tetrads along the metaphase plate. Model independent assortment. Anaphase I
Move the homologous chromosomes to opposite ends of the cell. Telophase I
You should have three chromosomes, knife, fork, and spoon, (each composed of two chromatids) in each of your two nuclei. Cytokinesis I
Divide your cell membrane so that you have two daughter cells (two string circles for cells). Prophase II
Check to make sure that each of your two cells contains three chromosomes made up of sister chromatids. Metaphase II
Line your chromosomes up along the middle of each of your cells. Anaphase II
Separate your sister chromatids (remove rubber band holding them together). Move one sister chromatid from each chromosome to opposite sides of your cell. Telophase II
You should now have four nuclei, with three single chromosomes in each of the cells. Cytokinesis II
Divide your cell membranes so that you have four daughter cells (use more string).
Take 3 photos or create 3 drawings of your model as it looked in 3 different stages of meiosis as described in the table above. Indicate which stage of meiosis is shown in each photo/drawing. Include the photos or a scan of the sketches when you upload your lab to the drop box.
Conclusion (6 points)
1. List the two code words that were given in the lab recording found on my message board.
Chromosomes and fertilization????
2. Does mitosis more closely resemble meiosis I or meiosis II? Explain your answer.
More like Meiosis I because it only made two cells.
3. What is crossing over? When does it occur during meiosis? Why did you have difficulty modeling this element of meiosis using your “chromosomes?”
Crossing over is the process in which homologous chromosomes exchange portions of their chromatids during meiosis. It occurs during prophase I. I had a hard time because you can’t really change the chromosomes so it was difficult to show.
4. Think about your modeling. Visually, what is the major difference between the separation of chromosomes between anaphase I and anaphase II
There were four to separate then just two.
5. What is independent assortment? When does it occur during meiosis? How
did you model this element of meiosis?
6. Which part of meiosis, meiosis I or meiosis II, is responsible for reducing the chromosome number by half in the cell? MEIOSIS II
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