Meiosis Project PDF

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Meiosis project for beginning of the course...

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INSTRUCTIONS: MEIOSIS MODELING This activity teaches the important events that take place in Meiosis and explains how they lead to genetic diversity. Prior to meiosis, the cell must go through interphase to grow and replicate the chromosomes in order to prepare for meiosis. Meiosis is a process that only occurs in a male testes and female ovaries. It starts with a specialized diploid cell (2N) called a germ cell, after two divisions it becomes a set of 4 haploid cells called gametes. Haploid cells eventually form into either sperm or eggs. With luck, a single sperm and egg meet during fertilization and combine their chromosomes to create a new fertilized diploid cell that has the potential to grow and develop into a new genetically unique individual. Since each sperm or egg cell is haploid, they contain only one half the parent genome. The fertilized embryo is made up of one half of the maternal genome and one half of the paternal genome. This genetic uniqueness is nearly infinite and primarily results from two critical processes that occur during Meiosis I: crossing over and random alignment of homologous chromosomes. Crossing over involves the exchange of segments between homologous chromosomes. Random alignment is due to the alignment of maternal and paternal chromosomes that will be divided into daughter cells. While the number of combinations from crossing over cannot be determined, for random alignment the formula 2^n is used where n represents the number of chromosomes. In this activity, you will make chromosomes from modeling clay and simulate meiosis so that you can understand genetic mixing. You will use only 6 colors to make 6 chromosomes arranged in 3 homologous pairs. The lighter color in each pair will represent the maternal chromosomes. To complete the assignment, you should submit a total of 4 pictures. Picture #1 – Prophase I. Picture #2 – Crossing over. Picture #3 – Random alignment. Picture #4 – End of Meiosis II. You also need to submit the answers to all five ponder questions.

You will need the following     

1 1 1 1 1

ball ball ball ball ball

of of of of of

pink clay (18g)—maternal chromosome 1 red clay (18g)—paternal chromosome 1 light blue (Sky) clay (14g)—maternal chromosome 2 blue clay (14g)—paternal chromosome 2 light green (Lime) clay (10g)—maternal chromosome 3

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1 ball of green clay (10g)—paternal chromosome 3 5 panels representing different stages of meiosis (attached) 2 plastic knives 1 penny Digital camera

1. Roll each of the balls of clay into a strand that is 5/16 of an inch thick. Use the spacer bars on the handout to determine the proper thickness. Make the long strands from pink and red, the medium strands from blue and sky and the short strands from green and lime. 2. Next, create a duplicated maternal chromosome 1. Cut the pink strand into two 3.75-inch segments. Use the cutting guides for measuring length (bottom of panels 2–5). Place the two segments side by side and lightly pinch them together in the middle to create a metacentric centromere. Do not pinch too tightly or they will be difficult to separate later (during cell division). Repeat the steps to make a duplicated paternal chromosome 1 with the red clay. Place the two chromosomes randomly on the Panel 1 sphere. 3. Make the maternal and paternal duplicated chromosome 2 from the sky and blue clay, respectively. Chromosome 2 is 2.75 inches long. It also has a metacentric centromere meaning the two strands are pinched in the middle. Place the two chromosomes randomly on the Panel 1 sphere. 4. Make the maternal and paternal duplicated chromosome 3 from the Lime and Green clay, respectively. Chromosome 3 is 1.75 inches long. It is has a telocentric centromere meaning the two strands are pinched closer to the end of the chromosome. Place the two chromosomes randomly on the Panel 1 sphere.

Take picture #1

5. Your panel 1 now represents the cell in the early stages of meiosis I. The nuclear membrane protecting the DNA is lost and the chromosomes are condensed. The next step involves each chromosome finding and linking with its homologous partner to form a structure called a tetrad because of the four pairs of arms. Each tetrad migrates and lines up along the center of the cell.

Take picture #2

6. Now that your chromosomes are paired with their homolog, you will simulate crossing over. Crossing over takes place in areas where the arms of two homologous chromosomes have stitched together to form a synapse. A special protein enzyme called Recombinase comes along and cuts the DNA in each chromosome, swaps the ends and then reseals the new pieces together. The result is a transfer of part of the maternal chromosome to the paternal chromosome and vice versa. This increases diversity. Simulate the action of recombinase during crossing over by cutting one arm from each chromosome, and then resealing the broken piece to the “stump” on the opposite homolog. Make sure you cut the chromosome at the same spot on each homolog or you will irreversibly remove information from one chromosome and duplicate it on the other. Also try and be random in your cut site. There is a tendency to always cut in the exact middle of the arm. Be more creative than that. Crossing over can actually occur more than once on each pair of homologs. If you feel adventurous, you can try a second crossing over on one of your homologous pairs. 7. During random alignment the maternal chromosome of one pair may end up on the either the right or the left side of the cell. The second chromosome pair may line up in the same or opposite orientation—the same for the third, fourth, etc. This random alignment of each tetrad creates potentially different combinations of maternal and paternal chromosomes in the resulting daughter cells. To simulate random alignment, pair each of your chromosomes with its homolog by overlapping an “arm and a leg” from each set. Then place the pair in the middle of the cell according to a coin toss and the following rules: If the toss is heads, place the maternal chromosome on the right; if tails, place the maternal chromosome on the left. The coin toss emphasizes the random nature of the alignment process.

Take picture #3

8. Now, separate your homologous pairs and perform cell division. Remember: Meiosis I separates homologous pairs of duplicated chromosomes. Simulate this end of this process by separating each pair and transferring the set on the right to panel 2 and the set on the left to panel 3. Place each chromosome randomly inside its new panel. 9. After Meiosis I your original diploid germ cell split into two new cells.

The next step is Meiosis II. Align each chromosome along the middle of the cell. Now separate the twin copies of each chromosome made during replication. (Each twin copy is a sister chromatid). Cut or pull apart the centromere and place each copy (chromatid) on opposite sides of the cell. Next, transfer each newly separated chromosome to one of the cells drawn on panels 4 and 5. That’s it! Meiosis II is complete. We now have four new haploid cells called gametes. Note that each of your four cells represents a different combination of genetic material.

Take picture #4

Ponder Questions 1. Using only random alignment, how many different combinations do you think are possible for a cell like ours but with only 3 pairs of chromosomes instead of 23 pairs of chromosomes? Explain. If the normal is 2^23=8,400,000 combinations. With 3 would be 3^23= 94,143,178,827 2. T/F. Chromosomes must be replicated during interphase prior to starting meiosis. True 3. Below is a scrambled list of events for meiosis. Number the events in the correct order in which they occur. 5. . Chromatids separate and produce four haploid daughter cells 1. Nuclear envelope dissolves, chromosomes condense, and homologous chromosomes pair together 2. Homologous chromosomes are randomly aligned along cell equator 3. Crossing over between homologous chromosomes occurs 4. Homologous chromosomes separate and are separated in two diploid daughter cells 6. What does it mean when a biologist uses the term crossing over? Why is crossing over a good thing? Crossing over is the process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis At that stage each chromosome has replicated into two strands called sister chromatids. Crossing over is essential for the normal segregation of chromosomes during meiosis. Crossing over also accounts for genetic variation, because due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical. 7. Your unique genetic identity comes from three different events: crossing over, random alignment, and random fertilization. Explain what each of these are and how they lead to genetic diversity. Crossing over is the process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis At that stage each chromosome has replicated into two strands called sister chromatids. Random alignment leads to new combinations of traits. The chromosomes that were originally inherited by the gamete-producing individual came equally from the egg and the sperm. In

metaphase I, the duplicated copies of these maternal and paternal homologous chromosomes line up across the center of the cell. Random fertilization refers to the fact that if two individuals mate, and each is capable of producing over 8million potential gametes, the random chance of any one sperm and egg coming together is a product of these two probabilities - some 70 trillion different combinations of chromosomes in a potential offspring....