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Using Paper Chromosomes to Simulate Meiosis Table of Contents Page Teaching notes ii Templates for paper chromosomes iii-vi Student Handout 1-8 Teaching Notes: I use this activity as a laboratory activity for a 100 level Introductory Biology course. Each section of this course has approximately 16 students. During the class meeting before this activity, we discuss sexual and asexual reproduction, the importance of meiosis for sexual reproduction, and the actual process of meiosis. We have also covered the cell cycle, specifically S phase, before discussing meiosis. At this point in the course we have not yet covered Mendelian genetics, so I assume that students do not know about dominant vs. recessive traits, though most college students already understand this concept. Lab Prep: I print, cut and package all chromosomes before class. Templates for the chromosomes are on the next four pages. Students work in pairs for this activity, and each pair receives a large envelope that contains 2 long red strips (chromatids) that encode traits for BLUE EYES and YELLOW FUR 2 long green strips that encode traits for white eyes and tan fur 2 short blue strips that encode traits for five fingers and green ears 2 short yellow strips that encode traits for SIX FINGERS and PURPLE EARS (The color scheme is somewhat arbitrary. I chose these colors because we use a similar color scheme in class the day before when learning about meiosis.) Each pair will also receive four envelopes that are labeled “Sperm” or “Egg.” (Each pair makes four sperm or four eggs.) If there is an odd number of pairs, I also make a set for myself so that there will be an equal number of sperm and eggs. These envelopes are distributed once students are close to completing meiosis. Each pair of students also needs a scissors and transparent tape. Compiling class data: When tallying co-segregation of chromosomes (page 3 of the Student Handout) students record their group’s information on the board. When we are compiling phenotypes of offspring, I write a table on the board that lists all possible phenotypes, students raise their hands if their offspring will have that phenotype, and I count and write the number on the board. (There will be four possible offspring phenotypes in Part 2, sixteen possible offspring phenotypes in Part 3.) Variations of this activity: While I have not yet written a formal activity, I have used these paper chromosomes in a 300-level Genetics class to introduce the concept of genetic linkage. Usually the eye color and fur color genes co-segregate more frequently because they are closer together on the chromosome. Students have also used these models to simulate nondisjunction of chromosomes. ii BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES BLUE EYES YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR YELLOW FUR centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere iii white eyes white eyes white eyes white eyes white eyes white eyes white eyes white eyes white eyes white eyes white eyes white eyes white eyes tan fur tan fur tan fur tan fur tan fur tan fur tan fur tan fur tan fur tan fur tan fur tan fur tan fur centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere iv five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere green ears green ears green ears green ears green ears green ears green ears green ears green ears green ears green ears green ears five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers five fingers centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere green ears green ears green ears green ears green ears green ears green ears green ears green ears green ears green ears green ears v SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS SIX FINGERS centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere centromere PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS PURPLE EARS vi MEIOSIS and SEXUAL REPRODUCTION Objectives: Students will review cell division Students will review meiosis Students will review independent assortment and homologous recombination and explain why they are important for sexual reproduction and the generation of diversity within a population. Activity In your envelope you have eight chromatids from a diploid organism (2n = 4). You will use these chromatids to simulate mitosis, meiosis, and crossing over between homologous chromosomes during meiosis. This activity will start with the cell in the G1 phase of the cell cycle. During G1 each chromosome consists of a single chromatid. Take out one of each color strip. (One red strip, one green strip, one yellow strip, and one blue strip). Each strip represents a chromatid. Because the cell is in G1, the four different colored chromatids represent the chromosomes of the cell. The red strip and green strip represent a pair of homologous chromosomes. The blue strip and yellow strip represent a pair of homologous chromosomes. Q1. Explain one reason why you know red-green and blue-yellow each represent a pair of homologous chromosomes. The boxes on each chromatid represent genes that encode physical traits. The genes in capital letters encode the dominant traits. If a gene in capital letters is present, the offspring will exhibit that dominant trait, if there are one OR two copies present. The genes in lowercase letters encode the recessive traits. If a gene in lowercase letters is present, the offspring will exhibit that recessive trait only if there is not a dominant trait (gene in capital letters) that can take over. 1 Look at one red chromatid, one green chromatid, one blue chromatid, and one yellow chromatid. Examine the traits on these chromatids. What would an organism with these chromatids look like? Fur color Eye color Number of fingers Ear color Part 1: Asexual Reproduction Let’s pretend that this organism reproduces asexually. Its cells divide by mitosis to produce an organism that is identical to itself (i.e. a clone) Before a cell divides (either by mitosis or by meiosis) it must make a complete replica of its DNA during S phase of the cell cycle. During S phase, the cells DNA replication machinery makes an identical sister chromatid for each chromatid in the cell. Simulate that the cell has passed through S phase by taking the remaining four chromatids out of the envelope and taping each pair of identical sister chromatids together. Once the cell has passed through S phase, each of its four chromosomes should be a pair of identical sister chromatids. Simulate mitosis. When you get to Anaphase I, use a scissors to cut the tape apart to represent sister chromatids separating. When you are finished with mitosis, you should have four identical cells. If this organism reproduces asexually, what will all of its offspring look like? Write your answers in the table below. (Double check with your instructor or other people in the classroom.) Fur color Eye color Number of fingers Ear color Q2. Describe the class population of offspring. Does asexual reproduction by mitosis generate any genetic variety (or variability in traits) in the class population? 2 Part 2: Sexual Reproduction, no crossing over during meiosis 1. You will begin this step with a diploid cell that has already progressed through S-phase. Tape the identical sister chromatids together again. 2. Now simulate meiosis without crossing over. Remember that Meiosis is two divisions. It’s OK to ask your instructor or classmates for help. Once you’ve completed meiosis, lay out the four haploid gametes separately and ask your instructor to check. Use a scissors to cut the tape between sister chromatids during Anaphase II. (At the end of Meiosis II you should have four haploid gametes, and each gamete contains two chromatids.) 3. Write below which colors of chromosomes are paired together in your gametes AND write this on the board. Record the data from the entire class in the spaces below: Class data of chromosome pairs in gametes: Number of groups that had gametes containing…. Red and yellow or green and blue chromatids Red and blue or green and yellow chromatids Q3. Why do gametes from different groups have different color combinations of chromatids? Is one combination more “correct” than the other combination? Q4. Explain in your own words what independent assortment means. 4. Once she has checked your gametes, your instructor will give you four envelopes. Each envelope represents an egg or a sperm. Put the chromatids from one gamete into one envelope. Repeat this until chromatids for each of the four gametes are packaged in their own envelopes. (You will have four envelopes, each envelope contains two chromatids: a long chromatid and a short chromatid.) 3 5. When the instructor lets you know, we will simulate mating! a. Give away two envelopes of gametes to other groups that have opposite gametes as you. b. You should receive two envelopes of gametes opposite of yours from other groups. c. Once you have four gametes (two from you; two from other groups), combine the chromatids from two envelopes (an egg and a sperm) in a small pile on the table. You should have two piles. Each pile represents a new diploid offspring. Q5. For each offspring, 2n = 6. Record how each diploid offspring will appear. Remember that the genes in capital letters are the dominant traits, meaning that if these traits are present, the offspring will exhibit these traits if there are one or two copies present. Offspring will exhibit lowercase (recessive) traits ONLY if both alleles (copies) of the traits are lowercase. Record your offspring’s appearances below and on the board. Offspring 1 Offspring 2 Fur color Eye color Number of fingers Ear color In the table below, record the variety seen in the entire class. Class data for Part 2. Traits Number of offspring Yellow fur, blue eyes, six fingers, purple ears Yellow fur, blue eyes, five fingers, green ears Tan fur, white eyes, six fingers, purple ears Tan fur, white eyes, five fingers, green ears Q6. Which method of reproduction—Part I: Asexual Reproduction (mitosis) or Part 2: Sexual Reproduction, no crossing (meiosis and fusion of gametes)--produced a wider variety of offspring? State one explanation for this result. 4 Q7. In your own words and in complete sentences explain why independent assortment of homologous chromosomes during Meiosis I generates diversity. Part 3: Sexual Reproduction, crossing over during meiosis 1. After each pair in the class has recorded the appearances of the offspring, swap chromatids among the groups so you have a complete set of chromosomes—2 red chromatids, 2 green chromatids, 2 blue chromatids, 2 yellow chromatids. Tape the identical sister chromatids together to represent that the cell has progressed through S phase. 2. Now simulate meiosis with crossing over. How to simulate crossing over: 1) During Prophase I, cut one red chromatid anywhere on the chromatid. 2) Take a green chromatid and cut it in the same spot where you had just cut the red chromatid. 3) Tape the red piece you cut off to the green chromatid you had cut. 4) Tape the green piece you cut off to the red chromatid you had cut. 5) Repeat steps 1-4 with a blue chromatid and a yellow chromatid. **You can simulate crossing over as much as you want; just be sure that homologous chromosomes equally exchange DNA! Q8. In your own words explain crossing over between homologous chromosomes. 3. Complete meiosis. Remember that meiosis is two divisions. It’s OK to ask your instructor or classmates for help. Use a scissors to cut the tape between sister chromatids during Anaphase II. (At the end of meiosis II you should have four haploid gametes, and each gamete contains two chromatids.) Once you’ve completed meiosis, lay out the four haploid gametes separately and ask your instructor to check. 4. Once she has checked your gametes, your instructor will give you four envelopes. Each envelope will represent an egg or a sperm. Put the chromatids from one gamete into one envelope. Repeat this until chromatids for each of the four gametes are packaged in their own envelopes. (You will have four envelopes, each envelope contains two chromatids: a long chromatid and a short chromatid.) 5 5. When everyone in the class is ready, we will simulate mating! a. Give away two envelopes of gametes to other groups that have opposite gametes as you. b. You should receive two envelopes of gametes opposite of yours from other groups. c. Once you have four gametes (two from you; two from other groups), combine the contents of two envelopes (an egg and a sperm) in a small pile on the table. You should have two piles. Each pile represents the chromosomes of a new diploid offspring. Q9. For each offspring, 2n = 6. Record how your two diploid offspring will appear. Remember that the genes in capital letters are the dominant traits, meaning that if these traits are present, the offspring will exhibit these traits if there is one or two copies present. Offspring will exhibit lowercase (recessive) traits ONLY if both copies (alleles) of the traits are lowercase (recessive). Record your offspring’s appearances below and on the board. Offspring 1 Offspring 2 Fur color Eye color Number of fingers Ear color 6 7. In the space below, design a table to report the variety of offspring within the entire class. 7 Q10. Compare the variation of offspring from Part 2: Sexual Reproduction, no crossing over to the variation of offspring from Part 3: Sexual Reproduction with crossing over. Offspring from which part-Part 2 or Part 3--displayed more variety? Why do you think this is? Q11. Explain in your own words why crossing over (meiotic recombination) causes genetic diversity. 8
