Sexual reproduction and gamete diversity or how to make unique babies without really trying.

Transcription

1 Meiosis lab 3 Sexual reproduction and gamete diversity or how to make unique babies without really trying. Cautions and Pitfalls Follow the directions step-by-step for each stage of meiosis. Ask your instructor if you are unsure before proceeding to the next step. Objectives Upon completion of this activity, you should be able to: 1. Explain why meiosis is necessary in sexually reproducing organisms. 2. Diagram and/or describe the activity of chromosomes during Meiosis I and Meiosis II. 3. Compare and contrast mitosis and meiosis. 4. Demonstrate crossing over, segregation and independent assortment and describe how each contributes to genetic variation. Introduction If a sexually reproducing species is to retain the original number of chromosomes from generation to generation, then the parents must have a mechanism for producing haploid (n) cells with half the species specific number of chromosomes. The production of these haploid cells is called meiosis. Meiosis consists of two rounds of cell division and occurs only in the germ cells of sexually mature organisms. Germ cells are diploid cells with the potential to form gametes and are only found in the reproductive organs of sexually reproducing species. Thus we would find germ cells in the ovaries and testes of animal species, in the anther and pistil of flowering plants, and in the sporangia of many fungi. Meiosis is a specialized kind of cell division that takes place in multicellular organisms in preparation for sexual reproduction. In meiosis, the nuclei of certain cells found in the reproductive organs divide twice, but the chromosomes replicate only once. This process results in four daughter nuclei with differing chromosomes and genes called eggs, sperm, spores, pollen, or ovules depending on the parent organism. During fertilization, offspring receive one set of chromosomes from each parent restoring the diploid number of chromosomes. A human zygote contains 23 chromosomes from dad (paternal) and 23 chromosomes from mom (maternal) while a fruit fly receives 4 chromosomes from dad and 4 from mom. Except for monozygotic twins we all inherit a unique combination of chromosomes. The remarkable diversity of form and function that multicellular organisms display is all stored in the original fertilized egg, or zygote.

2 Procedure To understand the mechanism involved in the production of gametes and to recognize sources of genetic variation, we will simulate meiosis using another type of model: chromosomes models. Work in pairs and obtain four model chromosomes that have already completed DNA replication and are therefore composed of identical sister chromatids. Model Chromosomes 1. Note the paternal and maternal set of chromosomes. 2. Identify the homologous pairs. 3. What is the diploid number of chromosomes in our simulation? 4. Observe that each of the 6 genes that we will follow has a specific locus. 5. The models represent chromosomes that have already replicated. Geneticists designate this genetic constitution as 2n/4c or diploid/4 DNA strands per tetrad. If this designation is confusing to you discuss it with your instructor. 6. Assume that dominate alleles are represented by all capital letters and that lower-case lettering represents a recessive allele. Determine the genotype and phenotype for each of the six traits. Trait Genotype Phenotype Cystic Fibrosis (C) CC CC No cystic fibrosis

3 Ear Shape (E) Finger Number (F) Blood Type (B) Insulin Production (I) Hair Color (D) The actual genotype for every cell (organism) under study is: CCEeFfBoIiDd This is the genetic makeup for all interphase cells in which DNA replication has not occurred. Therefore we will all begin this lab activity as clones. Your model chromosomes represent those found in a single germ cell that has already completed S of interphase, thus DNA replication has occurred producing double copies of all the genes.

4 7. All the genes along the same sister chromatid are called a linkage group. These genes are physically linked together on the same chromosome because they are found on the same DNA helix. Pick up your large maternal chromosome and write down the order of the linkage group found on this chromosome. Keep in mind that linkage groups can be altered during the process of meiosis. Linkage group on large maternal chromosome 8. Use a large sheet of newspaper to represent the cell cytoplasm. You are now ready to simulate the process of meiosis. MEIOSIS I Reduction Division Prophase I. Synapsis and crossing over. Pair up homologous chromosomes. Because the chromosomes are double-stranded each homologous pair will be composed of four strands of DNA and can be called a tetrad. Crossing over is the exchange of equivalent segments of DNA an can occur several times in a prophase cell. The crossover site forms a chiasma or X shape at the point of exchange. Simulate this by completing a total of three crossovers within your genome.

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6 If you are having difficulty envisioning the behavior of chromosomes during any part this meiosis simulation discuss the events with your lab partner, if needed, ask questions of your instructor before proceeding. Metaphase I. Homologous pairs line up independent of other homologous pairs. Demonstrate two possible ways that homologous pairs could align along the midline of your cell.

7 Anaphase I. Homologous chromosomes separate. This is when segregation and independent assortment occur. Can you demonstrate the concept of independent assortment?

8 Telophase I. Cytokinesis - cut or tear your paper to simulate division of the cytoplasm. Table 2. Comparison of Genetic Data

9 Long Xsome

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12 MEIOSIS II Equilateral Division Prophase II. no replication of DNA, no interphase (interkinesis). The cells prepare for a second round of division.

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15 Metaphase II. Chromosomes consisting of two strands of DNA line up at the midline.

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19 Anaphase II. Spindle fibers attach to the kinetochore region and pull sister chromatids apart, now referred to as chromosomes. Simulate this in each cell. Telophase II. Results in the formation of four haploid cells called gametes. Divide your paper into four separate cells. n/1c Gamete 1 ong

20 Fertilization. See your instructor for guidance. Those gametes designated as male will carry an X or a Y chromosome in addition to the six genes we have been working with. Gametes derived from females will carry only X sex chromosomes. Use a pen or pencil to mark each gamete with a single sex chromosome. Male gametes will then swim toward female gametes to simulate the process of fertilization. Caution: Once a zygote is formed by fusion of the chromosomes from the male parent with the chromosomes from the female parent fill in Table 4 below. Do not mix-up male and female derived chromosomes or it will be difficult to return the chromosome sets in their original configuration.

21 Cystic Fibrosis Ear Shape Finger Number Blood Type Insulin Production Hair Color Sex Chromosomes Name your offspring and put its genotype and phenotype on the board for a class discussion on genetic diversity. Arrange the chromosome models as they were when you first received them, then answer the post-lab questions.

22 Name Meiosis Post Lab Questions Answer as completely as possible. 1. How many hypothetical offspring produced during this lab activity had the same genotype? The same phenotype? 2. List all of the factors observed in this simulation that contributed to genetic variation among the offspring. 3. Many people use the term gene and allele interchangeably. Are they the same? Explain 4. Name the most significant event of meiosis. This process ensures that meiosis is different from mitosis. 5. When does DNA replication take place in both mitotic and meiotic cells? 6. How do the results of meiosis and mitosis differ in terms of chromosome number? Fill in the diagram below using 46 human chromosomes. 7.