Explain the purpose and location of meiosis in organisms.

Explain the purpose and location of meiosis in organisms.

Explain the purpose and location of meiosis in organisms. 150 150 Nyagu

EXERCISE 9.3 The Process of Meiosis Objectives After completing this exercise, you should be able to 1. Explain the purpose and location of meiosis in organisms. 2. Describe and recognize the phases of meiosis. 3. Explain how the process of meiosis results in genetic variation of offspring. Mitosis produces an exact copy of a cell and is an essential process for growth and repair in multicellular organisms. But multicellular organisms also need a special type of cell division to produce gametes (eggs and sperm), and that process is called meiosis. Unlike mitosis, which occurs throughout the body, meiosis occurs only in specialized reproductive tissues. One outcome of meiosis is reduction of chromosome number. Look again at Figure 9.1. the human karyotype, which shows the homologous pairs of a diploid cell. Gametes made from this cell will be haploid with 23 chromosomes. How should 23 chromosomes be selected to go into each gamete? They are selected at random. To see the impact of this randomness, try the exercise below. On the karyotype in Figure 9.1, randomly label each chromosome as maternal (M) or paternal (P). Then randomly choose one member of each pair to put in a gamete (or flip a coin to choose) and write your selections (M or P) below for each chromosome. _ _ _ _ _ _ _ _ _ __ __ __ __ __ __ __ __ __ __ __ __ __ __ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 2122 sex If you did this a second time, would the results be the same? Enormous genetic variation of gametes is another outcome of meiosis. Again, each chromosome is a DNA molecule that encodes information and each segment of information is called a gene. The position of the gene is fixed, for the most part, on the chromosome, at a site called a locus. Distributed on the 46 chromosomes in the karyotype in Figure 9.1 are the 30,000 to 40,000 genes that make up the human genome, the set of instructions that produces a human being. In the next lab topic you will consider the distribution of genes during meiosis, but for now we are going to examine how the chromosomes themselves are sorted into gametes. 223 Activity A: Pop Bead Simulation of Meiosis To see how the process of meiosis results in haploid gametes that differ from each other genetically, you will take a cell through the steps of meiosis as you did for mitosis. The mechanisms of mitosis, such as the centrosomes and spindle fibers, are also used in meiosis, and the same names — prophase, metaphase, anaphase, telophase — are given to the “snapshot” phases. But meiosis will result in four cells rather than two, so two divisions take place. The two divisions are designated meiosis I and meiosis 11. Also, while many different types of cells can undergo mitosis, only specific cells can undergo meiosis. In animals, meiosis is carried out in the cells that produce gametes. In plants, meiosis produces spores (pollen). Where in the human body does meiosis occur? Procedure Begin with the same three homologous pairs of chromosomes that you used for mitosis. Interphase Before meiosis begins, the chromosomes must replicate. Make an exact copy of each of your chromosomes, then draw the interphase nucleus in the cell below. Prophase I As in mitosis, the chromosomes become visible and the nuclear envelope disintegrates during this phase. But something very different happens, too. Homologous pairs associate closely with each other, even to the point of entwining their “arms.” Each pair of pairs is called a tetrad. Group your pop bead chromosomes into tetrads. 224 While they are in close contact, the chromatids of homologous chromosomes can exchange pieces of DNA. The size of these exchanged segments varies. You can simulate this process, called crossing over, by popping a few beads off of one chromatid and exchanging them for the same number of beads on a chromatid from its homologue. What is the result of crossing over in a homologous pair? (Hint: are all the genes in one sister chromatid now from the same parent?) Sketch prophase I below Metaphase I Recall that metaphase of mitosis is when the chromosomes are attached to the spindle apparatus and aligned along the center of the cell. Metaphase I in meiosis is similar, but each tetrad stays together. Line up your tetrads and sketch them below. 225 Anaphase I In anaphase I, homologous pairs are separated from each other and move to opposite poles, but sister chromatids remain attached. How does this differ from anaphase of mitosis? Move your chromosomes to simulate anaphase I and sketch it below Telophase I and Cytokinesis Once the homologous pairs of chromosomes are at opposite poles, the cytoplasm divides to form two daughter cells. Move your chromosomes into the two new daughter cells that will be formed when cytokinesis is completed. Make a sketch below 226 List by color the chromosomes that are in each daughter cell you made. For example, if your chromosomes are red and yellow, your daughter cells might be red, red, yellow and yellow, yellow, red. (For chromosomes that underwent crossing over, list the predominant color.) Check with other students to see whether their daughter cells are the same as yours. If there are different combinations, can you explain why? (Hint: Review the metaphase I sketch of someone whose daughter cells turned out differently than yours.) Crossing over during prophase I is one source of genetic variation that results from meiosis. The random alignment of tetrads at metaphase I is another source. With only three pairs of chromosomes, there are eight different combinations of chromosomes in the gametes (two for each possible alignment). Imagine how many possibilities exist with 23 pairs of chromosomes! Each cell that results from meiosis I has one member of each homologous pair, which is the desired outcome for gametes. Why isn’t this the end of meiosis? Prophase II During prophase II, a spindle apparatus forms again. 227 Metaphase II With each chromatid attached to spindle fibers, the chromosomes line up in the center of the cell. . Anaphase II During this anaphase, the sister chromatids move to opposite poles. 228 Telophase II and Cytokinesis The division of chromatids is completed, and the cytoplasm divides to form new daughter cells. Move your chromosomes into the new daughter cells that will be formed when cytokinesis is completed. Make a sketch below. How many chromosomes does each cell have? How many homologous pairs does each cell have? Are these cells haploid or diploid? List by color the chromosomes that are in each daughter cell you made. Check with other students to see whether their gametes turned out the same as yours. Make a complete list of all possible chromosome combinations in the gametes. 229 Activity B: Viewing Meiosis in Organisms You will view professionally prepared slides to see what actual cells look like during the phases of meiosis. If you were to use animal tissue for this exercise, what parts of the animal would be used to show meiosis? If you were to use plant material, which parts would you look at to see meiosis? (Hint: Where does sexual reproduction take place?) Procedure 1. Your instructor will supply you with prepared microscope slides to examine. Begin by using scanning power to locate the specimen. Switch to low power to identify an area that includes cells undergoing meiosis. 2. Using high power, sketch the positions of the chromosomes during different stages of meiosis in the spaces below. Label your drawing with the name of the stage, as best you can identify it. Why isn’t it as easy to identify the stages of meiosis in cells as it was to identify the stages of mitosis? 230 Questions for Review Answer these questions about your experiences in Lab Topic 9. Use other paper or a computer if you wish. 1. What purpose does mitosis serve in development and growth? (Hint: A zygote is the first cell formed after conception, but by adulthood a human body has many trillions of cells.) 2. After an adult has all his trillions of cells, does he still need the process of mitosis? Explain. 3. Which phase(s) of mitosis could the karyotype in Figure 9.1 have been made from? Explain your reasoning. 4. Besides the root tips, where else in plants would you expect mitosis to take place? 5. Meiosis is from a Greek root meaning “to diminish or lessen.” Why is this an appropriate term? 6. Summarize the sources of genetic variation in meiosis. 7. Which phase of meiosis is responsible for the genetic variation of gametes demonstrated in the introduction to Exercise 9.2? 8.Most people who have Down syndrome have an extra copy of chromosome 21 (trisomy 21), which is a result of an egg or sperm that carried an extra copy of chromosome 21. How could a gamete with an extra chromosome come about? 9. You have homologous pairs of chromosomes because you got one member of each pair from each parent. Your parents got their chromosomes from your grandparents, and so on. But are the chromosomes that are passed along through the generations exactly the same? From example, is your maternal chromosome #1 exactly the same one that your mother received from one of her parents? Explain. 231 232 LAB TOPIC 10 Heredity : Classic Genetics Adapted from Perry, Morton and Perry, Laboratory Manual for General Biology (8th Edition), Dickey, and Peter Lanzetta, (Bio 13 Laboratory Manual 2006) Laboratory Objectives After completing this exercise, you will be able to 1. Define genotype, phenotype, diploid, haploid , dominant, recessive, homozygous, heterozygous, true-breeding, hybrid, monohybrid cross, dihybrid cross, law of segregation, complete dominance, probability 2. Solve monohybrid and dihybrid cross problems; 3. Determine your phenotype and give your probable genotype for some common traits. 4. Be able to use the stereo (dissecting) microscope to recognize and be able to identify sex differences, the results of mutations, and (if requested) the metamorphic stages in fruit flies. Outline Pre-Laboratory Definitions Exercise 10.1 Activity A. Monohybrid Crosses Activity B. Predicting the Outcome of a Monohybrid Cross Exercise 10.2 Activity A. The Chromosomal Basis of Independent Assortment Activity B. Predicting the Outcome of a Dihybrid Cross Exercise 10.3 Activity A. Some Readily Observable Human Traits Exercise 10.4 Activity A. Microscopy: Theory and Practice of the Dissection (Stereo) Microscope Activity B. Drosophila Phenotypes Activity C. Protein Molecular Genetics: Fly Eye Pigment Chromatography • Before to coming to lab, you should read the corresponding chapter in your textbook and this laboratory exercise. • Use those sources to write a definition of the 12 terms below. 233 Gene: Locus: Allele: Dominant allele: Recessive allele: Genotype: Phenotype: Homologous chromosomes: Diploid: Haploid: Heterozygote: Homozygote: Introduction In 1866, an Austrian monk, Gregor Mendel, presented the results of painstaking experiments on the inheritance of the garden pea, but the scientific community ignored them, possibly because they didn’t understand their significance. He determined that diploid individuals have two alternate forms of a gene (two alleles, in modern terminology) for each trait in their body cells. Today, we know that alleles are on chromosomes (Figure 10.1). Figure 10.1. A few genetic terms illustrated. 234 An individual can be homozygous dominant (two dominant alleles, GG), homozygous recessive (two recessive alleles, gg) or heterozygous (one dominant and one recessive allele, Gg). Genotype refers to an individual’s genes, while phenotype refers to an individual’s appearance. Homozygous dominant and heterozygous individuals show the dominant phenotype; homozygous recesive individuals show the recessive phenotype. Now, more than a century later, Mendel’s work seems elementary to modem-day geneticists, but its importance cannot be overstated. The principles generated by Mendel’s pioneering experimentation are the foundation for the genetic counseling so important today to families with genetically based health disorders. They are also the framework for the modern research that is making inroads into treating diseases previously believed to be incurable. In this era of genetic engineering-the incorporation of foreign DNA into chromosomes of other species-it’s easy to lose sight of the concepts underlying the processes that make it all possible. These experiments and genetics problems should give you a good basic understanding of these processes. Assume that the chromosomes shown in Figure 10.1 are those of a fruit fly, and G = gray body and g =ebony (black) body. What is the genotype of the fly?________ What is the phenotype of this fly?_________ Explain_______________________________________________________________ Activity A. Monohybrid Crosses For his experiments, Mendel chose parental plants that were true-breeding, meaning that all self-fertilized offspring displayed the same form of a trait as their parent. When parents that are true-breeding for different forms of a trait are crossed — for example, purple flowers and white flowers — the offspring are called hybrids. When only one trait is being studied, the cross is a monohybrid cross. We’ll look first at monohybrid problems and crosses. Procedure 1. Most organisms are diploid; that is, they contain homologous chromosomes in pairs with genes for the same traits. The location of a gene on a chromosome is its locus (plural: loci). Two genes at homologous loci are called a gene pair. Each chromosome has numerous genes, as we saw in Figure 10.1 (above). Gametes, on the other hand, are haploid; they contain only one of the two homologues and thus only one of the two alleles for a specific trait. According to Mendel’s first law of inheritance, the law of segregation, each organism contains two alleles for each trait, and the alleles segregate (separate) during the formation of gametes during meiosis. Each gamete then contains only one allele of the pair. 235 The genotype of an organism represents its genetic constitution-that is, the alleles present, either for each locus, or taken cumulatively as the genotype of the entire organism. In Table 10.1, for each of these diploid genotypes, indicate all possible gamete genotypes that can be produced by the organism: Diploid Genotype Potential Gamete Genotype(s) GG Gg gg Table 10.1 Gamete genotypes 2. During fertilization, two haploid gamete nuclei fuse, and the diploid condition is restored. Give the diploid genotype produced by fusion of the following gamete genotypes. Gamete Genotype Gamete Genotype G G G g g g Diploid Genotype(s) Table 10.2 Diploid genotypes Activity B: Predicting the Outcome of a Monohybrid Cross 1. The genotype is the actual genetic makeup of the organism. The phenotype is the outward expression of the genotype-that is, what the organism looks like because of its genotype, as well as its physiological traits and behavior. (Although phenotype is determined primarily by genotype, in many instances environmental factors can modify phenotype.) Human earlobes are either attached or free (Figure 10.2). This trait is determined by a single gene consisting of two alleles, F and f. An individual 236 whose genotype is FF or Ff has free earlobes. This is the dominant condition. Note that the presence of one or two F alleles results in the dominant phenotype, free earlobes. The allele F is said to be dominant over its allelic partner, f. The recessive phenotype, attached earlobes, occurs only when the genotype is ff. In the case of complete dominance, the dominant allele completely masks the expression or effect of the recessive allele. When both alleles in a nucleus are identical, the nucleus is homozygous. Those with both dominant alleles are homozygous dominant. When both recessives are present in the same nucleus, the individual is said to be homozygous recessive for the trait. When both the dominant and recessive alleles are present in a single nucleus, the individual is heterozygous for that trait. Figure 10.2 Free and attached earlobes in humans. A man has the genotype FF. What is the genotype of his gamete (sperm) nuclei?_____ A woman has attached earlobes. What is her genotype?_________________________ What allele(s) does her gametes (ova) carry?____________ These two individuals produce a child. Show the genotype of the child by doing the cross: sperm genotype X ovum genotype child’s genotype What is the phenotype of the child? (That is, does this child have attached or free earlobes?)_______________ 2. In garden peas, purple flowers are dominant over white flowers. Let A represent the allele for purple flowers, a the allele for white flowers. What is the phenotype (color) of the flowers with the following genotypes? Note: Always distinguish clearly between upper and lowercase letters. Genotype AA Phenotype aa Aa 237 3. When the genotypes of the parents are known, we may determine what gametes the parents can make and in what proportion the gametes will occur. This information allows us to predict the genotypes and phenotypes of the offspring. The prediction is simply a matter of listing all of the possible combinations of gametes. We are still concentrating on monohybrid crosses, so only one trait is followed. By convention, the parental generation is called P. The first generation of offspring is called F1. F stands for filial, which refers to a son or daughter, so F1 is the first filial generation. If members of the F1 generation are crossed, their offspring are called the F2 generation, and so on. The Punnett square was devised to keep track of all possible conbinations of genotypes when more than one type of gamete can be produced. Genotypes of gametes are entered along the edges, and the boxes in the middle contain the genotypes of the offspring. Fill out the Punnett square in Figure 10.3 by crossing F1 offspring (Gg x Gg) (remember, these are the body color genes of the fruit flies from earlier in the Topic). Figure 10.3 Punnett Square of F1 x F1 cross A. What are the possible genotypes in the F2 generation? B. What is the phenotype of each genotype in the F2 generation? C. What is the phenotypic ratio for this cross? (that is, the ratio of grey to ebony flies) 238 4. A heterozygous purple plant (Pp) is crossed with a white-flowered plant (pp). Fill in the Punnett square, then give the genotypes, phenotypes, and phenotypic ratio of all the possible genetic outcomes. (This is known as a test cross.) Genotypes____________________ Phenotypes___________________ Phenotypic ratio_______________ 5. It’s unlikely that every cross between two pea plants will produce four seeds that will in turn grow into four offspring every time. Rather, one of the most useful facets of problems such as these is that they allow you to predict the chances of a particular genetic outcome occurring. Genetics is really a matter of probability, the likelihood of the occurrence of a particular outcome. To take a simple example, consider that the probability of coming up with heads in a single toss of a coin is one chance in two, or 1/2. Now let’s apply this idea to the probability that offspring will have a certain genotype. Look at your Punnett square in Problem 3. The probability of having a genotype is the sum of all occurrences of that genotype. For example, the genotype Gg occurs in two of the four boxes. The probability that the genotype Gg will be produced from that particular cross is thus 2/4, or 50%. It is half of whatever number of offspring are produced (that is, 50 out of 100 for example, or 10 out of 20). What is the probability of an individual from Problem 3 having the genotype gg?________________ Exercise 10.2 Activity A. The Chromosomal Basis of Independent Assortment Genes that are located on the same chromosome are linked with each other. if genes are located on separate, nonhomologous chromosomes, they are not linked, or unlinked. Unli…
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