BioSci 2200 General Genetics Problem Set 1 Answer Key Introduction and Mitosis/ Meiosis

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1 BioSci 2200 General Genetics Problem Set 1 Answer Key Introduction and Mitosis/ Meiosis Introduction - Fields of Genetics To answer the following question, review the three traditional subdivisions of genetics from Chapter 1, the lecture notes and the syllabus, then read the story about albinism in the Hopi tribe of Native Americans on pg 1-2 of the book. Additional Q1 1. Which aspects of the story about Albinism in the Hopis illustrates each of the three major fields of genetics. Answer: a) Transmission or Mendelian Genetics Determining that albinisim is inherited as an autosomal recessive trait. b) Molecular Genetics Determining that albinism is due to a lack of production of melanin likely caused by a mutation in the OCA2 gene on chromosome 15 c) Population Genetics Determining that the frequency of albinism in the Hopis is one hundred times more frequent than other populations and what forces may have contributed to the increased frequency (positive selection, small population size). Mitosis and Meiosis Once you have convinced yourself that you know what occurs at each stage of mitosis and meiosis, test your understanding by solving the following problems from Chapter 2. To compare and contrast mitosis and meiosis, it may be helpful to review Fig and Connecting concepts pg List the similarities and differences between mitosis and meiosis. Which differences do you think are most important and why? Mitosis A single cell division produces two genetically identical progeny cells. Chromosome number of progeny cells and the original cell remain the same. Daughter cells and the original cell are genetically identical. No separation of homologous chromosomes or crossing over takes place. Meiosis Two cell divisions usually result in four progeny cells that are not genetically identical. Daughter cells are haploid and have half the chromosomal complement of the original diploid cell as a result of the separation of homologous pairs during anaphase I. Crossing over in prophase I and separation of homologous pairs during anaphase I produce daughter cells that are genetically different from each other and from the original cell. 1

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3 Additional Q2 2. The cells in the following figure were all taken from the same individual (a mammal). Identify the cell division events occurring in each cell and explain your reasoning. What is the diploid chromosome number? Answer: To reconcile how all the cells illustrated could come from one individual, consider that the cells shown could come from either somatic or germ-line cells. Cell (a) shows three pairs of previously synapsed homologs disjoining and must therefore illustrate anaphase I of meiosis. That three pairs of chromosomes are present indicates that the organism has 2N = 6 chromosomes, so that N = 3. Cell (b) shows the disjoining of chromatids. Since the organism has 2N = 6 chromosomes, and the daughter cells that will form as a result of this cell division will have 6 chromosomes, (b) must illustrate part of mitosis, specifically anaphase. Cell (c) also shows the disjoining of chromatids. Since the daughter cells will receive three chromosomes, this must be anaphase II of meiosis. Cell (d) shows the pairing of homologs and therefore illustrates metaphase I of meiosis. Because the individual has three pairs of identically appearing chromosomes, there are two identical sex (= X) chromosomes, indicating that the animal is female. Additional Q3 3. DNA synthesis occurs prior to mitosis in the cell cycle. During mitosis, the 2 daughter cells each receive half of the DNA in the parent cell. If the parent has 2N chromosomes prior to mitosis (but after DNA synthesis) and a daughter cell receives half of the DNA, why is the chromosome number of the daughter 2N? Shouldn t each daughter receive N chromosomes? 3

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5 chromosome number in the now haploid daughter cell (compare to f). The number of chromosomes and the number of DNA molecules present will both be 12. f. Prophase II of meiosis. The daughter cells in prophase II of meiosis are haploid. The haploid cells will contain six chromosomes and 12 DNA molecules. g. After cytokinesis following mitosis. After cytokinesis following mitosis the daughter cells will enter G1. Each cell will contain 12 chromosomes and 12 DNA molecules. h. After cytokinesis following meiosis II. After cytokinesis following meiosis II, the haploid daughter cells will contain six chromosomes and six DNA molecules. 29. The amount of DNA per cell of a particular species is measured in cells found at various stages of meiosis, and the following amounts are obtained (3.7picogram, 7.3pg, 14.6pg). Match the amounts of DNA with the corresponding stages of the cell cycle. You may use more than one stage for each amount of DNA. Stage of meiosis a. G1 b. Prophase I c. G2 d. Following telophase II and cytokinesis e. Anaphase I f. Metaphase II Amount of DNA per cell a. 7.3 pg b pg c pg d. 3.7 pg e pg f. 7.3 pg Answer: The amount of DNA in the cell will be doubled after the completion of S phase in the cell cycle and prior to cytokinesis in either mitosis or meiosis I. At the completion of cytokinesis following meiosis II, the amount of DNA will be halved. a. G1 occurs prior to S phase and the doubling of the amount of DNA and prior to the completion of the meiosis II and cytokinesis, which will result in a haploid cell containing one-half the amount of DNA that was contained in the cell in G1. b. During prophase I of meiosis, the amount of DNA in the cell is two times the amount in G1. The homologus chromosomes are still located within a single cell, and there are two sister chromatids per chromosome. c. G2 takes place directly after the completion of S phase, so the amount of DNA is two times the amount prior to the S phase. d. Following cytokinesis associated with meiosis II, each daughter cell will contain only one-half the amount of DNA of a mother cell found in G1 of interphase. By the completion of cytokinesis associated with meiosis II, both homologous pairs of chromosomes and sister chromatids have been separated into different daughter cells. Therefore, each daughter cell will contain only one-half the amount of DNA of the original cell in G1. e. During anaphase I of meiosis, the amount of DNA in the cell is two times the amount in G1. The homologus chromosomes are still located within a single cell, and there are two sister chromatids per chromosome. f. Metaphase II takes place after the cytokinesis associated with meiosis I and results in the daughter cells receiving only one-half the DNA found in their mother cell. In metaphase II of 5