Chapter 10: Meiosis and Sexual Reproduction

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1 Chapter 10: Meiosis and Sexual Reproduction AP Curriculum Alignment The preservation and continuity of genetic material that is being passed from generation to generation in sexually reproducing organisms is accomplished through the processes of meiosis and fertilization. Big Idea 3 concentrates on a storage, retrieval, and transmission of information that is essential for life processes. Detailed information that students must know about the process of meiosis is spelled out in Enduring Understanding 3. A. Special emphasis should be placed on the methods in which genetic variation can occur and which are crossing over, independent assortment of alleles, and fertilization. Of course mutations can always be a source of variation. Chapter 10 clearly outlines the vocabulary and concepts that students must master in order to understand the formation of gametes, also known as sex cells. When the process meiosis experiences mistakes, disease in humans can result. Several of the disease conditions caused by mistakes in meiosis are explained in chapter 10. ALIGNMENT OF CONTENT TO THE CURRICULUM FRAMEWORK Big Idea 1: The process of evolution drives the diversity and unity of life. Essential knowledge 1.A.2: Natural selection acts on phenotypic variations in populations. b. Phenotypic variations are not directed by the environment but occur through random changes in the DNA and through new gene combinations. Big Idea 3: Living systems store, retrieve, transmit, and respond to information essential to life processes. Enduring understanding (EU) 3.A: Heritable information provides for continuity of life. Essential knowledge 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms. Evidence of student learning is a demonstrated understanding of each of the following: 1. Meiosis ensures that each gamete receives one complete haploid (1n) set of chromosomes. 2. During meiosis, homologous chromosomes are paired, with one homologue originating from the maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is random with respect to the cell poles. 3. Separation of the homologous chromosomes ensures that each gamete receives a haploid (1n) set of chromosomes composed of both maternal and paternal chromosomes. 4. During meiosis, homologous chromatids exchange genetic material via a process called crossing over, which increases genetic variation in the resultant gametes. 150 Mader, Biology, 12 th Edition, Chapter 10

2 5. Fertilization involves the fusion of two gametes, increases genetic variation in populations by providing for new combinations of genetic information in the zygote, and restores the diploid number of chromosomes. Enduring understanding (EU) 3.C: The processing of genetic information is imperfect and is a source of genetic variation. Essential knowledge 3.C.1: Changes in genotype can result in changes in phenotype. c. Errors in mitosis or meiosis can result in changes in phenotype. Evidence of student learning is a demonstrated understanding of each of the following: 1. Changes in chromosome number often result in new phenotypes, including sterility caused by triploidy and increased vigor of other polyploids. 2. Changes in chromosome number often result in human disorders with developmental limitations, including Trisomy 21 (Down syndrome) and XO (Turner syndrome). Essential knowledge 3.C.2: Biological systems have multiple processes that increase genetic variation. c. Sexual reproduction in eukaryotes involving gamete formation, including crossing-over during meiosis and the random assortment of chromosomes during meiosis, and fertilization serve to increase variation. Reproduction processes that increase genetic variation are evolutionarily conserved and are shared by various organisms. The details of sexual reproduction cycles in various plants and animals are beyond the scope of the course and the AP Exam. However, the similarities of the processes that provide for genetic variation are relevant and should be the focus of instruction. Concepts covered in Chapter 10 also align to the learning objectives that provide a foundation for the course, an inquiry-based laboratory experience, class activities, and AP exam questions. Each learning objective (LO) merges required content with one or more of the seven science practices (SP), and one activity or lab can encompass several learning objectives. The learning objectives and science practices from the Curriculum Framework that pertain to the meiosis and sexual reproduction are shown in the table below. Note that other learning objectives may apply as well. LO 3.9 The student is able to construct an explanation, using visual representations or narratives, as to how DNA in chromosomes is transmitted to the next generation via mitosis, or meiosis followed by fertilization. LO 3.10 The student is able to represent the connection between meiosis and increased genetic diversity necessary for evolution. LO 3.11 The student is able to evaluate evidence provided by data sets to support the claim that heritable information is passed from one generation to another generation through mitosis, or meiosis followed by fertilization. LO 3.27 The student is able to compare and contrast processes by which genetic Mader, Biology, 12 th Edition, Chapter

3 variation is produced and maintained in organisms from multiple domains. LO 3.28 The student is able to construct an explanation of the multiple processes that increase variation within a population. Key Concepts Summary Diploid and haploid cells In sexually reproducing organisms, meiosis is the type of nuclear division that reduces the number of chromosomes from diploid (2n) to the haploid (n) number. In many animals, meiosis is used to produce gametes, haploid reproductive cells. When gametes combine during fertilization, they produce a diploid offspring (2n). The process of meiosis Meiosis requires two cell divisions and results in four unique haploid cells. Meiosis starts with DNA replication. Meiosis I resembles mitosis, and is followed by meiosis II where cells divide without additional DNA replication. During prophase 1 of meiosis, homologous chromosomes line up next to each other and can exchange pieces of the chromosome in a process called crossing over. Key Terms alleles aneuploidy Barr body bivalent crossing-over deletion diploid duplication euploidy fertilization gametes gametogenesis gametophyte genetic recombination haploid homologues independent assortment interkinesis inversion karyotype life cycle meiosis meiosis I meiosis II monosomy nondisjunction oocyte oogenesis polar body sexual reproduction spermatogenesis sporophyte synapsis translocation trisomy zygote 152 Mader, Biology, 12 th Edition, Chapter 10

4 Teaching Strategies The College Board s AP Biology Investigative Labs contains Investigation 7 - Cell Division: Mitosis and Meiosis. Part 4 of this lab is modeling meiosis. You probably modeled mitosis during the Chapter 9 activities. You may use the suggested method for modeling meiosis from Investigation 7, or you may choose Activity 1 below. I suggest ordering slides or using images from the internet for the Sordaria activity. Class time: Three 45-minute class periods Day 1: Lecture on meiosis 20 minutes Investigation 7, Part 4 OR Activity 1 25 minutes Day 2: Lecture on the difference between meiosis and fertilization 5 minutes Activity 2 as a demonstration to emphasize the difference between meiosis and fertilization 15 minutes Lecture on non-disjunction, the human diseases that result from chromosomal mutations, and karyotype analysis 25 minutes Day 3: Lecture on Sordaria, crossing over, and map units in preparation for Part 5 of Lab Investigation 7 20 minutes Investigation 7, Part 5 25 minutes Suggested Approaches College Boards Investigation 7 is rich with activities that provide students with the science practices and concepts that will help make students successful on the AP Biology Exam. Slides for Part 5 can be obtained from a biological supply company. Student should be clear that fertilization is not a part of meiosis, that they are separate events. Even though meiosis occurs, fertilization does not always follow. Gametes either engage in a fertilization event or die after 72 hours, Pointing out the differences between oogenesis that produces only one viable egg and spermatogenesis that produces 4 viable sperm is important knowledge for students. Emphasize the areas in meiosis that provide genetic variability. This would include crossing over where homologous chromosomes exchange pieces of DNA and independent assortment which is the random movement of homologous chromosomes to the metaphase plate in Meiosis I and subsequent separation in Anaphase I. Mader, Biology, 12 th Edition, Chapter

5 Student Misconceptions and Pitfalls Students sometimes confuse the process called meiosis with fertilization and write about unfertilized chromosomes. They are two separate events. Meiosis is intended to the amount of genetic material in a sex cell so the two sex cells (gametes) can combine during fertilization to produce a zygote with the correct amount of genetic material. Providing activities that show both meiosis and fertilization can dispel this misconception. Suggested Activities Activity 1: Modeling Meiosis and Crossing Over This activity can be completed as an alternative to Investigation 7, Part 4. There are many ways in which to reach visual and kinesthetic learners. One way is to construct chromosomes using pool noodles. I would suggest preparing two paired chromosomes to represent XY chromosomes. You will need a total of 8 pool noodles- 4 for each parent. To represent the male (4 noodles total), you will need 2 for replicated X chromosome and 2 for the replicated Y chromosome. To represent the female (4 noodles total), you will need 2 noodles to represent one replicated X chromosomes and 2 noodles to represent the other X chromosome. Use duct tape of a unique color to circle all X chromosomes, 10 cm from the upper end of all X chromosomes. The duct tape should be of the same color on two each of the X chromosomes so you will need two different colors of duct tape. Cut these ends off and place toothpicks on both sides of the cut chromosomes and then reconnect the chromosomes. The bottoms of the X chromosomes should be marked with a different color duct tape. These chromosomes can now be used to simulate the formation of a tetrad and crossing over. The Y chromosomes form a homologous pair. You show independent assortment by the movement of one X and one Y, or 2 X chromosomes into each of 4 unique haploid cells. The cell membrane can be made using yarn or chalk on the floor. This can be conducted as a demonstration or you can make enough noodles for the entire class. 154 Mader, Biology, 12 th Edition, Chapter 10

6 Activity 2: The Great Sperm Race Game This game simulates the movement of sperm to the human egg for fertilization. It can be used as a demonstration for emphasizing the difference between meiosis and fertilization. Activity 3: Optional Meiosis Activity The DNA learning center presents meiosis and gene shuffling in a manner in which students can easily understand the importance of meiosis in genetic variation. Go to for this activity. Mader, Biology, 12 th Edition, Chapter

7 Student Edition Chapter Review Answers Answers to Assess Questions 1. d; 2. b; 3. a; 4. d; 5. a; 6. d; 7. b; 8. c; 9. d; 10. b; 11. a; 12. d; 13. a; 14. d; 15. c; 16. d; 17. d; 18. c Answers to Applying the Big Ideas Questions 1. Outwardly visible variations (phenotype) in species are not directed by the environment but occur through random changes in DNA and through new gene combinations. Some phenotypic variations significantly increase or decrease the fitness of the organism and the population. a) Describe TWO kinds of data that could be collected by scientists to provide a direct answer to the question, how can scientists determine that a change in chromosome number or structure will decrease the fitness of an organism? b) Explain how the data you suggested in part (a) would provide a direct answer to the question. Essential Knowledge Science Practice Learning Objective 1.A.2: Natural selection acts on phenotypic variations in populations. 5.3: The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 1.4: The student is able to evaluate data-based evidence that describes evolutionary changes in the genetic makeup of a population over time. 4 points maximum. Description of the appropriate kind of data and the appropriately linked explanation of its selection may include: Descriptions of kind of data (1 point each) A karyotype may be used to identify if an aneuploid (extra or missing copies of chromosomes) condition exists. Nondisjunction during meiosis I or meiosis II may result in aneuploidy. Trisomy occurs when an individual has three of a particular type of chromosome (2n+1). Down syndrome is a well-known trisomy in humans Explanations (1 point each) If phenotypic variations hinder attraction by potential mates, then reproduction is hindered and fitness is decreased. For example, individuals with Down syndrome have an increased tendency toward leukemia, cataracts, intellectual disability, and an accelerated rate of aging. 156 Mader, Biology, 12 th Edition, Chapter 10

8 resulting in an extra copy of chromosome 21. Monosomy occurs when an individual has only one of a particular type of chromosome (2n-1) and is usually lethal. Aneuploidy of sex chromosomes, while not necessarily lethal, may result in syndromes such as Turner syndrome, Klinefelter syndrome, and Jacobs syndrome. Abnormalities in crossing-over may result in deletions, duplications, inversions and translocations within chromosomes. These changes in chromosome structure maybe result in syndromes such as Williams syndrome, cri du chat syndrome, and Alagille syndrome. These can be identified by doing a karyotype or by studying the inheritance pattern of a disorder in a particular family. Look for when crossing-over occurs during parthenogenesis (a form of asexual reproduction, where only one parent contributes genetic information to the next generation). If monosomy is lethal, this will certainly decrease fitness as the organism will die before reproduction. Often, aneuploidy of sex chromosomes is not evident outwardly until puberty or when trying to reproduce. They may not hinder attracting a mate, but may cause difficulty in reproduction. Turner females can usually lead fairly normal lives if they receive hormone suppliments and some have given birth following in vitro fertilization using donor eggs. Klinefelter syndrome causes sterility, which decreases an individual s fitness. Jacobs males are fertile and may have children, and therefore their fitness is not necessarily impacted. Changes in chromosome structure may lead to intellectual disability or facial abnormalities or congenital heart conditions and liver problems (as seen with the syndromes listed). If phenotypic variations hinder attraction by potential mates, then reproduction is hindered and fitness is decreased. However, these individuals may live to reproduce, and thereby pass on their genetic material to their offspring. Lowered genetic diversity with parthenogensis can make it more difficult for populations to respond to environmental changes. However, if crossing-over during meiosis occurs between the sister chromatids instead of the nonsister chromatids of homologous chromosomes, small amounts of variation are maintained in the genome. Mader, Biology, 12 th Edition, Chapter

9 2. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms. In a paragraph, explain the connection between meiosis and increased genetic diversity necessary for evolution. Essential Knowledge Science Practice Learning Objective 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization. 7.1: The student can connect phenomena and models across spatial and temporal scales. 3.10: The student is able to represent the connection between meiosis and increased genetic diversity necessary for evolution. 3 points maximum. Connections between meiosis and increased genetic diversity may include (1 point each): Natural selection operates on populations through the phenotypic differences (traits) that individuals display; meiosis followed by fertilization provides a spectrum of possible phenotypes on which natural selection acts, and variation contributes to the long-term continuation of species. Independent assortment: During meiosis, homologous chromosomes are paired, with one homologue originating from the maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is random with respect to cell poles. Separation of the homologous chromosomes ensures that each gamete receives a haploid (1n) set of chromosomes composed of both maternal and paternal chromosomes. Crossing over: During meiosis, homologous chromatids exchange genetic material via a process called crossing over; thus, the resulting four gametes, while carrying the same number of chromosomes, are genetically unique with respect to individual alleles and allele combinations. Sexual reproduction of diploid organisms involves the recombination of heritable information from both parents through fusion of gametes during fertilization. This increases the genetic variation in populations by providing for new combinations of genetic information in each zygote. 158 Mader, Biology, 12 th Edition, Chapter 10

10 3. The variation produced by meiosis at the cellular level affects all levels of an organism s physiology. a) Describe TWO kinds of data that could be collected by scientists to provide a direct answer to the question, how does variation in molecular units provide cells with a wider range of functions? b) Explain how the data you suggested in part (a) would provide a direct answer to the question. Essential Knowledge Science Practice Learning Objective 4.C.1: Variation in molecular units provides cells with a wider range of functions. 5.3: The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 4.22: The student is able to construct explanations based on evidence of how variation in molecular units provides cells with a wider range of functions. 4 points maximum. Description of the appropriate kind of data and the appropriately linked explanation of its selection may include: Descriptions of kind of data (1 point each) Genetic material is exchanged l between nonsister chromatids during meiosis I (crossing over). Homologous chromosome pairs separate independently (randomly) during independent assortment. Explanations (1 point each) This allows for a new combination of genetic material in the daughter chromosomes, because the members of a homologous pair can carry slightly different alleles for the same genetic traits. When the genetic material is swapped, the chromatids are no longer identical and so the daughter cells receive daughter chromosomes with recombined alleles. The offspring can then have a different set of alleles (and therefore genes) than their parents. A maternal or paternal homologue may be oriented toward either pole of the cell during alignment at the metaphase plate. This allows for a significanly large number of possibilities for assorment and combination of genes to the different daughter cells. (In humans Mader, Biology, 12 th Edition, Chapter

11 with 23 pairs of chromosomes, the possible combinations are 2 23! The union of male and female gametes during fertilization creates another avenue for diversity in the resulting zygote cell. Zygotes have (2 23 ) 2 possible chromosome combinations, assuming no crossing-over occurred! This provides great opportunity for genetic diversity within a population, with both positive (allowing the population greater stability in responding to environmental change), and negative (typically just when harmful mutations occur for an individual that lead to disease) outcomes. Answers to Applying the Science Practices Questions 1. Yes. The yeast that cannot make Cik1p produces far fewer haploid cells. 2. No. Cells without ability to make Kar3p still make haploid cells. 3. No. Of the two major proteins tested, Kar3p does not appear to play a role in meiosis. 160 Mader, Biology, 12 th Edition, Chapter 10

12 Additional Questions for AP Practice 1. Select the description that indicates how meiosis can increase genetic variability. A) The union of two gametes creates a new genetic combination. B) Non-disjunction can occur during meiosis to create abnormal gametes. C) Homologous chromosomes can line up and exchange pieces of DNA. D) Sister chromatids are separated in Anaphase II of Meiosis. 2. Describe homologous chromosomes and justify how they provide variation during meiosis. 3. Select the correct method that can provide genetic variation in all Domains. A) meiosis B) crossing over C) crossing over and independent assortment D) DNA mutations 4. Which process is not considered to be part of meiosis? A) crossing over B) independent assortment C) separatiohn of homologous chromosomes D) fertilization 5. What does this karyotype show? Mader, Biology, 12 th Edition, Chapter

13 Grid-In Questions 1. A diploid cell with 52 chromosomes undergoes meiosis to form four daughter cells. How many chromosomes are found in the daughter cells? 162 Mader, Biology, 12 th Edition, Chapter 10

14 Answers to Additional Questions for AP Practice 1. The correct answer is C. B would not create viable gametes. 2. Homologous chromosomes can provide variation through the processes of crossing over and independent assortment during meiosis. 3. D is the correct answer. 4. Correct answer is D. 5. It shows the chromosomes of a normal male. Answers to Grid-In Questions 1. Chapter: 10 Meiosis and Sexual Reproduction Answer: 26 52/2 = 26 Mader, Biology, 12 th Edition, Chapter