(Solution Manual) Genetics: A Conceptual Approach

Choi,McCallum, 6

Biology

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Author(s): Choi, McCallum

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SOLUTIONS AND PROBLEM -SOLVING MANUAL FOR Genetics: A Conceptual Approach SIXTH EDITION JUNG H. C HOI MARK E. MCCALLUM W. H. FREEMAN AND COMPANY NEW YORK A MACMILLAN HIGHER EDUCATION COMPANY ISBN -13:978 -1-3190 -8870 -5 ISBN -10:1-3190 -8870 -8 © 2017, 2014, 2012, 2008, 2006 by W. H. Freeman and Company All rights reserved. Printed in the United States of America First printing W. H. Freeman and Company Once New York Plaza, Suite 4500 New York, NY 10004 Houndmills, Basingstoke RG21 6XS England www.macmillanlearning.com Contents Chapter 1 Introduction to Genetics 1 Chapter 2 Chromosomes and Cellular Reproduction 14 Chapter 3 Basic Principles of Heredity 35 Chapter 4 Sex Determination and Sex-Linked Characteristics 69 Chapter 5 Extensions and Modifications of Basic Principles 93 Chapter 6 Pedigree Analysis, Applications, and Genetic Testing 122 Chapter 7 Linkage, Recombination, and Eukaryotic Gene Mapping 143 Chapter 8 Chromosome Variation 174 Chapter 9 Bacterial and Viral Genetic Systems 202 Chapter 10 DNA: The Chemical Nature of the Gene 224 Chapter 11 Chromosome Structure and Organelle DNA 243 Chapter 12 DNA Replication and Recombination 261 Chapter 13 Transcription 280 Chapter 14 RNA Molecules and RNA Processing 299 Chapter 15 The Genetic Code and Translation 315 Chapter 16 Control of Gene Expression in Prokaryotes 336 Chapter 17 Control of Gene Expression in Eukaryotes 351 Chapter 18 Gene Mutations and DNA Repair 362 Chapter 19 Molecular Genetic Analysis and Biotechnology 386 Chapter 20 Genomics and Proteomics 404 Chapter 21 Epigenetics 422 Chapter 22 Developmental Genetics and Immunogenetics 434 Chapter 23 Cancer Genetics 445 Chapter 24 Quantitative Genetics 455 Chapter 25 Population Genetics 475 Chapter 26 Evolutionary Genetics 494 Chapter One: Introduction to Genetics COMPREHENSION QUESTIONS Section 1.1 *1. How did Hopi culture contribute to the high incidence of albinism among members of the Hopi tribe? Solution: In Hopi culture, albino individuals were considered special and awarded special status in the village. Hopi male albinos were not required to work the fields, thus avoiding extensive exposure to sunlight that could prove damaging or deadly. Because the male albinos remained in the village during the day, they had mating advantages over the males who participated in farming and other duties outside of the village. Albini sm was considered to be a positive trait reflecting the purity of the villagers. Finally, the small population size of the Hopi tribe may have helped increase the allele frequency of the albino gene due to chance. 2. Outline some of the ways in which genetics is important to all of us. Solution: Genetics directly influences our lives and is fundamental to what and who we are. For example, genes affect our appearance (e.g., eye color, height, weight, skin pigmentation, and hair color). Our susceptibili ty to diseases and disorders is affected by our genetic makeup. Genetics plays a significant role in researching and developing techniques to diagnose and treat these hereditary diseases. Genes may also influence our intelligence and personality. Genetic t echniques have enabled scientists to develop genetically modified crops that are resistant to pesticides and herbicides or that have enhanced nutritional properties making it possible to produce larger quantities of food for the world’s growing population. Lastly, genetic engineering has made it possible to mass produce pharmaceuticals and other substances of commercial value. 3. Give at least three examples of the role of genetics in society today. Solution: Genetics plays important roles in the diagnosis and treatment of hereditary diseases: in breeding plants and animals for improved production and disease resistance; and in producing pharmaceuticals and novel crops through genetic engineering. 4. Briefly explain why genetics is crucial to modern biology. 2 Chapter One: Introduction to Genetics Solution: Genetics is crucial to modern biology in that it provides unifying principles: The genetic code is universal, meaning that all organisms use nucleic acid as their genetic material, and all organisms encode genetic information in the same manner. The study of many other biological disciplines, such as developmental biology, ecology, and evolutionary biology, is supported by genetics. 5. List the three traditional subdisciplines of genetics and summarize what each covers. Solution: 1) Transmission (classical) genetics is concerned with the inheritance of genes from one generation to the next. It also deals with the location of genes on chromosomes and gene - mapping. 2 ) Molecular genetics focuses on the structure, organization, and function of genes at the molecular level. Molecular genetics is also concerned with the processes by which genetic information is transferred and expressed. 3) Population genetics studies genetic variation and changes in genes and allele frequencies within groups of individuals of the same species over time. 6. What are some characteristics of model genetic organisms that make them useful for genetic studies? Solution: Model genetic organisms have relatively short generation times, produce numerous progeny, are amenable to laboratory manipulations, and can be maintained and propagated inexpensively. Section 1.2 7. When and where did agriculture first arise? What role did genetics play in the development of the first domesticated plants and animals? Solution: Agriculture first arose 10,000 to 12,000 years ago in the area now referred to as the Middle East (i.e., Turkey, Iran, Iraq, Syria, Jordan, and Israel). Early farmers selectively bred individual wild plants or animals that had useful characteristics with o thers that had similar useful traits. The farmers then selected for offspring that contained those useful features. Early farmers did not completely understand genetics, but they clearly understood that breeding individual plants or animals with desirable traits would lead to offspring that contained these same traits. This selective breeding led to the development of domesticated plants and animals. 8. Outline the notion of pangenesis and explain how it differs from the germ -plasm theory. Chapter One: Introduction to Genetics 3 Solution: Pangenesis theorizes that information for creating each part of the offspring’s body originates from each respective part of the parent’s body and is passed through the reproductive organs to the embryo at conception. Pangenesis suggests that changes in parts of the parent’s body may be passed to the offspring’s body. The germ -plasm theory, in contrast, states that the reproductive cells (eggs and sperm) possess all of the information required to make the complete body; the rest of the body contributes no i nformation to the next generation. 9. What does the concept of the inheritance of acquired characteristics propose and how is it related to the concept of pangenesis? Solution: The theory of inheritance of acquired characteristics postulates that traits acquired during one’s lifetime can be transmitted to offspring. It developed from pangenesis, which postulates that information from all parts of one’s body is transmitted to the next generation. Thus, according to the theory of inheritance of acquired characteristics, the development of large bicep muscles through exercise would produce children with large biceps. 10. What is preformationism? What did it have to say about how traits are inherited? Solution: Preformationism is the theory that the offspring results from a miniature adult that is already preformed in the sperm or the egg. All traits would thus be inherited from only one parent, either the father or the mother, depending on whether the homunculus (the preformed miniature adult) resided in the sperm or the egg. 11. Define blending inheritance and contrast it with preformationism. Solution: The theory of blending inheritance proposes that the egg and sperm from two parents contains material that blend s upon conception, influencing the development of the offspring. This theory indicates that the offspring is an equal blend of the two parents. In preformationism, the offspring inherits all of its traits from one parent. 12. How did developments in botany in the seventeenth and eighteenth centuries contribute to the rise of modern genetics? Solution: Botanists of the seventeenth and eighteenth centuries discovered that plants reproduce sexually, enabling them to develop new techniques for crossing plants and creating plant hybrids. These early experiments provided essential background work for Mendel’s plant crosses. Mendel’s work laid the foundation for the study of modern genetics. 13. List some advances in genetics made in the twentieth century. 4 Chapter One: Introduction to Genetics Solution: 1902 Proposal that genes are located on chromosomes by Walter Sutton 1910 Discovery of the first genetic mutation in a fruit fly by Thomas Hunt Morgan 1930 The foundation of population genetics by Ronald A. Fisher, John B. S. Haldane, and Sewall Wright 1940s The use of viral and bacterial genetic systems 1953 Three -dimensional structure of DNA described by Watson and Crick 1966 Deciphering of the genetic code 1973 Recombinant DNA experiments 1977 Chemical and enzymatic methods for DNA sequencing developed by Walter Gilbert and Frederick Sanger 1986 PCR develope d by Kary Mullis 1990 Gene therapy 14. Briefly explain the contribution that each of the following people made to the study of genetics. Solution: a. Matthias Schleiden and Theodor Schwann Proposed the concept of the cell theory, which indicated that the cell is the fundamental unit of living organisms. Caused biologists interested in heredity to examine cell reproduction. b. August Weismann Proposed the germ -plasm theory, which holds that cells in reproductive organs carry a complete set of genetic information. c. Gregor Mendel First discovered the basic rules of inheritance. d. James Watson and Francis Crick Along with Rosalind Franklin and Maurice Wilkins, described the three -dimensional structure of DNA. e. Kary Mullis Developed the polymerase chain reaction, used to quickly amplify small amounts of DNA. Section 1.3 15. What are the two basic cell types (from a structural perspective) and how do they differ? Solution: The two basic cell types are prokaryotic and eukaryotic. Prokaryotic cells have neither membrane -bound organelles nor a true nucleus. Their chromosomes are found within the cytoplasm. Eukaryotic cells possess a membrane -bound nucleus and other membrane - bound organelles such as an endomembrane system and mitochondria. 16. Summarize the relations between genes, DNA, and chromosomes. Chapter One: Introduction to Genetics 5 Solution: A gene is a particular DNA nucleotide sequence that encodes an individual trait. Genes are located at specific positions on chromosomes. APPLICATION QUESTIONS AND PROBLEMS Section 1.1 *17. How are genetics and evolution related? Solution: In essence, evolution is change in the genetic composition of a population over generations. Mutations generate new genetic variants, recombination generates new combinations of genetic variants, and natural selection or other evolutionary processes cause a change in the proportions of specific genetic variants in the population. *18. For each of the following genetic topics, indicate whether it focuses on transmission genetics, molecular genetics, or population genetics. a. Analysis of pedigrees to determine the probability of someone inheriting a trait Solution: Transmission genetics b. Study of people on a small island to determine why a genetic form of asthma is prevalent on the island Solution: Population genetics c. Effect of nonrandom mating on the distribution of genotypes among a group of animals Solution: Population genetics d. Examination of the nucleotide sequences found at the ends of chromosomes Solution: Molecular genetics e. Mechanisms that ensure a high degree of accuracy in DNA replication Solution: 6 Chapter One: Introduction to Genetics Molecular genetics f. Study of how the inheritance of traits encoded by genes on sex chromosomes (sex - linked traits) differs from the inheritance of traits encoded by genes on nonsex chromosomes (autosomal traits) Solution: Transmission genetics 19. How does the picture in Figure 1.6a illustrate transmission genetics? Solution: The picture likely shows a female cat with her litter of kittens. As evidenced from the picture, the kittens have characteristics that are similar to their mother. The traits of these kittens were inherited from the mother who is pictured and from the fath er who is not pictured. The inheritance of these traits is explained by transmission genetics or essentially the inheritance of genes from one generation to the next. 20. Describe some of the ways in which your own genetic makeup affects you as a person. Be as specific as you can. Solution: Answers will vary but should include observations similar to those in the following example: Genes affect my physical appearance; for example, they probably have largely determined the fact that I have brown ha ir and brown eyes. Undoubtedly, genes have affected my height of five feet, seven inches, which is quite close to the height of my father and mother, and my slim build. My dark complexion mirrors the skin color of my mother. I have inherited susceptibiliti es to certain diseases and disorders that tend to run in my family; these include asthma, a slight tremor of the hand, and vertigo. 21. Describe at least one trait that appears to run in your family (appears in multiple members of the family). Does this trait run in your family because it is an inherited trait or because it is caused by environmental factors that are common to family members? How might you distinguish between these possibilities? Solution: Answers will vary but should include observation s similar to those in the following example: My two brothers and I share two traits: we are all three taciturn (we don’t speak much) and smart (just don’t ask my teenage daughter). Although the literature provides evidence for a genetic component for intel ligence, I’m not aware of any studies on the heritability of being taciturn. If I were to investigate to what extent these traits are determined by the environment or by heredity, I would look at studies of twins who had been separated at birth and lived i n different environments to adulthood. Such studies would separate environmental factors from genetic factors, whereas studies of family members reared in the same household are confounded by the fact that the family members experienced similar environment s. If the trait had a strong genetic component, we would expect identical twins reared apart to be similarly taciturn or similarly intelligent. One Chapter One: Introduction to Genetics 7 would have to devise some objective measure of these traits —degrees of being taciturn or smart. Section 1.2 *22. Genetics is said to be both a very old science and a very young science. Explain what is meant by this statement. Solution: Genetics is old in the sense that humans have been aware of hereditary principles for thousands of years and have applied them since the beginning of agriculture and the domestication of plants and animals. It is very young in the sense that the fundamenta l principles were not uncovered until Mendel’s time, and the discovery of the structure of DNA and the principles of recombinant DNA have occurred within the last 60 years. *23. Match each of the descriptions (a through d) with the correct theory or conce pt listed below. Preformationism Pangenesis Germ -plasm theory Inheritance of acquired characteristics a. Each reproductive cell contains a complete set of genetic information. Solution: Germ -plasm theory b. All traits are inherited from one parent. Solution: Preformationism c. Genetic information may be altered by use of a characteristic. Solution: Inheritance of acquired characteristics d. Cells of different tissues contain different genetic information. Solution: Pangenesis 24. Briefly explain why each of the following theories is incorrect: a. pangenesis b. preformationism 8 Chapter One: Introduction to Genetics c. blending inheritance d. inheritance of acquired characteristics. Solution: (a) Germ cells have a complete set of information; therefore, genetic information does not need to travel from different body parts to the reproductive organs as predicted by pangenesis. (b) A tiny adult organism (homunculus) does not reside inside the egg or sperm; rather, embryos develop through differentiation of tissues and structures. Furthermore, each offspring inherits genetic material from both parents. (c) Genes do not blend; rather, genes remain discreet, although their effects may produce a phenotype that appears to be a mixture of parental traits. (d) Genetic traits are not acquired through one’s lifetime and the genetic material is not modified by the acquisition of these traits; a n individual’s genotype remains stable, except in the rare case of mutation. Section 1.3 *25. Compare and contrast the following terms: a. Eukaryotic and prokaryotic cells Solution: Both cell types have lipid bilayer membranes, DNA genomes, and machinery fo r DNA replication, transcription, translation, energy metabolism, response to stimuli, growth, and reproduction. Eukaryotic cells have a nucleus containing chromosomal DNA and possess internal membrane -bound organelles. b. Gene and allele Solution: A gene is the basic unit of heredity, which is dictated by the nucleotide sequence, and typically encodes a functional RNA or polypeptide. An allele is a variant form of a gene, arising through mutation. c. Genotype and phenotype Solution: The genotype is the set of genes or alleles an organism has inherited from its parent(s). The expression of the genes of a particular genotype, through interaction with environmental factors, produces the phenotype, the observable trait. d. DNA and RNA Soluti on: Both are nucleic acid polymers. RNA contains ribose, whereas DNA contains deoxyribose. RNA also contains uracil as one of the four bases, whereas DNA contains thymine. The other three bases are common to both DNA and RNA. Chapter One: Introduction to Genetics 9 Finally, DNA is usually double -stranded, consisting of two complementary strands and very little secondary structure, whereas RNA is single -stranded with regions of internal base-pairing to form complex secondary structures. e. DNA and chromosome Solution: Chromosomes are structures formed of DNA and associated proteins. The nucleotide sequence of the DNA contains the genetic information. CHALLENGE QUESTIONS *26. The type of albinism that arises with high frequency among Hopi Native Americans (discussed i n the introduction to this chapter) is most likely oculocutaneous albinism type 2, due to a defect in the OCA2 gene on chromosome 15. Do some research on the Internet to determine how the phenotype of this type of albinism differs from the phenotypes of other forms of albinism in humans and to identify the mutated genes that result in those phenotypes. Hint: Visit the Online Mendelian Inheritance in Man web site (http://www.ncbi.nlm.ni h.gov/omim/) and search the database for albinism. Solution: Type of albinism Phenotype Gene mutated OCA2 Pigment reduced in skin, hair, and eyes, but small amount of pigment acquired with age; visual problems OCA2 OCA1B General absence of pigment in hair, skin, and eyes, but may be small amount of pigment; does not vary with age; visual problems Tyrosinase OCA1A Complete absence of pigment; visual problems Tyrosinase OCA3 Some pigment present, but sun sensitivity and visual problems Tyrosinase -related protein 1 OASD Lack of pigment in the eyes and deafness later in life Unknown OA1 Lack of pigment in the eyes but normal elsewhere GPR143 gene, G protein -coupled receptor ROCA Bright copper red coloration in skin and hair of Africans; dilution of color in iris Tyrosinase -related protein 1 OCA4 Reduced pigmentation MATP gene, membrane - associated transporter protein 10 Chapter One: Introduction to Genetics Section 1.1 27. We now know a great deal about the genetics of humans, and humans are the focus of many genetic studies. What are some reasons humans have been the focus of intensive genetic study? Solution: Humans are intensively interested in how humans function biolog ically. Because of this intense interest, we know more about human anatomy, physiology, genetics, and biochemistry than we know about many other organisms. Many human diseases and disorders are associated with human genes. Understanding how to treat and di agnose these diseases and disorders requires intensive studies to identify the gene(s) responsible for the disorder as well as understanding how they are inherited and expressed. Recent advances in the understanding of genetic risk factors associated with diseases such as heart disease and cancer have enabled the development of predictive genetic tests for some of these disorders. These successes continue to stimulate a focus in identifying genetic risk factors for other diseases. The ability of families to keep careful records about members extending back many generations has facilitated the study of human inheritance aiding the ability of researchers to identify genetic markers within families. In addition, these detailed records have provided some humans who are intensely interested in their own heredity the ability to trace their ancestry. Section 1.3 *28. Suppose that life exists elsewhere in the universe. All life must contain some type of genetic information, but alien genomes might not consist of nucleic acids and have the same features as those found in the genomes of life on Earth. What might be the co mmon features of all genomes, no matter where they exist? Solution: All genomes must have the ability to store complex information, the capacity to vary and yet be transmissible in large part faithfully to subsequent generations. The blueprint for the entire organism must be contained within the genome of each reproductive cell. The information has to be in the form of a code that can be used as a set of instr uctions for assembling the components of the cells. The genetic material of any organism must be stable, be replicated precisely, and be transmitted faithfully to the progeny, but must be capable of mutating. 29. Choose one of the ethical or social issues in parts a through e and give your opinion on the issue. For background information, you might read one of the articles on ethics marked with an asterisk in the Suggested Readings section for Chapter 1 in your LaunchPad. a. Should a person’s genetic makeup be used in determining his or her eligibility for life insurance? Chapter One: Introduction to Genetics 11 Solution: Arguments pro: Genetic susceptibility to certain types of diseases or conditions is relevant information regarding consequences of exposure to certain occupational hazards. Genes that will result in neurodegenerative diseases, such as Huntington disease, Alzheimer disease, or breast cancer, could logically be considered preexisting condit ions. Insurance companies have a right, and arguably a duty to their customers, to exclude people with genetic preconditions so that insurance rates can be lowered for the general population. Arguments con: The whole idea of insurance is to spread the risk and pool assets. Excluding people based on their genetic makeup would deny insurance to people who need it most. Indeed, as information about various genetic risks accumulates, more people would become excluded until only a small fraction of the population is insurable. Further, many genes are incompletely penetrant and variably expressed. Not all women who have mutant BRCA1 or BRCA2 genes develop breast cancer. And some women who have breast cancer lack mutations in those genes. b. Should biotechnology companies be able to patent newly sequenced genes? Solution: Pro: Patenting genes provides companies with protection for their investment in research and development of new drugs and therapies. Without such patent protection, companies would have less incentive to expend large amounts of money in genetic research and thu s would slow the pace of advancement of medical research. Such a result would be detrimental to everyone. Con: Patents on human genes would be like allowing companies to patent a human arm. Genes are integral parts of our selves, so how can a company paten t something that every human has? c. Should gene therapy be used on people? Solution: Pro: Gene therapy can be used to cure previously incurable or intractable genetic disorders and to relieve the suffering of millions of people. Con: Gene therapy may lead to genetic engineering of people for unsavory ends. Who determines what is a genetic defect? Is short stature a genetic defect? Should genetic testing be made available for inherited conditions for which there is no treatment or cure? d. Should genetic testing be made available for inherited disorders for which there is no treatment or cure? 12 Chapter One: Introduction to Genetics Solution: Pro: Information will provide relief from unnecessary anxiety (if the test is negative). Even if the test result is positive for a genetic disorder, it provides the individual, the family, and friends with information and time to prepare. Information about one’s own genetic makeup is a right; every person should be able to make his or her own choice as to whether he or she wants this information. Con: If there is no treatment or cure, a positive test result can have no good consequences. It’s like receiving a death sentence or sentence of extended punishment. It will only engender feelings of h opelessness and depression and may cause some people to terminate their own lives prematurely. 30. A 45 -year-old woman undergoes genetic testing and discovers that she is at high risk for developing colon cancer and Alzheimer disease. Because her children hav e 50% of her genes, they may also be at an increased risk for these diseases. Does she have a moral or legal obligation to tell her children and other close relatives about the results of her genetic testing? Solution: Legally, she is not required to inform her children or other relatives about her test results, but people may have different opini ons about her moral and parental responsibilities. On the one hand, she has the legal right to keep private the results of any medical information, including the results of genetic testing. On the other hand, her children may be at increased risk of develo ping these disorders and might benefit from that knowledge. For example, the risk of colon cancer can be reduced by regular exams, so that any tumors can be detected and removed before they become cancerous. Some people might argue that her parental respon sibilities include providing her children with information about possible medical problems. Another issue to consider is the possibility that her children or other relatives might not want to know their genetic risk, particularly for a disorder like Alzheimer for which there is no cure. 31. Suppose that you could undergo genetic testing at age 18 for susceptibility to a genetic disease that would not appear until middle age and has no available treatment. a. What would be some of the possible reasons for h aving such a genetic test and some of the possible reasons for not having the test? Solution: Having the genetic test removes doubt about the potential for the disorder —either you are susceptible or not. By knowing about the potential of a genetic disorder lifestyle changes could possibly be made to lessen the impact of the disease or lessen the risk. The types and nature of future medical tests could be positively impacted by the genetic testing, thus allowing for early warning and screening for the disease. The knowledge could also have impact on future family plans and allow for informed decisions regarding future offspring and the potential of passing the trait to your offspring. Also by knowing the future, one could plan one’s life accordingly. Chapter One: Introduction to Genetics 13 Reasons for not having the test typically revolve around the potential for testing positive for the susceptibility to the genetic disease. If the susceptibility was detected, the potential for discrimination could exist. For example, your employer (or possibly future employer) might see you as a long -term liability, thus affecting employment options. Insurance companies may not want to insure you fo r that condition or symptoms of the disorder, and potentially social stigmatism associated with the disease could be a factor. Knowledge of the potential future condition could lead to psychological difficulties in coping with the anxiety of waiting for th e disease to manifest. b. Would you personally want to be tested? Explain your reasoning. Solution: There is no “correct” answer, but for me, yes, I would personally want to be tested. The test would remove doubt about the susceptibility particularly if the genetic disease had been demonstrated to occur in my family. Either a positive or negative result would allow for informed planning of lifestyle, medical testing, and family choices in the future. Chapter Two: Chromosomes and Cellular Reproduction COMPREHENSION QUESTIONS Section 2.1 1. What are some genetic differences between prokaryotic and eukaryotic cells? Solution: Prokaryotic cell Eukaryotic cell No nucleus No paired chromosomes (haploid) Typically single circular chromosome containing a single origin of replication Single chromosome is replicated with each copy moving to opposite sides of the cell No histone proteins bound to DNA Nucleus present Paired chromosomes common (diploid) Typically multiple linear chromosomes containing centromeres, telomeres, and multiple origins of replication Chromosomes are replicated and segregate during mitosis or meiosis to the proper location Histone proteins are bound to DNA 2. Why are viruses often used in the study of genetics? Solution: The close relationship between a virus and its cell host, along with the simpler structure of the viral particle, makes it useful in studying the genetics of mammals. The viral genome will have a simi lar genetic structure to its cell host, but because it has fewer genes, it will be easier to decipher the interactions and regulation of the viral genes. Section 2.2 3. List three fundamental events that must take place in cell reproduction. Solution: (1) A cell’s genetic information must be copied. (2) The copies of the genetic information must be separated from one another. (3) The cell must divide into two daughter cells. 4. Outline the process by which prokaryotic cells reproduce. Solution: (1) Replication of the circular chromosome takes place . (2) The two replicated chromosomal copies attach to the plasma membrane. Chapter Two: Chromosomes and Cellular Reproduction 15 (3) The plasma membrane grows, which results in the separation of the two chromosomes. (4) A new cell wall is formed between the two chromosomes, producing two cells, each with its own chromosome. 5. Name three essential structural elements of a functional eukaryotic chromosome and describe their functions. Solution: (1) Centromere: serves as the point of attachment for the kinetochore to which spindle fibers (microtubules) attach (2) Telomeres, or the natural ends of the linear eukaryotic chromosome: serve to stabilize the ends of the chromosome; may have a role in limiting cell division (3) Origins of replication: serve as the starting place for DNA synthesis 6. Sketch and identify four different types of chromosomes based on the position of the centromere. Solution: metacentric submetacentric acrocentric telocentric 7. List the stages of interphase and the major events that take place in each stage. Solution: Three predominant stages are found in interphase of cells active in the cell cycle. (1) G1 (Gap 1): In this phase, the cell grows and synthesizes proteins necessary for cell division. During G 1, the G 1/S checkpoint takes place. Once the cell has passed this checkpoint, it is committed to divide. (2) S phase: During S phase, DNA replication takes place. (3) G2 (Gap 2): In G 2, additional biochemical reactions take place that prepare the cell for mitosis. A major checkpoint in G 2 is the G 2/M checkpoint. Once the cell has passed this checkpoint, it enters into mitosis. A fourth stage is frequently found in cells prior to the G 1/S checkpoint. Cells may exit the active cell cycle and enter into a nondividing stage called G0. 8. What are checkpoints? List some of the important checkpoints in the cell cycle. 16 Chapter Two: Chromosomes and Cellular Reproduction Solution: Checkpoints function to ensure that all the cellular components, such as important proteins and chromosomes, are present and functioning before the cell moves to the next stage of the cell cycle. If components are missing or not functioning, the checkpoint will prevent the cell from moving to the next stage. The checkpoints prevent defective cells from replicating and malfunctioning. These checkpoints occur throughout the various stages of the cell cycle. Important checkpoints include the G 1/S checkpoint, which o ccurs during G 1 prior to the S phase; the G 2/M checkpoint, which occurs in G 2 prior to mitosis; and the spindle -assembly checkpoint, which occurs during mitosis. 9. List the stages of mitosis and the major events that take place in each stage. Solution: (1) Prophase: The chromosomes condense and become visible, the centrosomes move apart, and microtubule fibers form from the centrosomes. (2) Prometaphase: The nucleoli disappear and the nuclear envelope begins to disintegrate, allowing for the cytoplasm and nucleoplasm to join. The sister chromatids of each chromosome are attached to microtubules from the opposite centrosomes. (3) Metaphase: The spindle microtubules are clearly visible and the chromosomes arrange themselves on the equatorial plane of the cell. (4) Anaphase: The sister chromatids separate at the centromeres after the breakdown of cohesin protein, and the newly formed daughter chromosomes move to the opposite poles of the cell. (5) Telophase: The nuclear envelope reforms around each set of daughter chromo somes. Nucleoli reappear. Spindle microtubules disintegrate. 10. Briefly describe how the chromosomes move toward the spindle poles during anaphase. Solution: Due to the actions of the microtubule subunits attached to the kinetochores of the chromosome and motor proteins (e.g., the protein kinesin is a motor protein), the chromosomes are pulled toward the spindle poles during anaphase. The spindle fibers are com posed of tubulin protein subunits. As the tubulin subunits are removed from the “–” end of the microtubule, the chromosome is pulled (or “reeled in”) toward the spindle pole as the microtubule is shortened. While at the “+” end, the kinetochore is removing tubulin subunits of the microtubule attached to the kinetochore with the net effect being the movement of the chromosome closer to the spindle pole. Molecular motor proteins, such as kinesin, are responsible for removing the subunits at the “+” and “ –” ends of the microtubules and thus generate the force needed to move the chromosomes. 11. What are the genetically important results of the cell cycle and mitosis? Solution: Chapter Two: Chromosomes and Cellular Reproduction 17 In the mitotic cell cycle, the genetic material is precisely copied and mitosis ensures that the identical copies of the genetic material are separated accurately into the new daughter cells, resulting in two cells containing the same genetic information. In other words, the cells have genomes identical to each other and t o the mother cell. 12. Why are the two cells produced by the cell cycle genetically identical? Solution: The two cells are genetically identical because during S phase an exact copy of each DNA molecule was created. These exact copies give rise to the two identical sister chromatids. Mitosis ensures that each new cell receives one of the two identical sister chromatids. Thus, the newly formed cells will contain identical daughter chromosomes. Section 2.3 13. What are the stages of meiosis and what major events take place in each stage? Solution: Meiosis I: Separation of homologous chromosomes Prophase I: The chromosomes condense and homologous pairs of chromosomes undergo synapsis. While the chromosomes are synapsed, crossing over occurs. The nuclear membrane disintegrates and the meiotic spindle begins to form. Metaphase I: The homologous pairs of chromosomes line up on the equatorial plane of the metaphase plate. Anaphase I: Homologous chromosomes separate and move to opposite poles of the cell. Each chromosome possesses two sister chromatids. Telophase I: The separated homologous chromosomes reach the spindle poles and are at opposite ends of the cell. Meiosis I is followed by cytokinesis, resulting in the division of the cytoplasm and the production of two haploid cells. These cells may skip directly into meiosis II or enter interkinesis, where the nuclear envelope reforms and the spindle fibers break down. Meiosis II: Separation of sister chromatids Prophase II: Chromosomes condense, the nuclear envelope breaks down, and the spindle fibers form. Metaphase II: Chromosomes line up at the equatorial plane of the metaphase plate. 18 Chapter Two: Chromosomes and Cellular Reproduction Anaphase II: The centromeres split, which results in the separation of sister chromatids. Telophase II: The daughter chromosomes arri ve at the poles of the spindle. The nuclear envelope reforms, and the spindle fibers break down. Following meiosis II, cytokinesis takes place. 14. What are the major results of meiosis? Solution: Meiosis involves two cell divisions, thus producing four new cells (in many species). The chromosome number of a haploid cell produced by meiosis I (haploid) is half the chromosome number of the original diploid cell. Finally, the cells produced by meiosis are genetically different from the original cell and genetically different from each other. 15. What two processes unique to meiosis are responsible for genetic variation? At what point in meiosis do these processes take place? Solution: (1) Crossing over, which begins during the zygotene stage of prophase I and is completed near the end of prophase I. (2) The random distribution of separated members of the homologous chromosomes (the maternal and paternal chromosomes) to daughter cells, which takes place in anaphase I of meiosis. The arrangement for separation is determined by the random alignment of homologs in metaphase I. 16. How does anaphase I of meiosis differ from anaphase of mitosis? Solution: In anaphase I of meiosis, homologous chromosomes separate whereas in anaphase of mitosis the sister chromatids separate. 17. Briefly explain why sister chromatids remain together in anaphase I but separate in anaphase II of meiosis. Solution: In meiosis, a similar process to mitosis occurs. Meiosis -specific cohesin complexes (different from cohesion proteins in mitosis) form at the centromeres of the sister chromatids during the S phase. At the beginning of meiosis, cohesin molecules are also f ound along the entire length of the chromosome arms assisting in the formation of the synaptonemal complex and holding together the two homologs. During anaphase I of meiosis, the cohesin molecules along the arms are cleaved by activated separase allowing the homologs to separate. However, the cohesin complexes at the centromeres of the sister chromatids are protected from the action of separase by the protein shugoshin and are unaffected. The result is that sister chromatids remained attached during anaphase I. At the end of metaphase II, the Chapter Two: Chromosomes and Cellular Reproduction 19 protection of the cohesin molecules at the centromeres is lost, and the separase proteins can now cleave the cohesin complex, which allows the sister chromatids to separate. 18. Outline the processes of spermatogenesis and oogenesis in animals. Solution: In animals, spermatogenesis occurs in the testes. Primordial diploid germ cells divide mitotically to produce diploid spermatogonia that can either divide repeatedly by mitosis or enter meio sis. A spermatogonium that has entered prophase I of meiosis is called a primary spermatocyte and is diploid. Upon completion of meiosis I, two haploid cells, called secondary spermatocytes, are produced. Upon completing meiosis II, the secondary spermatoc ytes produce a total of four haploid spermatids. Female animals produce eggs through the process of oogenesis. Similar to what takes place in spermatogenesis, primordial diploid cells divide mitotically to produce diploid oogonia that can divide repeatedly by mitosis, or enter meiosis. An oogonium that has entered prophase I is called a primary oocyte and is diploid. Upon completion of meiosis I, the cell divides, but unequally. One of the newly produced haploid cells receives most of the cytoplasm and is called the secondary oocyte. The other haploid cell receives only a small portion of the cytoplasm and is called the first polar body. Ultimately, the secondary oocyte will complete meiosis II and produce two haploid cells. One cell, the ovum, will receive most of the cytoplasm from the secondary oocyte. The smaller haploid cell is called the second polar body. Typically, the polar bodie s disintegrate, and only the ovum is capable of being fertilized. 19. Outline the processes of male gamete formation and female gamete formation in plants. Solution: Plants alternate between a multicellular haploid stage called the gametophyte and a multicellular diploid stage called the sporophyte. Meiosis in the diploid sporophyte stage of plants produces haploid spores that develop into the gametophyte. The gametoph yte produces gametes by mitosis. In flowering plants, the microsporocytes found in the stamen of the flower undergo meiosis to produce four haploid microspores. Each microspore divides by mitosis to produce the pollen grain, or the microgametophyte. Withi n the pollen grain are two haploid nuclei. One of the haploid nuclei divides by mitosis to produce two sperm cells. The other haploid nucleus directs the formation of the pollen tube. Female gamete production in flowering plants takes place within the megagametophyte. Megasporocytes found within the ovary of a flower divide by meiosis to produce four megaspores. Three of the megaspores disintegrate, while the remaining megaspore divides mitotically to produce eight nuclei that form the embryo sac (or fe male gametophyte). Of the eight nuclei, one will become the egg. 20 Chapter Two: Chromosomes and Cellular Reproduction APPLICATION QUESTIONS AND PROBLEMS Introduction *20. Answer the following questions about the blind men’s riddle, presented in the introduction to this chapter. a. What do the two socks of a pair represent in the cell cycle? Solution: The two chromatids of a chromosome b. In the riddle, each blind man buys his own pairs of socks, but the clerk places all the pairs in one bag. Thus, there are two pairs of socks of each color in th e bag (two black pairs, two blue pairs, two gray pairs, etc.). What do the two pairs (four socks in all) of each color represent? Solution: The two chromosomes of a homologous pair c. What is the thread that connects the two socks of a pair? Solution: Cohesin d. What is the molecular knife that cuts the thread holding the two socks in a pair together? Solution: The enzyme separase e. What in the riddle performs the same function as spindle microtubles? Solution: The hands of the two blind men f. What would happen if one man failed to grasp his sock of a particular pair? How does that outcome relate to events in the cell cycle? Solution: If one man failed to grasp his sock, it would be difficult for the knife to cut the string holding them together. The two socks of a pair would not be separated and both would end up in one man’s bag. Similarly, if each chromatid is not attached to spindle fibers and pulled in opposite directions, the two chromatids will not separate and both would migrate to the same cell. This cell would have two copies of one chromosome. Chapter Two: Chromosomes and Cellular Reproduction 21 Section 2.1 21. A cell has a circular chromosome and no nuclear membrane. Its DNA is complexed with some histone proteins. Does this cell belong to a eubacterium, an archaean, or a eukaryote? Explain your reasoning. Solution: This cell is most likely an archaea. The cell is not eukaryotic because it lacks a nuclear membrane and has a single circular chromosome. The cell is not a eubacterium because it has histone proteins, which are present in archaea and eukaryotes but lacking in eubacteria. Section 2.2 22. A certain species has three pairs of chromosomes: an acrocentric pair, a metacentric pair, and a submetacentric pair. Draw a cell of this species as it would appear in metaphase of mitosis. Solution: 23. Examine Figure 2.6a. What type of chromosome (metacentric, submetacentric, acrocentric, or telocentric) is chromosome 1? What about chromosome 4? Solution: The centromere in chromosome 1 is centrally located, so it is metacentric. The centromere of chromosome 4 is located between the center and the end of the chromosome, so it is submetacentric.