Focus on the Concepts

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1 Focus on the Concepts ~is chapter describes how genes are controlled and how this relates to differentiation, development, cloning, signal transduction, and what can happen when cells escape their normal controls--cancer. Focus on the following concepts: The flow of information from genes to proteins is called gene expression. Gene regulation helps organisms respond to environmental changes. In bacteria such as E. coli, groups of genes called operons respond to changes. In the absence of the sugar lactose, a repressor protein keeps the opera ~or sequence c. ontrolling a group of genes turned off. When lactose is present, it pulls the repressor from the DNA, RNA polymerase attaches to the promoter, the genes are transcribed, and proteins are made that enable the bacterium to use lactose. In eukaryotes, differentiation of cells results from differences in gene expression. Regulation can occur at many points from gene to protein: Coiling or "packing" can control access to genes, even inactivating an entire X chromosome. DNA can be chemica!ly altered. The most important level of control is carried out by transcription factors (and other proteins) that enhance or inhibit attachment of RNA polymerase and thus regulate gene transcription, mrna can be spliced, blocked, or broken down before translation. Fina!ly, polypeptides and proteins may be modified, activated, or destroyed. Cascades of gene expression direct animal development. For example, communication between a fruit fly egg and surrounding cells determines where in the egg genes are transcribed, where certain RNAs and proteins accumulate, and as a result, which end of the fly will be the head and which end the tail. Similar changes determine left and right and division of the fly into segments. Master control genes called homeotic genes regulate batteries of other genes that determine where and when appe.ndages and organs develop. Signal transduction pathways are important in gene control. Typically, the pathway is activated by a signal molecule that binds to a receptor in the cell membrane. The receptor activates a chain of relay proteins, which actives a transcription factor. The transcription factor triggers a specific gene, which codes for a protein that carries out some function in the cell. Cloning demonstrates that differentiated cells may retain all of their genetic potential. Individual plant cells can be made to dedifferentiate and develop into whole plants. Animals are typically cloned via nuclear transplantation-- transferring the nucleus of a somatic cell into an egg or zygote. Reproductive cloning can produce genetically identical individuals. Therapeutic cloning generates stem cells, which retain the ability to give rise to all or most kinds of cells in the organism. ~42

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3 How Genes Are Controlled Cancer results from mutations in genes that control the cell cycle, causing uncontrolled multiplication of cells. The affected genes usually code for proteins in signal transduction pathways. Mutation of a proto-oncogene into an oncogene results in manufacture of an overactive protein or excess of a protein that stimulates cell division. Mutation of a tumor-suppressor gene alters a protein that normally activates the production of a cell-division inhibitor. Review the Concepts Work through the following exercises to review the concepts in this chapter. For additional review, refer to the activities at The website offers a pre-test that will help you plan your studies. Natural selection has favored bacteria that expre.ss only those genes whose functions are needed by the cell--in other words, bacteria that can turn genes on and off in response to changes in their environment. In bacteria, genes are grouped, with control sequences called operators and promoters, into clusters called operons. The lac and trp operons are two such gene clusters that enable the bacterium E. coli to respond to its environment. Study the diagrams in Module 11.1 and then match each of the components of the lac and trp operon systems with its function. lac operon: 1. Regulatory gene 2. Repressor protein + lactose 3. Repressor protein without lactose 4. RNA polymerase 5. Promoter 6. Opera[or 7. Operon genes 8. Enzymes trp operon: 1. Regulatory gene 2. Repressor protein + tryptophan 3. Repressor protein without tryptophan 4. RNA polymerase 5. Promoter 6. Operator 7. Operon genes 8. Enzymes A. Keeps RNA polymerase from attaching to promoter and transcribing genes B. Transcribes genes into mrna for protein synthesis C. Repressor protein attaches here D. Use lactose E. Information for making repressor protein F. Where RNA polymerase starts transcribing genes G. Allows RNA polymerase to transcribe genes H. Information for making enzymes that use lactose A. Keeps RNA polymerase from attaching to promoter and transcribing genes B. Transcribes genes into mrna for protein synthesis C. Repressor protein attaches here D. Make tryptophan E. Information for making repressor protein F: Where RNA polymerase starts transcribing genes G. Allows RNA polymerase to transcribe genes H. Information for making enzymes that make tryptophan

4 Chapter 11 The first level of genetic control is a variety of mechanisms that can "lock" and "unlock" the DNA itself by controlling access to genes and allowing or disallowing RNA transcription. Complete this crossword puzzle to review the roles of DNA packing and protein activators in gene expression. Across 1. All cells in a eukaryotic organisms have the same genes, but in different kinds of cells different genes are 3. Methylation patterns are passed on to The default setting for most genes seems to be"." The color pattern of a cat reflects the influence of chromosome inactivation. Scientists think most eukaryotic regulatory proteins act as One X chromosome in each of a woman s cells is Proteins called silencers sometimes bind to DNA and transcription. 16. The DNA-histone beaded fiber is further wrapped into a tight fiber.

5 How Genes Are Controlled t 4*3 19. Nucleosomes may control gene by limiting access to DNA. 20. is a chemical modification of DNA that turns genes off. 21. The DNA supercoil is further folded and compacted to form a 23. In eukaryotes, many proteins interact with DNA and one another to turn genes on and off. 25. Epigenetic variations may account for differences between identical 27. The folding and coiling of DNA into a chromosome is called 28. In eukaryotes, genes coding for the enzymes of a metabolic pathway are often around the genome. 29. Twisted DNA further coils into a with a diameter of 300 nm. Down 2. Specialization of cell structure and function is called 4. Besides helping to regulate genes, DNA packing enables DNA to into the nucleus. 5. The DNA-histone complex looks like " on a string." 8. DNA is wound around small proteins called 10. The first step in initiating gene transcription is binding of activators to sites called. 11. A transcription is a protein that assists RNA polymerase. 13. A ~ is a complex of DNA wrapped around eight histone molecules. 15. DNA packing seems to control gene expression at the stage. 17. Activators and other proteins help trigger RNA to begin transcription. 18. Inheritance of traits transmitted by mechanisms not directly involving the nucleofide sequence--such as methylafion and histone changes--are called inheritance. 22. Activators and enhancers may help position RNA polymeraseon a gene s 24. In most eukaryofic cells, most are not expressed. 26. The inactive X chromosome in each cell of a female compacts into an object called a body. Often gene expression is controlled at the transcription step, but in eukaryotes, gene expression is also regulated after transcription of genes into mrna and during and after translation of mrna into protein. Review these processes by matching each of the processes on the left (listed in order of occurrence) with a description on the right. 1. First step in RNA splicing 2. Second step in RNA splicing 3. Alternative RNA splicing 4. RNA interference 5. Selective breakdown of mrna 6. Control of initiation of translation 7. Activation of finished protein 8. Selective breakdown of proteins A. Altering a protein to form an active final product B. Retaining or destroying mrna molecules, controlling how much they are translated C. Action of proteins that may control the start of protein synthesis D.. Joining exons in different ways [o produce more than one kind of mrna (and polypeptide) from a single gene E. Removal of noncoding introns from RNA F. Joining of exons to produce mrna G. Retaining or destroying proteins, depending on cell s needs H. Binding of microrna (rnirna) to mrna, blocking translation

6 Chapter 11 This module summarizes the major steps in gene expression in eukaryotes and the key mechanisms that regulate gene expression. After reviewing the module, match each of the mechanisms of regulation with the stage of gene expression at which it acts. Choose from: mrna breakdown, DNA unpacking and changes, cleavage/modification]activation, protein breakdown, addition of cap and tail, TRANSCRIFrION, splicing, ~SLATION, and flow through nuclear envelope. mrna in cytoplasm CYTOPLASM Polypeptide O oo 0o o o

7 How Genes Are Controlled As an animal develops, master genes activate other genes, and these genes signal still others. A chain reaction of gene expression shapes the body from head to tail. Review this cascade of gene expression, how cells in a developing embryo signal each other, and how researchers are unraveling this complex process, by filling in the blanks in the story that follows. Powerhfl new techniques of molecular biology have enabled scientists to explore how gene regulation controls animal development. Researchers have found that one of the first events in fruit fly development is a sequence of changes that determine which end of an egg will develop into the fly s 1 and which will develop into the 2. One of the first 3 that "turns on" in the egg cell codes for a protein that leaves the egg and signals nearby cells in its follicle, or egg chamber. In a follicle cell, the egg protein activates a signal-transduction pathway. The signal protein binds to a specific 4 in the membrane of a follicle target cell, which in turn activates a series of relay proteins in the target cell. The last relay protein activates a 5 factor that triggers transcription of a specific target Cell gene. The mrna produced is then 6 into a protein. Via this mechanism, the egg cell signals the follicle cells. The new protein formed in the follicle cells then 7 follicle cell genes, and they produce proteins that signal back to the egg cell. One of the egg, cell s responses is to localize "head" 8 at the opposite end of the egg cell. This marks where the fly s 9 end will develop. The other end of the egg will become the 10. Similar processes establish the other body axes and thus the layout of the overall body plan of the fly. After the egg is fertilized, the zygote is transformed into a multicellular embryo by repeated 11. Further signaling and translation creates a cascade of 12 that diffuse through the cell layers of the fly embryo, activating further signa! transduction pathways. One of the outcomes is to subdivide the body into a series of sections, or 13 Protein products of the axis-forming and segment-forming genes now activate another set of 14 that shape the details of the fly. Master control genes called 15 genes determine what body parts--antennae, legs, and so on--will develop in each segment. For example, one set of homeotic genes causes 16 to develop on the head of the fly and 17 on the thorax. Errors in homeotic genes produce 18 flies with spectacular changes in body structure, such as extra pairs of wings, or heads bearing legs instead of antennae. How can a researcher know which genes are expressed and which are inactive in particular cells during particular stages of development? One way is to analyze the activity of genes in different cells using a DNA 19, also called a DNA chip or gene chip. This is a glass slide that can hold many different 2o -stranded DNA fragments in a checkerboard array. Each DNA segment on the chip is obtained from a particular gene. There may be thousands of DNA sequences on the chip, perhaps representing every gene ".m the organism s genome! To find out which genes are active in a particular cell, for example, a fruit fly follicle cell, a researcher collects all the different mrnas transcribed by that kind of cell.

8 Chapter 11 This collection of mrnas is mixed with 21 transcriptase enzyme, which produces a mix of DNA fragments called copy DNAs or 22 These cdnas are modified so they will fluoresce, or glow. Then a small amount of the cdna mixture from the cell in question (the follicle cell in this example) is added to each of the DNA fragments on the chip. If a molecule in the cdna mixture is 23 to a DNA fragment at a particular spot on the chip, it will bind to it. After nonbinding DNA is rinsed away, the pattern of glowing spots enables the researcher to identify which genes are being 24 in the ceils from which the mrna was obtained. In this way, one can find out which genes are active in different parts of the organism during different stages in development. DNA microarrays are valuable tools in other areas of research and medicine. For example, a DNA chip can test for many different kinds of infectious bacteria at one time, or distinguish among different subtypes of 25 based on the activity of 17 genes. Cell signaling systems evolved early in the history of life, and the signal transduction pathways of many organisms, from microorganisms to multicellular eukaryotes, show many colnmon features. Review signal transduction by matcl~ing each of the parts.and processes in the diagram below with its name (A-H) and description (P-W). Signaling cel! Target cell Names A. Transcription B. Relay protein C. Gene D. Receptor protein E, Transcription factor E Translation G. New protein H, Signal molecule Descriptions P. Making a new protein Q. Information for making target cell protein R. Binds to receptor S. Conducts message within target cell T. Carries out new function in target cell U. Synthesis of specific mrna coding for new protein V. When activated, it binds to DNA and "turns on" a gene W. Receives signal molecule

9 How Genes Are Controlled Cloning demonstrates that differentiated cells retain all of their genetic potential. Stem cells of embryos and adults are able to differentiate into many kinds of cells--useful for reproduction and treating disease. Review cloning and stem cells by matching each phrase with a term from the list on the right. 1. Partially differentiated cells present in mature animals 2. Producing genetically identical organisms for agriculture, research, or saving endangered species 3. Naturally occurring animal or human clones 4. Cells that give rise to all specialized cells in the body 5. Regrowth 0f lost body parts 6. The proces s of cell specialization 7. Ability of adult cell to reverse specialization 8. Growing cells for replacement or repair of damaged or diseased organs 9. Genetically identical organisms 10. Replacing the nucleus of an egg or zygote with a nucleus from a differentiated cell A. Reproductive cloning B. Nuclear transplantation C. Differentiation D. Embryonic stem cells E. Dedifferentiation F. Adult stem cells G. Regeneration H. Clones I. Therapeutic cloning J. Identical twins Review the causes and mechanisms of cancer by filling in the blanks in the following story. In the United States, lung cancer kills about 160,000 people per year. Long one of the most common kinds of cancer in men, lung cancer has passed breast cancer to become the most frequent cancer in women. Cancer is uncontrolled multiplication of cells. Cancer cells have escaped from the normal 1 systems responsible for regulating cell 2 Cancer cells form abnormal masses called tumors, which displace nearby normal tissues and can spread through the body. The cell changes that lead to cancer are caused by accumulated 3 in genes that 4 cell division and other genes that 5 cell division. Because the growing tumors block breathing passages, the first symptoms of lung cancer are usually coughing and difficulty breathing. The tumorous masses show up on chest X-rays, and usually a small sample of lung tissue is taken to examine the tumor cells. What causes lung cancer? Cancer-causing agents are called 6 Radiation, such as X-rays and UV light, are known to cause some cancers, but most are caused by chemicals. Carcinogens in tobacco smoke appear to be the major cause of lung cancer. An increase in cigarette smoking over the last century was paralleled by a rise in ltmg cancer rates. Tobacco has also been linked to other forms of cancer, such as cancer of the mouth and throat. Chemicals in tobacco smoke act as 7, triggering mutations in lung cells exposed to the smoke. Scientists have learned a lot about the cellular mechanisms of cancer by studying cancers caused by viruses in humans and other animals. Researchers were surprised to find that cancer-causing viruses carry cancer-causing genes, called 8, as part of their genome. When the viruses insert their genes into the chromosomes of a host cell, the cancer-causing genes are inserted as well. Even more surprising, researchers

10 Chapter 11 found that oncogenes are simply altered versions of genes normally found in all cells. These normal genes, called 9, usually code for proteins called 10 factors--which normally stimulate cell 11, or for other proteins that affect the cell cycle. Most cancers seem to begin with a in a proto-oncogene in a body (somatic) cell, turning it into an oncogene. The altered gene may produce a hyperactive growth stimulating protein, or an of the normal protein. 14 Changes in genes whose products normally inhibit cell division--so-called genes--also contribute to the development of cancer. One kind of tumor-suppressor gene acts to 15 damaged DNA. Others work through signal-transduction pathways to create inhibitory proteins. It appears that it takes at least 16 active oncogene and the mutation or loss of 17 tumor-suppressor genes for a cell to become fully cancerous. This may explain why the likelihood of cancer increases with 18 Generally, the normal products of proto-oncogenes and tumor-suppressor genes are involved in 19 -transduction pathways. Normally, the 20 product of a proto-oncogene (such as one called ras) might act to conduct a signal from a 21 factor to the interior of a cell, activating a gene that makes a protein that causes 22 to occur. When the protooncogene mutates into an oncogene, it might produce a hyperactive protein that signals an increase in cell division even in the 23 of the growth factor. Cell division can also be affected by a mutant tumor-suppressor gene. The tumor suppressor gene p53 produces a protein that acts as a 24 factor, which normally acts at the end of a pathway that promotes production of a protein that blocks cell division. A mutation in p53 could produce a defective transcription factor, which cannot trigger transcription. In this case, the inhibitory protein is not transcribed, allowing an 25 rate of cell division. We are beginning to understand the genetic and cellular changes that cause cancer to develop. Lifestyle choices can decrease the chances of developing cancer. Regular 26 can detect tumors early and increase the chances for successful treatment. Not 27, avoiding overexposure to the 28 and a 29 -fiber, low -3 diet all reduce cancer risk. After reading this module, match each of the following human cancers with the correct associated risk factor(s). Some answers are used more than once. 1. Lung A. Ultraviolet light 2. Colon and rectum B. African heritage, possibly dietary fat 3. Breast C. Tobacco smoke 4. Prostate D. Estrogen 5. Urinary bladder E. Alcohol; hepatitis viruses 6. Kidney F. High dietary fat, tobacco smoke, alcohol 7. Lymphomas G. Viruses (some types). 8. Melanoma (skin) 9. Liver 10. Uterus

11 How Genes Are Controlled Test Your Knowledge Multiple Choice o o Your muscle and bone cells are different because a. they contain different sets of genes. b. they are differentiated. c. they contain different operons. d. different genes are expressed in each. e. they contain different histones. Operons enable bacteria to a. function in frequently changing environments. b. increase their genetic diversity. c. correct mutations that might interfere with their genetic instructions. d: differentiate. e. mutate and evolve more rapidly. If the nucleus of a frog egg is destroyed and replaced with the nucleus of an intestine cell from a tadpole, the egg can develop into a normal tadpole. This demonstrates that a. intestine cells are fi~y differentiated. b. there is little difference between an egg cell and an intestine cell. c. an intestine cell possesses a full set of genes. d. intestine cells are not differentiated. e. frogs can regenerate lost parts. DNA packing--the way DNA is folded into chromosomes--affects gene expression by a. controlling access to DNA. b. positioning related genes near each other. c. protecting DNA from mutations. d. enhancing recombination of genes. e. allowing "unpacked" genes to be eliminated from the genome: The genes that malfunction in cancer normally a. function in DNA replication. b. are responsible for organizing DNA packing. c. code for enzymes that transcribe DNA. d. are not present in most body cells unless inserted by a virus. e. regulate cell division. In most eukaryotic cells, most genes are not expressed. This suggests that most eukaryotic regulatory proteins act as a. exons. b. repressors. c. introns. d. enhancers. e. activators. 7. After an rnrna molecule is transcribed from a eukaryotic gene, portions called are removed and the remaining are spliced together to produce an mrna molecule with a continuous coding sequence. a. operators.., promoters b. exons.., introns c. silencers.., enhancers d. introns.., exons e. promoters.., operators 8. Which of the following mechanisms of gene regulation operates after mrna transcription but before translation of mrna into protein? a. RNA splicing b. DNA packing c. repressors and activators d. protein degradation e. all of the above 9. Homeotic genes a. are responsible for.the cellular changes that occur in cancer. b. coordinate development by controlling other genes. c. are found only.in certain cells. d. are any genes transferred from cell to cell by viruses. e. repair DNA and defend against mutations. 10. In a eukaryote, which of the following may block gene expression by binding to DNA? a. an operon b. an oncogene c. an enhancer d. a promoter e. a silencer Which of the following appear to have the most potential for use in therapeutic cloning? a. adult stem cells b. somatic cells c. gametes d. cancer cells e. embryonic stem cells Gene expression in animal development seems to be regulated largely by a. controlling gene packing and unpacking. b. controlling the transcription of genes into mrnas. c. controlling the translation of mrnas into protein. d. selectively eliminating certain genes from the genome. e. selectively breaking down certain proteins so they cannot function.

12 Chapter Which of the following is a known or likely carcinogen? a. ultraviolet light b. chemicals in cigarette smoke c. alcohol d. X-rays e. all of the above 14. Which of the following is the first thing that happens when a signal molecule acts on a target cell? a. A transcription factor acts on the DNA. b. The signal molecule binds to the DNA. c. A new protein is made in the target cell. d. A specific gene is transcribed. e. The signal mole4ule binds to a receptor. 15. Researchers want to test whether a particular combination of 56 suspected genes are active in cancer cells. They might find out by using a. reverse transcriptase. b. a signal transduction pathway. c. a DNA microarray. d. therapeutic cloning. e. nuclear transplantation. Essay 1. In the proper growth medium, a single cell from a Boston fern can be stimulated to grow into an entire plant. (This is how nurseries propagate many houseplants.) What does this signify with regard to cellular differentiation in plants? 2. What are introns and exons? Discuss three possible biological functions of introns. 3. What is a homeotic gene? Why does a mutation in a homeotic gene have a much more drastic effect on the organism than a mutation in other genes? 4. Briefly explain how genes control development of the head-to-tail axis of a fruit-fly embryo. 5. Describe the changes in a cell that can make the cell become cancerous. What is the difference between reproductive and therapeutic cloning? Compare the relative advantages and disadvantages of using embryonic stem cells versus adult stem cells for therapeutic cloning. If a person wishes to avoid cancer, what factors in the environment should he or she try to avoid? What dietary and health habits would you recommend? Apply the Concepts Multiple Choice o When a certain bacterium encounters the antibiotic tetracycline, the antibiotic molecule enters the cell and attaches to a repressor protein. This keeps the repressor from binding to the bacterial chromosome, allowing a set of genes to be transcribed. These genes code for enzymes that break down the antibiotic. This set of genes is best described as a. an exon. b. a signal transduction pathway. c. an operon. d. a homeotic gene. e. a nucleosome. A genetic defect in humans results in the absence of sweat glands in the skin. Some men have this defect all over their bodies, but in women it is usually expressed in a peculiar way. A woman with the defect typically has small patches of skin with sweat glands and other patches where sweat glands are lacking. This pattern suggests the phenotypic effect of a. a mutation. b. chromosome inactivation. c. RNA splicing. d. an operon. e. mirnas. A bacterium either makes the amino acid glycine or absorbs it from its surroundings. A biochemist found that glycine binds to a repressor protein and causes the repressor to bind to the bacterial chromosome, "turning off" an operon. If this is like other operons, the genes of this operon probably code for enzymes that a. control bacterial cell division. b. break down glycine. c. produce glycine. d. cause the bacterium to differentiate. e. manufacture the repressor protein. In humans, the hormone testosterone enters cells and binds to specific proteins, which in turn bind to specific sites on the cells DNA. These proteins probably act to a. help RNA polymerase transcribe certain genes. b. alter the pattern of DNA splicing. c. stimulate protein synthesis. d. unwind the DNA so that its genes can be transcribed. e. cause mutations in the DNA,

13 How Genes Are Controlled 5. It is possible for a cell to make proteins that last for months; hemoglobin in red blood cells is a good example. However, many proteins are not this long-lasting. They may be degraded in days or even hours. Why do you think cells make proteins with such short lifetimes if it is possible to make them last longer? a. Most proteins are used only once. b. Most cells in the body live only a few days. c. Cells lack the raw materials to make most of the proteins they need. d. Only cancer cells, which can keep dividing, contain long-lasting proteins. e. This enables cells to control the amount of protein present. 6. Dioxin~ produced as a by-product of various industrial chemical processes, is suspected of causing cancer and birth defects in animals and humans. It apparently acts by entering cells and binding to proteins, altering the pattern of gene expression. The proteins affected by dioxin are probably a. enzymes. b. DNA polymerases. c. transcription factors. d. enhancers. e. nucleosomes. Researchers studying medical records in a Swedish village found that a famine at a critical time in the lives of grandparents affected the life expectancy of their grandchildren. This appears to be an instance of a. differentiation. b. epigenetic inheritance. c. alternative RNA splicing. d. regeneration. e. X chromosome inactivation. Which of the following would be most likely to lead to cancer? a. multiplication of a proto-oncogene and inactivation of a tumor-suppressor gene b. hyperactivity of a proto-oncogene and activation of a tumor-suppressor gene c. inactivation of a proto-oncogene and multiplication of a tumor-suppressor gene d. inactivation of both a proto-oncogene and a tumor-suppressor gene e. hyperactivity of both a proto-oncogene and a tumor-suppressor gene A cell biologist found that two different proteins with largely different structures were translated from two different mrnas. These mrnas, however, were transcribed from the same gene in the cell nucleus. What mechanism that follows could best account for this? a. Different systems of DNA unpacking could result in two different rnrnas. b. A mutation might have altered the gene. c. Exons from the same gene could be spliced in different ways to make different ~As. d. The two mrnas could be transcribed from different chromosomes. e. Different chemicals activated different operons. Researchers have found homeotic genes in humans, but they are not yet certain how these genes shape the human phenotype. Considering the functions of homeotic genes in other animals, which of the following is most likely to be their function in humans? a. determining skin and hai~ color b. regulating cellular metabolic rate c. determining head and tail, back and front d. determining whether an individual is male or female e. regulating the rate and timing of cell division LOok at the signal transduction pathway in Figure 11.18A. Which of the following changes might cause a speed-up in cell division? A mutation that a. alters the middle relay protein so that it does not respond to the first relay protein. b. alters the receptor protein so that it. overreacts to growth factor. c. alters the receptor protein so that growth factor does not fit. d. alters the transcription factor so that it does not attach to the DNA. e. alters growth factor molecules so they do not fit the receptor. A researcher has modified a virus so that it can "infect" cells with a known nucleic acid sequence that turns off expression of a selected gene. It appears that this technique exploits the phenomenon of a. tumor suppression. b. RNA interference. c. epigenetic inheritance. d. DNA methylation. e. signal transduction.

14 Chapter 11 Essay 1. Mutations sometimes affect operons. Imagine a. mutation in the regulatory gene that produces the repressor of the lac operon in E. coli. The altered repressor is no longer able to bind to the operator. What effect will this have on the bacterium? Describe how th~ree different types of cells in your body are speci~ed for different functions. How do their differences reflect differences in gene expression? Suggest a gene that might be active in each of the cells but none of the others. Suggest a gene that might be active in all the cells. Suggest a gene that is probably not active in any of the cells. A biochemist was studying a membranetransport protein consisting of 258 amino acids. She found that the gene coding for the transport protein consisted of 3,561 nucleotides. The mrna molecule from which the transport protein was transcribed contained 1,455 o nucleotides. What is the minimum number of nucleotides needed to code for the protein? How can the protein be transcribed from an nurna that is larger than necessary? How can this mrna be made from a gene that is so much larger? Explain how, in a eukaryotic cell, a gene on one chromosome might affect the expression of a gene on a different chromosome. How might a gene in a certain cell affect expression of a gene in a different cell? A certain kind of leukemia can be caused by a virus, a chemical, or radiation. Explain how these different factors can all trigger identical forms of cancer in the same kind of tissue. Researchers have suggested that it might be possible to clone an extinct woolly mammoth (an ice age elephant) from a tissue sample obtained from a mammoth frozen in a glacier. Describe how this might be done. Put Words to Work Correctly use as many of the following words as possible when reading, talking, and writing about biology: activator, adult stem cell, alternative RNA splicing, Barr body, cancer, carcinogen, clone, differentiation, DNA packing, DNA microarray, mutagen, embryonic stem cell (ES cell), enhancer, epigenetic inheritance, exon, gene expression, gene regulation, histone, homeotic gene, intron, methylation, microrna (MiRNA) nuclear transplantation, nucleosome, oncogene, operator, operon, promoter, proto-oncogene, regeneration, regulatory gene, relay protein, repressor, reproductive cloning, RNA interference (RNAi), signal transduction pathway, silencer, therapeutic cloning, transcription factor, tumor-suppressor gene, X chromosome inactivation Use the Web For further review of gene control, be sure to access the exercises and questions at