13.4 Gene Regulation and Expression

Home , Gene expression, Lac operon, Lac repressor, Operon, TATA box

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Lesson Objectives Describe gene regulation in prokaryotes. Explain how most eukaryotic genes are regulated. Relate gene regulation to development in multicellular organisms. Lesson Summary Prokaryotic Gene Regulation Prokaryotes do not need to transcribe all of their genes at the same time. They can conserve energy and resources by regulating their activities, producing only those genes necessary for the cell to function. In prokaryotes, DNA-binding proteins regulate genes by controlling transcription. An operon is a group of genes that are regulated together. An example is the lac operon in the bacterium E. coli: ▶ This group of three genes must be turned on together before the bacterium can use lactose as food. ▶ When lactose is not present, the DNA-binding protein called lac repressor binds to a region called the operator, which switches the lac operon off. ▶ When lactose binds to the repressor, it causes the repressor to fall off the operator, turning the operon on.

Eukaryotic Gene Regulation Transcription factors are DNA-binding proteins. They control the expression of genes in eukaryotes by binding DNA sequences in the regulatory regions. Gene promoters have multiple binding sites for transcription factors, each of which can influence transcription. ▶ Complex gene regulation in eukaryotes makes cell specialization possible. ▶ The process by which microRNA (miRNA) molecules stop mRNA molecules from passing on their protein-making instructions is RNA interference (RNAi). ▶ RNAi technology holds the promise of allowing scientists to turn off the expression of genes from viruses and cancer cells, and it may provide new ways to treat and perhaps even cure diseases.

Genetic Control of Development Regulating gene expression is especially important in shaping the way a multicellular organism develops. Gene regulation helps cells undergo differentiation, becoming specialized in structure and function. Master control genes are like switches that trigger particular patterns of development and differentiation in cells and tissues. ▶ Homeotic genes are master control genes that regulate organs that develop in specific parts of the body. ▶ Homeobox genes share a similar 130-base DNA sequence called homeobox. They code for transcription factors that activate other genes that are important in cell development and differentiation in certain regions of the body. ▶ Hox genes are a group of homeobox genes that tell the cells of the body how to differentiate as the body grows. Environmental factors can also affect gene expression.

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Prokaryotic Gene Regulation 1. How do prokaryotes conserve energy? Prokaryotes regulate their activities, producing only those genes necessary for the cell to function. 2. How do DNA-binding proteins in prokaryotes regulate genes? They control transcription. Some of these regulatory proteins help switch genes on, while others turn genes off. 3. What is an operon? It is a group of genes that are regulated together. 4. What is in the lac operon in E. coli? three genes 5. What is the function of the genes in the lac operon of E. coli? They allow E. coli to use lactose for food when it is present.

6. What turns the lac operon off? A repressor protein turns the operon off.

7. How does a repressor protein turn off the lac operon? It binds to the operating region, blocking RNA polymerase from transcribing the lac genes. 8. How does lactose turn on the lac operon? It attaches to the repressor, which causes the repressor to fall off the operator. Transcription can take place. 9. Complete the table to describe the role of each regulatory region or molecule in the operation of the lac operon.

Regulatory Region What It Does or Molecule Repressor protein Binds to the operator, preventing transcription of the lac genes

Operator Binding site for the repressor protein

RNA polymerase When the repressor is not present, this enzyme carries out the transcription of lac genes. Lactose Causes the repressor to drop off the operator so transcription of the lac genes can begin

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Eukaryotic Gene Regulation 10. In what two ways is gene regulation in eukaryotes different from gene regulation in prokaryotes? a. Most eukaryotic genes are controlled individually. b. Eukaryotic cells have more complex regulatory sequences than those of the lac repressor system.

11. What is a TATA box? What does a TATA box do? It is a short region of DNA that contains a sequence of T and A base pairs. The protein that binds to this site helps position RNA polymerase.

12. What are transcription factors and what do they do? They are DNA-binding proteins that bind to DNA sequences in the regulatory regions of genes and help control gene expression.

13. Explain how gene regulation makes cell specialization possible. It allows particular genes to be expressed in some kinds of cells but not others.

14. What is microRNA and how is it related to mRNA? Small RNA molecules are called microRNA. They attach to certain mRNA molecules and stop them from passing on their protein-making instructions.

15. Explain how the process of RNA interference works. Certain small RNA molecules fold into loops. The Dicer enzyme cuts them into microRNA (miRNA). The strands then separate. An miRNA piece attaches to a cluster of proteins to form a silencing complex. The silencing complex binds to and destroys an mRNA molecule that contains a base sequence complementary to the miRNA. In this way, it blocks gene expression.

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Genetic Control of Development For Questions 16–23, write the letter of the correct answer on the line at the left. C 16. As an embryo develops, different sets of genes are regulated by A. mRNA and lac repressors. C. transcription factors and repressors. B. operons and operators. D. promoters and operators. C 17. The process through which cells become specialized in structure and function is A. transcription. C. differentiation. B. gene expression. D. RNA interference. B 18. Homeotic genes are A. regulator genes that bind to operons in prokaryotes. B. master control genes that regulate organs that develop in specific parts of the body. C. parts of the silencing complex that regulates gene action through RNA interference. D. base sequences complementary to sequences in microRNA. A 19. What role do homeobox genes play in cell differentiation? A. They code for transcription factors that activate other genes important in cell development and differentiation. B. They block certain gene expression. C. They cut double-stranded loops into microRNA. D. They attach to a cluster of proteins to form a silencing complex, which binds to and destroys certain RNA. D 20. I n flies, the group of homeobox genes that determines the identities of each segment of a fly’s body is the group known as A. silencing complexes. C. operators. B. promoters. D. Hox genes. D 21. Clusters of Hox genes are found in A. flies only. C. plants only. B. flies and frogs only. D. nearly all animals. B 22. The “switches” that trigger particular patterns of development and differentiation in cells and tissues are A. mRNA molecules. C. silencing complexes. B. master control genes. D. Dicer enzymes. A 23. Metamorphosis is A. a series of transformations from one life stage to another. B. the master switch that triggers development and differentiation. C. the product of interactions among homeotic genes. D. the process by which genetic information is passed from one generation to the next.

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24. Environmental factors can influence gene expression. Fill in the table below to show how organisms respond to conditions in their environment.

Environmental Factor Influencing Gene How the Organism Responds Expression E. coli with limited food nutrient availability The lac operon is switched on supply when lactose is the only food source.

A tadpole in a drying lack of water The tadpole may speed up its pond metamorphosis.

25. Many research studies have shown that different species may possess some of the exact same genes but show vastly different traits. How can that happen? The difference arises not from the genes themselves but from how they are regulated and expressed. For example, a gene may be turned on at a different time in one species than in another. Perhaps environmental factors have an effect, too.

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Chapter Vocabulary Review For Questions 1–7, write True if the statement is true. If the statement is false, change the underlined word or words to make the statement true. RNA 1. DNA contains the sugar ribose. True 2. Messenger RNA carries copies of the instructions for making proteins from DNA to other parts of the cell. Transfer RNA 3. RNA polymerase transfers amino acids to ribosomes. True 4. The process of transcription produces a complementary strand of RNA on a DNA template. True 5. The enzyme that assembles a complementary strand of RNA on a DNA template is RNA polymerase. promoter 6. The region of DNA where the production of an RNA strand begins is called the intron. True 7. Exons are spliced together in forming messenger RNA.

For Questions 8–16, match the term with its deﬁ nition.

Definition Term B 8. The sequence of bases that serves as the A. polypeptide “language” of life B. genetic code E 9. A sequence of three bases on a tRNA molecule C. codon that is complementary to a sequence of bases on an mRNA molecule D. translation F 10. How genetic information is put into action in a E. anticodon living cell F. gene expression I 11. Having extra sets of chromosomes G. mutation D 12. The decoding of an mRNA message into a H. mutagen protein I. polyploidy G 13. A heritable change in genetic information A 14. A chain of amino acids C 15. The three consecutive bases that specify a single amino acid to be added to the polypeptide chain H 16. A chemical or physical agent that causes a change in a gene

For Questions 17–19, complete each statement by writing the correct word or words. 17. A group of genes that are regulated together is called a(n) operon . 18. A region of DNA where a repressor can bind is a(n) operator . 19. Master control genes, called homeotic genes, regulate organs that develop in specific parts of the body.

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In the Chapter Mystery, you learned about a fly that was genetically manipulated to grow eyes in different places on its body. Scientists continue to develop techniques MOUSE-EYED FLY to modify a variety of other animals. But should they?

Learning

Should Genetic Experiments Be Performed on Animals? Some people think that the type of research that produced the mouse-eyed fly is perfectly acceptable. Others think that it’s a terrible thing to do even to a fly. In the biological sciences, animal testing has always been controversial, and it probably always will be.

Perspectives on Animal Genetic Experiments Animal rights groups have come a long way in recent years. More people are responding to the issues brought up by animal rights groups. For example, animal testing of cosmetics has been banned in a number of countries, and in the United States many companies have voluntarily stopped testing on animals. So what’s the next frontier for animal rights activists? According to Edward Avellone of Animal Rights Now!, it’s genetic experimentation. “What purpose is there in creating a mouse with six legs or a sheep with one eye in the middle of its forehead?” asks Avellone. “Scientists are just playing around with a new technology. Th ey’re creating horribly deformed animals for no real reason.” Some people disagree with this point of view. Says Ann Wilber of Scientists for the Ethical Treatment of Animals, “We’re responsible professionals, not monsters.” Wilber explains that the one-eyed sheep was the unintended result of an attempt to understand how the eye developed and how it works. “We’ve also developed a sheep whose milk contains a protein that might cure emphysema. Th ere are reasons for what we do.” But to Avellone, the point is not simply the motivation behind the experimentation. It’s also the process of the experiment. “Only 10 percent of the animals they breed have the gene they want to study. Th e remaining 90 percent [of the animals] are simply killed.” Wilber admits that this situation is “sad, but true.” Still, she says, “We’re working every day to improve our techniques and therefore our success rate.” Even if the success rate never tops 10 percent, she asks, “Isn’t that a small price to pay for a cure for cancer, or multiple sclerosis, or Parkinson’s disease?”

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Themes Science and Civic Literacy 1. Edward Avellone makes several arguments against animal testing in genetics experiments. Why does he think the process of animal genetic experimentation is flawed? He says that only 10 percent of animals scientists breed have the gene they want to study. He says the remaining 90 percent of the animals are killed because they are not useful. 2. Avellone argues that genetic experiments are unnecessary. What claim does he make about scientists’ motivations for doing such experiments? He says that scientists are performing experiments with no real scientific purpose, and that they are “playing with new technology.” 3. What is Ann Wilber’s main argument in favor of genetic testing on animals? It could lead to cures for serious human diseases.

4. Which of her arguments could be summed up as, “It’s a necessary evil”? Killing 90 percent of the animals they breed is a small price to pay to cure diseases.

5. Do you agree with Avellone, with Wilber, or with neither of them? Why? Accept any answers that are clearly argued, logical, and based on evidence, either from the article or from other reliable sources.

Evaluating an Issue The skills used in this activity include creativity and intellectual curiosity, communication skills, interpersonal and collaborative skills, information and media literacy, and social responsibility. The issue of experimenting on animals, especially genetic experimentation, is complex; you can’t construct an informed opinion after reading just one article. Working with a group, use library and Internet resources to collect opinions on both sides of the issue. Work through the material thoroughly and then make up your mind about how you feel about the issue.

If everyone in the group agrees, create a multimedia presentation for your class in which you present your point of view. If the members of your group disagree, stage a debate in front of the class, with the same number of students arguing each side of the issue. Evaluate students’ presentations based on the thoroughness of their research, the inclusion of sources from both sides of the issue, and the quality of the sources they used; also, consider the cogency of the arguments they make to support their point of view.

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