Gene Expression

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Chapter 11 Gene Expression

Table of Contents Section 1 Control of Gene Expression

Section 2 Gene Expression in Development and Cell Division

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Objectives

• Explain why cells regulate gene expression.

• Discuss the role of operons in prokaryotic gene expression.

• Determine how repressor proteins and inducers affect transcription in prokaryotes.

• Describe the structure of a eukaryotic gene.

• Compare the two ways gene expression is controlled in eukaryotes.

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Role of Gene Expression

• Gene expression is the activation of a gene that results in transcription and the production of mRNA.

• Only a fraction of any cell’s genes are expressed at any one time.

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Gene Expression in Prokaryotes

• An operon is a series of genes that code for specific products and the regulatory elements that control these genes. In prokaryotes, the structural genes, the promoter, and the operator collectively form an operon.

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Operon

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Gene Expression in Prokaryotes, continued

• A promoter is the segment of DNA that is recognized by the enzyme RNA polymerase, which then initiates transcription.

• An operator is the segment of DNA that acts as a ―switch‖ by controlling the access of RNA polymerase to the promoter.

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Gene Expression in Prokaryotes, continued

• Operon “Turned Off” – Repressor proteins are coded for by regulator genes and these proteins inhibit genes from being expressed. – A repressor protein attaches to the operator, physically blocking the advancement of RNA polymerase.

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Repression of Transcription in the lac Operon

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Gene Expression in Prokaryotes, continued

• Operon “Turned On” – An inducer is a molecule that initiates gene expression. In E. coli, lactose serves as an inducer. – An inducer binds to the repressor protein and the repressor protein detaches from the operator. RNA polymerase can then advance to the structural genes.

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Activation of Transcription in the lac Operon

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Mechanism of lac Operon

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Gene Expression in Eukaryotes

• Structure of a Eukaryotic Gene – Eukaryotes do not have operons. – The genomes of eukaryotes are larger and more complex than those of prokaryotes. – Eukaryotic genes are organized into noncoding sections, called introns, and coding sections, called exons.

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Gene Expression in Eukaryotes, continued • Control After Transcription – In eukaryotes, gene expression can be controlled after transcription—through the removal of introns from pre-mRNA.

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Removal of Introns After Transcription

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Gene Expression in Eukaryotes, continued • Control at the Onset of Transcription – In eukaryotes, gene expression can be controlled at the onset of transcription—through the action of regulatory proteins known as transcription factors.

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Enhancers for Control of Gene Expression

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Controlling Transcription in Eukaryotes

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Objectives • Summarize the role of gene expression in an organism’s development.

• Describe the influence of homeotic genes in eukaryotic development.

• State the role of the homeobox in eukaryotic development.

• Summarize the effects of mutations in causing cancer.

• Compare the characteristics of cancer cells with those of normal cells.

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Gene Expression in Development

• The development of cells with specialized functions is called cell differentiation.

• The development of form in an organism is called morphogenesis.

• Both cell differentiation and morphogenesis are governed by gene expression.

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Section 2 Gene Expression in Chapter 11 Development and Cell Division

Gene Expression in Development, continued

• Homeotic Genes – Homeotic genes are regulatory genes that determine where anatomical structures will be placed during development.

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Gene Expression in Development, continued

• Homeobox Sequences – Within each homeotic gene, a specific DNA sequence known as the homeobox regulates patterns of development.

– The homeoboxes of many eukaryotic organisms appear to be very similar.

Gene Expression in Development, continued

• Tracking Changes in Gene Expression – In the 1990s, researchers developed a tool for tracking gene expression called a DNA chip.

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Gene Expression, Cell Division, and Cancer

• Mutations of proto-oncogenes, which regulate cell growth, or tumor-suppressor genes, which prevent cell division from occurring too often,may lead to cancer.

• Cancer is the uncontrolled growth of abnormal cells.

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Effect of Mutation on Gene Expression

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Gene Expression, Cell Division, and Cancer, continued

• Gene Expression in Cancer – Unlike normal cells, cancer cells continue to divide indefinitely, even if they become densely packed. – Cancer cells will also continue dividing even if they are no longer attached to other cells.

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Gene Expression, Cell Division, and Cancer, continued

• Causes of Cancer – A carcinogen is any substance that can induce or promote cancer. – Most carcinogens are mutagens, substances that cause mutations.

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Multiple Choice

1. Which of the following codes for a repressor protein? A. enhancer B. promoter C. regulator gene D. structural gene

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Multiple Choice, continued

1. Which of the following codes for a repressor protein? A. enhancer B. promoter C. regulator gene D. structural gene

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Multiple Choice, continued

2. Which component of an operon controls the advancement of RNA polymerase? F. exon G. operator H. promoter J. structural gene

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Multiple Choice, continued

2. Which component of an operon controls the advancement of RNA polymerase? F. exon G. operator H. promoter J. structural gene

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Multiple Choice, continued

3. Pre-mRNA contains which of the following? A. exons only B. introns only C. both introns and exons D. neither introns nor exons

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Multiple Choice, continued

3. Pre-mRNA contains which of the following? A. exons only B. introns only C. both introns and exons D. neither introns nor exons

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G. As the number of cigarettes smoked decreased, the incidence of lung cancer increased. H. As the number of cigarettes smoked increased, the incidence of lung cancer increased. J. As the number of cigarettes smoked increased, the incidence of lung cancer decreased.

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Multiple Choice, continued The graph below shows the number 4. What was the relationship between of cigarettes smoked per capita per number of cigarettes smoked and year between 1920 and 2000 and incidence of lung cancer? the annual incidence of lung cancer F. There was no relationship among women. Use the graph to between cigarette smoking and lung answer the question that follows. cancer. G. As the number of cigarettes smoked decreased, the incidence of lung cancer increased. H. As the number of cigarettes smoked increased, the incidence of lung cancer increased. J. As the number of cigarettes smoked increased, the incidence of lung cancer decreased.

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Multiple Choice, continued

5. skin : carcinoma :: blood-forming tissue : A. sarcoma B. leukemia C. lymphoma D. carcinogen

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Multiple Choice, continued

5. skin : carcinoma :: blood-forming tissue : A. sarcoma B. leukemia C. lymphoma D. carcinogen

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Multiple Choice, continued

The diagram below shows 6. What does X represent? how mutations in certain genes can lead to cancer. Use F. mutagens the diagram to answer the G. carcinogens questions that follow. H. proto-oncogenes

J. tumor-suppressor genes

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Multiple Choice, continued

The diagram below shows 6. What does X represent? how mutations in certain genes can lead to cancer. Use F. mutagens the diagram to answer the G. carcinogens questions that follow. H. proto-oncogenes

J. tumor-suppressor genes

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Multiple Choice, continued

The diagram below shows 7. What does Y represent? how mutations in certain genes can lead to cancer. Use A. mutagens the diagram to answer the B. carcinogens questions that follow. C. proto-oncogenes

D. tumor-suppressor genes

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Multiple Choice, continued

The diagram below shows 7. What does Y represent? how mutations in certain genes can lead to cancer. Use A. mutagens the diagram to answer the B. carcinogens questions that follow. C. proto-oncogenes

D. tumor-suppressor genes

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Short Response

A biologist isolates mRNA from a mouse brain and liver and finds that the two types of mRNA differ. Can these results be correct, or has the biologist made an error? Explain your answer.

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Short Response, continued

A biologist isolates mRNA from a mouse brain and liver and finds that the two types of mRNA differ. Can these results be correct, or has the biologist made an error? Explain your answer.

Answer: The biologist did not make an error. The gene expression, and thus pre-mRNA processing is different, which results in different mRNA for different organs. This is done so that the specific proteins for that organ could be made.

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Extended Response

Mutations may occur in gametes or in body cells. Part A In which cell type could a mutation cause genetic variation in a population? Part B Explain how genetic variation could result from a mutation in this cell type.

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Extended Response, continued

Answer: Part A Mutations that occur in the gametes can be a source of genetic variation in a population. Mutations in body cells are restricted to the individual and, unlike gametic mutations, cannot be passed on to future generations (with the exception of asexually reproducing species.)

Part B Mutations arising in the gametes can be passed on to offspring during sexual reproduction and the formation of the zygote. The resulting offspring would be different (genetically) from the parent, and may show the resulting characteristics of the particular mutation.

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