chapter 11 objectives explain why cells regulate gene expression. discuss the role of operons in...
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Chapter 11
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.
Section 1 Control of Gene Expression
Section 1 Control of Gene ExpressionChapter 11
Role of Gene Expression
• Gene expression is the activation of a gene that results in transcription and the production of mRNA.
• Cells control gene expression so that their genes will only be expressed when needed
• Cells control the expression of their genes:• With regulatory sites found on each genes• With specific regulatory proteins• By determining when individual genes are to be
transcribed
Section 1 Control of Gene ExpressionChapter 11
Gene Expression in Prokaryotes, continued
• A promoter is the segment of DNA that is recognized by the enzyme RNA polymerase, which then initiates transcription.
• In order for RNA polymerase to attach to a DNA molecule, the RNA polymerase must recognize a promoter
• An operator is the segment of DNA that acts as a “switch” by regulating the access of RNA polymerase to the structural genes
Section 1 Control of Gene ExpressionChapter 11
Gene Expression in Prokaryotes
• An operon is a series of genes that code for functionally related proteins and the regulatory elements that control these genes. In prokaryotes, the structural genes, the promoter, and the operator collectively form an operon.
• lac operon -gene system whose operator gene and three structural genes control lactose metabolism in E. coli
Chapter 11
Click below to watch the Visual Concept.
Visual Concept
Operon
Section 1 Control of Gene Expression
Section 1 Control of Gene ExpressionChapter 11
Gene Expression in Prokaryotes, continued
• Operon “Turned Off” (LACTOSE IS ABSENT)– 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.
Chapter 11
Click below to watch the Visual Concept.
Visual Concept
Repression of Transcription in the lac Operon
Section 1 Control of Gene Expression
Section 1 Control of Gene ExpressionChapter 11
Gene Expression in Prokaryotes, continued
• Operon “Turned On” (LACTOSE PRESENT)– 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/removes from the operator. RNA polymerase can then advance to the structural genes.
– Inducer molecules allow transcription to proceed by changing the shape of repressor proteins
Chapter 11
Click below to watch the Visual Concept.
Visual Concept
Activation of Transcription in the lac Operon
Section 1 Control of Gene Expression
Section 1 Control of Gene ExpressionChapter 11
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 2 segments:
• Introns - noncoding sections• exons - coding sections
Section 1 Control of Gene ExpressionChapter 11
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 (form of messenger RNA that contains both introns and exons)
Section 1 Control of Gene ExpressionChapter 11
Gene Expression in Eukaryotes, continued
• Control After Transcription– After mRNA has been transcribed:
• Its introns are cut out• Its exons are joined together • It leaves the nucleus
Section 1 Control of Gene ExpressionChapter 11
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.
– Enhancer – sequence of nucleotides in a DNA molecule that aids in arranging RNA polymerase in the correct position on the promoter
Chapter 11
Click below to watch the Visual Concept.
Visual Concept
Enhancers for Control of Gene Expression
Section 1 Control of Gene Expression
Section 2 Gene Expression in Development and Cell DivisionChapter 11
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.
Section 2 Gene Expression in Development and Cell DivisionChapter 11
Gene Expression in Development
• The development of cells with specialized functions is called cell differentiation.
• As organisms grow and develop, organs and tissues develop to produce a characteristic form. This development of form in an organism is called morphogenesis.
• Both cell differentiation and morphogenesis are governed by gene expression.
Section 2 Gene Expression in Development and Cell DivisionChapter 11
Gene Expression in Development
• Examples of Morphogenesis:• The formation of cellular extensions in nerve cells
and the functioning of these cells in receiving and transmitting signals
• The formation of long, thin muscle cells that are able to respond to the proper stimulus by contracting
• The formation of liver cells that produce enzymes that break down fat
Section 2 Gene Expression in Development and Cell DivisionChapter 11
Gene Expression in Development, continued
• Homeotic Genes– Homeotic genes are regulatory genes that
determine where anatomical structures will be placed during development.
Section 2 Gene Expression in Development and Cell DivisionChapter 11
Gene Expression in Development, continued
• Homeobox Sequences– Within each homeotic gene, a specific DNA
sequence known as the homeobox regulates patterns of development.
– Pg 224, Figure 11-5
– The homeoboxes of many eukaryotic organisms appear to be very similar.
Section 2 Gene Expression in Development and Cell DivisionChapter 11
Gene Expression in Development, continued
• Characteristics of Homeoboxes– They are part of genes– They produce regulatory proteins that switch on or
off groups of developmental genes– Each controls the development of a specific part of
the adult organism
Section 2 Gene Expression in Development and Cell DivisionChapter 11
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.
Section 2 Gene Expression in Development and Cell DivisionChapter 11
Gene Expression, Cell Division, and Cancer
• Proto-oncogenes: genes that regulate the division of cells, cell growth, and ability to adhere to one another
• A mutation in a proto-oncogene can change the gene into a oncogene
• Oncogene is a gene that can cause uncontrolled cell proliferation, which can lead to CANCER
Section 2 Gene Expression in Development and Cell DivisionChapter 11
Gene Expression, Cell Division, and Cancer
• Tumor: abnormal proliferation of cells that results from uncontrolled, abnormal cell division
• Benign: generally pose no threat to life• Malignant: can cause cancer
• Cancer: uncontrolled growth of cells • Tumor-suppressor genes: genes the code for
proteins that prevent cell division from occurring too often
• Act as “brakes” to suppress tumor formation
Chapter 11
Effect of Mutation on Gene Expression
Section 2 Gene Expression in Development and Cell Division
Mutations in proto-oncogenes or tumor-suppressor genes can destroy normal gene functioning, possibly resulting in cancer. A mutation in a proto-oncogene may cause the gene to become an oncogene, a gene that triggers cancer.
Section 2 Gene Expression in Development and Cell DivisionChapter 11
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.– Metastasis: spread of malignant cells beyond
their original site
Section 2 Gene Expression in Development and Cell DivisionChapter 11
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.
Section 2 Gene Expression in Development and Cell DivisionChapter 11
Gene Expression, Cell Division, and Cancer, continued
• Kinds of Cancer– Malignant tumors can be categorized according to
the types of tissues affected– Carcinomas: grow in the skin and the tissues
that line the organs of the body– Saracomas: grow in bone and muscle tissue– Lymphomas: solid tumors that grow in the
tissues of the lymphatic system– Leukemia: grow in blood-forming tissues
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