chapter 11 objectives explain why cells regulate gene expression. discuss the role of operons in...

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


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


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




• 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


Visual Concept

Repression of Transcription in the lac Operon




• 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


Visual Concept

Activation of Transcription in the lac Operon


Chapter 11

Mechanism of lac Operon



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


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)

Chapter 11

Removal of Introns After Transcription




• Control After Transcription– After mRNA has been transcribed:

• Its introns are cut out• Its exons are joined together • It leaves the nucleus



• 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


Visual Concept

Enhancers for Control of Gene Expression


Chapter 11

Controlling Transcription in Eukaryotes


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.


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.


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


Gene Expression in Development, continued

• Homeotic Genes– Homeotic genes are regulatory genes that

determine where anatomical structures will be placed during development.



• 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.



• 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



• Tracking Changes in Gene Expression– In the 1990s, researchers developed a tool for

tracking gene expression called a DNA chip.


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


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.


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



• Causes of Cancer– A carcinogen is any substance that can induce or

promote cancer. – Most carcinogens are mutagens, substances that

cause mutations.



• 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

chapter 11 objectives explain why cells regulate gene expression. discuss the role of operons in...

Documents

operator gene

cells control gene expression

prokaryotic gene expression

ways gene expression

lac operon gene system

eukaryotic genes

individual genes

series of genes