Chapter 11: Gene Expression

11-1 Control of Gene Expression

11-2 Gene Expression and Development

I. Role of Gene Expression (3 Key Points)

• Cells use different genes to build different proteins.

• NOT all proteins are required at same time REGULATING gene expression, cells are able to control WHEN each protein is made.

11-1 Control of Gene Expression

• Gene expression is thus the activation of a gene resulting in the synthesis of a protein.

(1) A molecular biologist isolates mRNA from the brain and liver of a mouse and finds that the two types of mRNA are different. Can these results be correct or has the biologist made an error? Explain your answer.

Critical Thinking

(1) Genome (e.g., Human Genome)

• Complete set of genes contained within an individual’s cells.

II. Gene Expression in Prokaryotes

• Francois Jacob and Jacques Monod were first to discover how genes control the metabolism of lactose in Escherichia coli (E. coli), a species of bacteria.

(1) Structural Genes

• DNA segments (genes) that code for specific polypeptides (proteins).

(2) Promoter• DNA segment that recognizes RNA polymerase and promotes (initiates) transcription.

(3) Operator• DNA segment that serves as a binding site for an “inhibitory protein” that blocks transcription and prevents protein synthesis.

(4) Operon (entire structure)

• The structural genes, the promoter, and the operator collectively form a series of genes called the “operon.”

(2) What region of a prokaryotic gene is analogous to the ENHANCER region of a eukaryotic gene?

Critical Thinking

(A) Repression (middle diagram)• Occurs when a repressor protein physically blocks

transcription (prevents protein synthesis).

(1) Repressor Protein (purple structure below)

• A protein that inhibits a specific set of structural genes from being expressed.

(2) Regulator Gene (yellow stretch of DNA below)

• DNA sequence codes for production of a repressor protein (i.e., it regulates whether or not transcription will be repressed)

(B) Activation (bottom diagram)

• The initiation of transcription as a result of the removal of a repressor protein (leads to protein synthesis)

(1) Inducer (see red molecule, middle diagram)• A molecule that initiates gene expression by removing the

repressor protein (e.g., lactose is an inducer)

(2) Activation

• Activation (bottom) works with repression (middle) as a means to conserve NRG and produce only proteins immediately desired.

II. Gene Expression in Eukaryotes

• DNAEuk is located in duplicated (X) chromosomes instead of a single circular chromosome (plasmid) like that of DNAPro

(A) Structure of a Eukaryotic Gene• In eukaryotes, gene expression is partly related to the coiling and uncoiling of DNA within each chromosome.

• AFTER M phase of cell cycle, certain regions of the DNA coils relax, making transcription possible.

(1) Euchromatin

• Uncoiled form of DNA; the site of active transcription of DNA to RNA (the coiled portion remaining is unable to be transcribed)

(2) Introns (mutation resistant)

• (Beyond the promoter), sections of a structural gene that do NOT code for amino acids (i.e., do not get translated to amino acids)

(3) Exons (mutation vulnerable)

• (Beyond the promoter), sections of a structural gene that DO get expressed and will be translated into amino acids (proteins)

Side Note: Evolutionarily speaking, it is believed that the intron-exon pattern could facilitate the exchange of exons among homologous chromosomes during crossing over in meiosis, leading to an additional source of genetic diversity essential for evolution.

(B) Control After Transcription• Unlike prokaryotes, eukaryotes can control gene expression by modifying RNA AFTER transcription.

(1) Pre-mRNA• Transcription results in both introns and exons being copied from DNA onto RNA; the pre-RNA is the large, resulting molecule.

• Final mRNA

• Formed when introns are removed and remaining exons are spliced to one another, resulting in mRNA with only the exons.

• Similar splicing occurs following the transcription of tRNA and rRNA (enzyme mediated)

(C) Enhancer Control

• Eukaryotic genes also have non-coding control sequences that facilitate transcription.

(1) Enhancer

• Non-coding control sequence of DNA that must be activated for its associated gene to be expressed.

(2) Transcription Factors

• Additional proteins that bind to enhancers and RNA polymerase and regulate transcription.

I. Cell Differentiation• Although EACH cell in a developing organism contains the SAME genes, only a FRACTION of the genes are expressed. In order to become specialized, cells must FIRST differentiate.

11-2 Gene Expression and Development

• As cells differentiate and organisms grow, organs and tissues develop to produce a characteristic form.

(1) Morphogenesis

(A) Homeotic Genes

• Regulatory genes that determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis.

• I.E., Master genes of development that determine the overall body organization (See mutant fruit fly below with a second thorax).Ex: In fruit flies, each homeotic gene shares a common DNA sequence of 180 b.p.; this specific sequence within a homeotic gene regulates patterns of development (i.e., a homeobox)

(1) Homeobox

• It is believed that all organisms may have similar homeoboxes that code their anatomy.

II. Cancer

• A condition of abnormal proliferation of defective, unregulated cells (i.e., a tumor) that result from a specific alteration to the cells’ DNA.

(3) Why might X rays be more dangerous to an ovary or a testis than to muscle tissue?

Critical Thinking

(1) Benign Tumor (non-cancerous growth)

• Cells remain within a mass and are unlikely to spread. Examples include fibroid cysts (breast tissue) and warts. (Surgery)

(2) Malignant Tumor (cancerous growth)

• Cells invade and destroy healthy tissues elsewhere in the body; Malignant cells tend to break away and form new tumors in other parts of the body, hijacking resources and out-competing healthy cells.

(3) Metastasis• Spread of cancer beyond its original site, results in cancerous growths in parts of body not originally affected. (Chemotherapy and/or Radiation Therapy)

(A) Types of Cancer

• Malignant tumors are categorized according to the types of tissues they affect in the organism.

(1) Carcinomas

• Grow in the skin and the tissues that line the organs of the body.

(2) Sarcomas (progressive bone cancer shown below in two x rays)• Grow in the bone and muscle tissue.

• Solid tumors that grow in the liquid tissues of the blood

(3) Lymphomas

• Malignant cancer of the white blood cells, a class of lymphoma

(4) Leukemia

(B) Cancer and the Cell Cycle

• Factors that normally govern the rate of cell division include…

• Cell must receive adequate nutrition

• Cell must be attached to other cells, to a membrane, or to fibers between cells.

• Normal cells stop dividing when they become too crowded, usually after 20-50 divisions

• Cancer cells will continue dividing despite crowded conditions and despite a lack of attachment (can lead to metastasis)

(C) Causes of Cancer

• Cancer results when a cell loses the ability to regulate cell growth and division due to a change in its regulatory genes.

(1) Growth Factors (see blue squares below)

• Are proteins made by regulatory genes to ensure the events of cell division occur in proper sequence and at correct rate (mutations of these genes can lead to cancerous cells)

Note: Cells that produce tumor angiogenesis factor (TAF) form some of the most malignant tumors. TAF affects nearby blood vessels, causing them to grow toward the tumor.

(2) Carcinogen• A class of molecules that are known to increase the risk of altering the regulatory genes in cells (i.e, have been known to lead to cancer)• Ex: tobacco smoke, air pollutants, asbestos, and radiation from X-rays or ultraviolet radiation (sun)

• A class of molecules that are known to cause mutations to occur within the cell (within sequences of DNA)

(3) Mutagen

(4) Mutations may occur in gametes or in body cells. In which cell type is a mutation likely to be a source of genetic variation for evolution? Why?

Critical Thinking

(D) Oncogenes

• A gene that causes cancer or other uncontrolled cell proliferation.

• Oncogenes begin as normal genes (proto-oncogenes) that control a cell’s growth and differentiation.

(1) Proto-oncogenes

• Normal genes that code for proteins that regulate cell growth, cell division, and the ability to adhere to other cells.

• A mutation in these genes could cause a change in protein production, leading to an increased division rate and possibly cancer.

(2) Tumor-suppressor genes

• Healthy genes that code for proteins that stop the uncontrolled rate of cell division; if THESE GENES mutate, a change in protein production may occur, predisposing the cell to becoming cancerous.

(E) Viruses and Cancer

• Many viral genes are oncogenes; viruses can also initiate cancer in an infected cell by causing mutations in proto-oncogenes or tumor-suppressor genes, altering the rate of cell division.

• Leukemia has shown ties to certain viral infections.

Extra Slides AND Answers for Critical Thinking Questions

(1) The operator region of a prokaryote is analogous to the enhancer region of a eukaryotic gene. Both operators and enhancers act as a switch that must be turned “on” to activate the expression of a gene.

(2) The ovaries and testes contain rapidly dividing cells that will become egg and sperm cells, respectively. A mutation due to X-ray exposure could thus be passed on to offspring.

(3) The results are probably correct because different genes were expressed in the brain and liver tissues, resulting in the production of two types of mRNA.

(4) A mutation in the gametes is likely to be a source of genetic variation because gametes pass on the mutation to the next generation when forming the zygote.