Chapter 11: Gene Expression 11-1 Control of Gene Expression 11-2 Gene Expression and Development
11-1 Control of Gene Expression 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. • Gene expression is thus the activation of a gene resulting in the synthesis of a protein.
Critical Thinking (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.
(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.”
Critical Thinking (2)What region of a prokaryotic gene is analogous to the ENHANCER region of a eukaryotic gene?
(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.
11-2 Gene Expression and Development 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.
(1) Morphogenesis • As cells differentiate and organisms grow, organs and tissues develop to produce a characteristic form.
(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.
Critical Thinking (3)Why might X rays be more dangerous to an ovary or a testis than to muscle tissue?
(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.