GENE EXPRESSION protein is being made AND - Some are expressed for - PDF document

2/18/2013 GENE EXPRESSION IN PROKARYOTES - A gene is being “expressed” or “activated” when a GENE EXPRESSION protein is being made AND - Some are expressed for a time and then turned off MUTATION How does a cell know how and when to turn on and off certain genes? The “Players” in Prokaryotic Gene Expression Discovery of Gene Expression - Operon : promoter, operator, structural (functional) genes 1961: Francois Jacob & Jacques Monod - Promoter : control sequence, site where replication starts - studied bacteria e. coli (normal flora in intestines) - Operator : DNA sequence between promoter and enzyme genes, acts as on/off switch for genes - bacteria will break down lactose (into glucose + galactose) from dairy products in intestine to use as energy source - Functional genes : coding sections (will only do so in presence of lactose) - Inducer: protein that initiates gene expression, must be present - three enzymes needed to do this (each has a different gene) - allows bacteria to conserve energy when gene is off - lac operon : cluster of genes that enables e. coli to build proteins needed for lactose metabolism when lactose is present Steps of Gene Expression in Prokaryotes I. Turning on the lac operon The lac operon in “on” mode A. RNA polymerase attaches to promoter region near the • The default mode for the operon is the “off” position genes “START HERE” Lactose present • Gene expression occurs only when the cell needs B. RNA polymerase moves along chromosome to genes specific proteins to be made DNA RNA polymerase bound to promoter C. Once it hits genes, it mRNA produces m RNA (transcription) Protein Operon Inactive D. mRNA instructs ribosomes to repressor Lactose Enzymes for lactose make enzymes (translation) utilization Operon turned on (repressor inactivated by lactose) RNA polymerase cannot attach to promoter 1

2/18/2013 III. Reactivation of lac operon ***if cell needs more enzyme*** II. Turning off the lac operon A. when inducer enters cell it binds to the repressor The lac operon in “off” mode B. repressor changes shape and cant bend to operator any ** repressor – protein that longer inhibits gene from being Lactose absent expressed Operon C. repressor falls off operator Regulatory Promoter Operator Lactose-utilization genes gene A. repressor attaches to D. RNA polymerase binds to promoter and again forms m RNA DNA operator and sits between which will instruct ribosome to again make enzyme promoter and the genes mRNA E. when inducer runs out- RNA polymerase cannot Protein attach to promoter - repressor binds to operator again, changes shape & falls off B. repressor blocks access of Active repressor - operon is turned off RNA polymerase to genes Operon turned off (default state when no lactose is present) *SYSTEM IS AUTOMATIC AND SELF-REGULATING* C. protein stops being made Lac operon animation Controlling Onset of Transcription GENE EXPRESSION IN EUKARYOTES - more complex than prokaryotes - Gene regulation controls onset of transcription (when RNA polymerase binds to beginning of gene) - nuclear envelope physically separates transcription from translation, more opportunities for regulation - Requires transcription factors (extra regulatory of gene expression proteins) - help arrange RNA polymerases in correct position on - eukaryotes have DNA on many chromosomes not one promoter circular DNA - many different cell types make many different proteins Proofreading - remember that before mRNA goes into cytoplasm to start protein synthesis, RNA polymerase proofreads strand - 1976: Philip Sharp & Susan Berget - discovered that m RNA not exactly complementary to strand of DNA Introns : non coding, non functional DNA , “junk DNA” Exons : – coding functional sections of DNA (1.5%) 2

2/18/2013 Differences in Prokaryotic and Eukaryotic Steps Gene Expression 1. RNA polymerase moves along gene and transcribes entire gene 2. pre mRNA is produced 3. A. RNA splicing occurs – introns are removed and exons are spliced back together B. chemical cap and tail are attached to RNA C. pre mRNA now called mRNA 4. molecular “gatekeeper” only allows processed mRNA to leave and go to cytoplasm to RNA splicing animation ribosome to make protein Gene Expression Theories: Differences in Prokaryotic and Eukaryotic Gene Expression Prokaryotes Eukaryotes - Introns have some type of function, but we are not sure what they are. Transcription in cytoplasm Transcription in nucleus Uses operons as functional No operons - The more complex the organism, the more introns it has. units - It doesn’t make sense for DNA to have introns if there is no Regulatory gene Regulatory gene recognizes function because it goes to so much work to keep them and causes inhibitor to make RNA polymerase and starts repressor which binds to transcription remove them. operator - Study done where they spliced out introns of a plant leaf and No proofreading- Proofreading occurs crossed it: the resulting leaf was very different than original mRNA goes directly to make (prevents mutations) proteins leaf. DNA Pre mRNA mRNA - It is thought that introns add evolutionary flexibility. Gene Expression during Development enhancer : non-coding control sequence of DNA that facilitates transcription - located 1000’s of nucleotides away from promoter During zygote development: - loop can bring enhancer and promoter together - transcription factors bound to RNA polymerase - genes expressed only when and enhancers activate transcription factors bound to specific proteins are needed promoter and allow transcription to begin - cell differentiation : development of different cells with specialized functions - forms tissues and organs - morphogenesis: development of form in an organism 3

2/18/2013 Homeotic Genes Homeobox Sequences Homeotic genes: • DNA sequences of many genes that control body pattern contain a regulatory genes that determine where certain common stretch of about 180 nucleotides within its sequence anatomical structures will develop in an organism during morphogenesis - homeobox: specific common DNA sequence, codes for proteins that regulate patterns of development - master genes for development of body organization - only a portion of each gene - regulate gene expression by turning genes on and off - homeotic genes: contain the homeobox - increases or decreases cell division rates - ex: if the words below were homeotic genes, the in areas of developing organism capital letters would represent the homeobox • every cell in an organism carries, within its togeTHEr DNA, all of the information necessary to build THEoretical the entire organism gaTHEring boTHEr MUTATIONS Somatic vs Germ Mutation Mutation : any sudden chemical change in genes or chromosomes (mistake) - most mutations are recessive - can occur in any cell - NOT normal occurrence like recombination - germ mutation : affect reproductive or germ cells (inherited) - somatic mutation : affect body cells (not inherited) Chromosome mutation : chemical alteration in segments of chrom, Mutant : organism that has a mutation and shows a whole chrom., or sets of chrom. completely different trait than its parents 1. deletion – piece of chrom. is broken off and - can also carry 1 recessive gene and not information is lost express mutation 2. duplication - segment of chrom. is repeated - can occur at the level of the chromosome or gene 3. inversion – pieces breaks from chrom and reattaches to same chrom. in reverse order 4. translocation – broken piece of one chrom. breaks off and attaches itself to another non- homologous (replicated) chromosome 4

2/18/2013 Cancer Gene Mutation : any chemical change in the base code of DNA molecule - can affect 1 or many nucleotides Tumor : abnormal proliferation of cell that results from uncontrolled, abnormal cell division 1. point mutation - single nucleotide is affected Benign : non cancerous, cells - substitution stay within the mass AUG methionine AUA isoleucine Malignant : uncontrolled dividing cells invade and 2. frameshift mutation – insertion or deletion of a single base destroy healthy tissues in body - shifts groupings of codons following mutation Metastasis : spread of cancer cells beyond **very serious- will completely change protein original site made by a single gene** Kinds of Cancer Genetics of Cancer Oncogenes: genes that cause cancer or other uncontrolled cell proliferation Carcinomas : grow in skin and tissues that line organs of the body - proto-oncogene : normal form of oncogene that controls cells growth and proliferation - mutation in proto oncongene causes uncontrolled Sarcomas : grow in bone and muscle tissue growth leading to cancer Lymphomas : solid tumors that grow in tissues that form blood cells - cause leukemia - tumor suppressor gene : codes for proteins that prevent uncontrolled cell division - we have two copies (both must be mutated) - mutation can cause suppressor expression not to work leading to uncontrolled growth 5

2/18/2013 Study for the test (lots)!! Causes of Cancer Exposure to the following: • Radiation • Viruses • Chemicals 6