Transcript Genes
1. 2. 3. What is the Central Dogma? How does prokaryotic DNA compare to eukaryotic DNA? How is DNA organized in eukaryotic cells? 1. Draw and label the 3 parts of an operon. 2. Contrast inducible vs. repressible operons. 3. How does DNA methylation and histone acetylation affect gene expression? 1. 2. 3. List and describe the 3 processes that are involved in transforming a zygote. Compare oncogenes, proto-oncogenes, and tumor suppresor genes. What are the roles of the ras gene and the p53 gene? Chapter 18 Transcription Bacteria need to respond quickly to changes in their environment ◦ if they have enough of a product, need to stop production STOP why? waste of energy to produce more how? stop production of enzymes for synthesis ◦ if they find new food/energy source, need to utilize it quickly GO why? metabolism, growth, reproduction how? start production of enzymes for digestion Feedback inhibition - = inhibition ◦ product acts as an allosteric inhibitor of 1st enzyme in tryptophan pathway ◦ but this is wasteful production of enzymes - - Gene regulation = inhibition ◦ instead of blocking enzyme function, block transcription of genes for all enzymes in tryptophan pathway saves energy by not wasting it on unnecessary protein synthesis - - Cells vary amount of specific enzymes by regulating gene transcription ◦ turn genes on or turn genes off turn genes OFF example if bacterium has enough tryptophan then it STOP doesn’t need to make enzymes used to build tryptophan turn genes ON example if bacterium encounters new sugar (energy GO source), like lactose, then it needs to start making enzymes used to digest lactose Operon: cluster of related genes with on/off switch Three Parts: 1. Promoter – where RNA polymerase attaches 2. Operator – “on/off”, controls access of RNA poly 3. Genes – code for related enzymes in a pathway Operon: Promoter, Operator & Genes they control serve as a model for gene regulation RNA polymerase RNA TATA repressor polymerase gene1 gene2 gene3 gene4 1 2 3 4 enzyme1 enzyme2 enzyme3 enzyme4 mRNA promoter DNA operator Repressor protein turns off gene by blocking RNA polymerase binding site. repressor = repressor protein Repressor protein ◦ ◦ ◦ ◦ binds to DNA at operator site blocking RNA polymerase blocks transcription Produced by regulatory gene Regulatory gene: produces repressor protein that binds to operator to block RNA poly Normally ON Anabolic (build organic molecules) Organic molecule product acts as corepressor binds to repressor to activate it Operon is turned OFF Eg. trp (tryptophan) operon Synthesis pathway model When excess tryptophan is present, it binds to trp repressor protein & triggers repressor to bind to DNA RNA polymerase RNA trp repressor TATA polymerase gene1 1 2 3 4 enzyme1 enzyme2 enzyme3 enzyme4 mRNA promoter ◦ blocks (represses) transcription gene2 gene3 gene4 DNA trp operator trp trp repressor repressor protein trp trp trp trp trp trp conformational change in repressor protein! trp repressor tryptophan trp tryptophan – repressor protein complex What happens when tryptophan is present? Don’t need to make tryptophan-building enzymes Tryptophan is allosteric regulator of repressor protein trp operon Normally OFF Catabolic (break down food for energy) Repressor is active inducer binds to and inactivates repressor Operon is turned ON Eg. lac operon lac RNA polymerase lac Digestive pathway model lac When lactose is present, binds to lac repressor protein & triggers repressor to release DNA lac lac lac lac RNA TATA lac repressor polymerase gene1 1 2 3 4 enzyme1 enzyme2 enzyme3 enzyme4 mRNA promoter ◦ induces transcription gene2 gene3 gene4 operator repressor lac conformational change in repressor protein! lac repressor DNA repressor protein lactose lactose – repressor protein complex What happens when lactose is present? Need to make lactose-digesting enzymes Lactose is allosteric regulator of repressor protein lac operon Repressible operon ◦ usually functions in anabolic pathways synthesizing end products ◦ when end product is present in excess, cell allocates resources to other uses Inducible operon ◦ usually functions in catabolic pathways, digesting nutrients to simpler molecules ◦ produce enzymes only when nutrient is available cell avoids making proteins that have nothing to do, cell allocates resources to other uses Many stages Typical human cell: only 20% of genes expressed at any given time Different cell types (with identical genomes) turn on different genes to carry out specific functions Differences between cell types is due to differential gene expression Chromatin Structure: Tightly bound DNA less accessible for transcription DNA methylation: methyl groups added to DNA; tightly packed; transcription Histone acetylation: acetyl groups added to histones; loosened; transcription Modifications on chromatin can be passed on to future generations Unlike DNA mutations, these changes to chromatin can be reversed (de-methylation of DNA) Explains differences between identical twins Transcription Initiation: Control elements bind transcription factors Enhances gene expression Enhancer regions bound to promoter region by activators Regulation of mRNA: • micro RNAs (miRNAs) and small interfering RNAs (siRNAs) can bind to mRNA and degrade it or block translation Section 18.4 1. 2. 3. Cell Division: large # identical cells through mitosis Cell Differentiation: cells become specialized in structure & function Morphogenesis: “creation of form” – organism’s shape Cytoplasmic determinants: maternal substances in egg distributed unevenly in early cells of embryo Induction: cells triggered to differentiate Cell-Cell Signals: molecules produced by one cell influences neighboring cells ◦ Eg. Growth factors Section 18.5 1. 2. Proto-oncogene = stimulates cell division Tumor-suppressor gene = inhibits cell division Mutations in these genes can lead to cancer Proto-Oncogene Gene that stimulates normal cell growth & division Oncogene Mutation in protooncogene Cancer-causing gene Effects: Increase product of proto-oncogene Increase activity of each protein molecule produced by gene Ras gene: stimulates cell cycle (proto- oncogene) ◦ Mutations of ras occurs in 30% of cancers p53 gene: tumor-suppresor gene ◦ Functions: halt cell cycle for DNA repair, turn on DNA repair, activate apoptosis (cell death) ◦ Mutations of p53 in 50+% of cancers Cancer results when mutations accumulate (57 changes in DNA) Active oncogenes + loss of tumor-suppressor genes The longer we live, the more likely that cancer might develop Embryonic development occurs when gene regulation proceeds correctly Cancer occurs when gene regulation goes awry