Chapter 18 Regulation of Gene Expression Regulation of Gene Expression • Important for cellular control and differentiation. • Understanding “expression” is a “hot” area in Biology. General Mechanisms 1. Regulate Gene Expression 2. Regulate Protein Activity Operon Model • Jacob and Monod (1961) - Prokaryotic model of gene control. • Always on the National AP Biology exam! Operon Structure 1. Regulatory Gene 2. Operon Area a. Promoter b. Operator c. Structural Genes Gene Structures Regulatory Gene • Makes Repressor Protein which may bind to the operator. • Repressor protein blocks transcription. Promoter • Attachment sequence on the DNA for RNA polymerase to start transcription. Operator • The "Switch”, binding site for Repressor Protein. • If blocked, will not permit RNA polymerase to pass, preventing transcription. Structural Genes • Make the enzymes for the metabolic pathway. Lac Operon • For digesting Lactose. • Inducible Operon - only works (on) when the substrate (lactose) is present. If no Lactose • Repressor binds to operator. • Operon is "off”, no transcription, no enzymes made If Lactose is absent If Lactose is present • Repressor binds to Lactose instead of operator. • Operon is "on”, transcription occurs, enzymes are made. If Lactose is present Enzymes • Digest Lactose. • When enough Lactose is digested, the Repressor can bind to the operator and switch the Operon "off”. Net Result • The cell only makes the Lactose digestive enzymes when the substrate is present, saving time and energy. Animation • http://www.biostudio.com/d_%20Lac%20Ope ron.htm trp Operon • Makes/synthesizes Tryptophan. • Repressible Operon. – Predict how it is different from the inducible operon… If no Tryptophan • Repressor protein is inactive, Operon "on” Tryptophan made. • “Normal” state for the cell. Tryptophan absent If Tryptophan present • Repressor protein is active, Operon "off”, no transcription, no enzymes • Result - no Tryptophan made If Tryptophan present Repressible Operons • Are examples of Feedback Inhibition. • Result - keeps the substrate at a constant level. Positive Gene Regulation • Positive increase of the level of transcription. • Uses CAP - Catabolite Activator Protein • Uses cAMP as a secondary cell signal. CAP - Mechanism • Binds to cAMP. • Complex binds to the Promoter, helping RNA polymerase with transcription. Result • If the amount of glucose is low (as shown by cAMP) and lactose is present, the lac operon can kick into high gear. Eukaryotic Gene Regulation • Can occur at any stage between DNA and Protein. • Be prepared to talk about several mechanisms in some detail. Chromatin Structure • Histone Modifications • DNA Methylation • Epigenetic Inheritance Histone Acetylation • Attachment of acetyl groups (-COCH3) to AAs in histones. • Result - DNA held less tightly to the nucleosomes, more accessible for transcription. DNA Methylation • Addition of methyl groups(-C H3) to DNA bases. • Result - long-term shut-down of DNA transcription. • Ex: Barr bodies, genomic imprinting Epigenetics • Another example of DNA methylation effecting the control of gene expression. • Long term control from generation to generation. • Tends to turn genes “off”. Do Identical Twins have Identical DNA? • Yes – at the early stages of their lives. • Later – methylation patterns change their DNA and they become less alike with age. Transcriptional Control • Enhancers and Repressors • Specific Transcription Factors • Result – affect the transcription of DNA into mRNA Enhancers • Areas of DNA that increase transcription. • May be widely separated from the gene (usually upstream). Posttranscriptional Control • Alternative RNA Processing/Splicing – Ex. - introns and exons • Can have choices on which exons to keep and which to discard. • Result – different mRNA and different proteins. Another Example Results • Bcl-XL – inhibits apoptosis • Bcl-XS – induces apoptosis • Two different and opposite effects!! DSCAM Gene • Found in fruit flies • Has 100 potential splicing sites. • Could produce 38,000 different polypeptides • Many of these polypeptides have been found Commentary • Alternative Splicing is going to be a BIG topic in Biology. • About 60% of genes are estimated to have alternative splicing sites. (way to increase the number of our genes) • One “gene” does not equal one polypeptide (or RNA). Other post transcriptional control points • RNA Transport - moving the mRNA into the cytoplasm. • RNA Degradation - breaking down old mRNA. Translation Control • Regulated by the availability of initiation factors. • Availability of tRNAs, AAs and other protein synthesis factors. (review Chapter 17). Protein Processing and Degradation • Changes to the protein structure after translation. • Ex: Cleavage – Modifications – Activation – Transport – Degradation Protein Degradation • By Proteosomes using Ubiquitin to mark the protein. Noncoding RNA • Small RNA molecules that are not translated into protein. • Whole new area in gene regulation. • Ex - RNAi Types of RNA • MicroRNAs or miRNAs. • RNA Interference or RNAi using small interfering RNAs or siRNAs. • Both made from RNA molecule that is “diced” into double stranded (ds) segments. RNAi • siRNAs or miRNAs can interact with mRNA and destroy the mRNA or block transcription. • A high percentage of our DNA produces regulatory RNA. Morphogenesis • The generation of body form is a prime example of gene expression control. • How do cells differentiate from a single celled zygote into a multi-cellular organism? Clues? • Some of the clues are already in the egg. • Cytoplasmic determinants – chemicals in the egg that signal embryo development. • Made by Maternal genes, not the embryo’s. Induction • Cell to cell signaling of neighboring cells gives position and clues to development of the embryo. Fruit Fly Studies • Have contributed a great deal of information on how an egg develops into an embryo and the embryo into the adult. Homeotic (Hox) Genes • Any of the “master” regulatory genes that control placement of the body parts. • Usually contain “homeobox” sequences of DNA (180 bases) that are highly conserved between organisms. Comment • Evolution is strongly tied to gene regulation. Why? • What happens if you mutate the homeotic genes? • Stay tuned for more “evo-devo” links in the future. When things go wrong Example case • Bicoid (two tailed) – gene that controls the development of a head area in fruit flies. • Gene produces a protein gradient across the embryo. Result • Head area develops where Bicoid protein levels are highest. • If no bicoid gradient – get two tails. Other Genes • Control the development of segments and the other axis of the body. Gene Expression and Cancer • Cancer - loss of the genetic control of cell division. • Balance between growth-stimulating pathway (accelerator) and growth-inhibiting pathway (brakes). Proto-oncogenes • Normal genes for cell growth and cell division factors. • Genetic changes may turn them into oncogenes (cancer genes). • Ex: Gene Amplification, Translocations, Transpositions, Point Mutations Proto-oncogenes Tumor-Suppressor Genes • Genes that inhibit cell division. • Ex - p53, p21 Cancer Examples • RAS - a G protein. • When mutated, causes an increase in cell division by over-stimulating protein kinases. • Several mutations known. Cancer Examples • p53 - involved with several DNA repair genes and “checking” genes. • When damaged (e.g. cigarette smoke), can’t inhibit cell division or cause damaged cells to apoptose. Carcinogens • Agents that cause cancer. • Ex: radiation, chemicals • Most work by altering the DNA, or interfering with control or repair mechanisms. Multistep Hypothesis • Cancer is the result of several control mechanisms breaking down (usually). • Ex: Colorectal Cancer requires 4 to 5 mutations before cancer starts. Colorectal Cancer News Flash • Severe damage to a chromosome that causes it to “shatter” can lead to immediate cancer. • Doesn’t always take a long time and multiple steps. Can Cancer be Inherited? • Cancer is caused by genetic changes but is not inherited. • However, oncogenes can be inherited. • Multistep model suggests that this puts a person “closer” to developing cancer. Example – BRAC1 • BRAC1 is a tumor suppressor gene linked with breast cancer. • Normal BRAC1 – 2% risk. • Abnormal BRAC1 – 60% risk. • Runs in families. Some will have breasts removed to avoid cancer risk. Summary • Know Operons • Be able to discuss several control mechanisms of gene expression. • Be familiar with gene expression and development of organisms. • How control of DNA can lead to cancer..