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BIOLOGY 207 - Dr. McDermid Lecture #9 Introduction to gene regulation and operons in Prokaryotes. Readings for lectures #4 and #5: Griffiths et al, 7th Edition: Ch. 11 pp 335-344, Ch 7 pp210-219 (skim only, beginning and end) Problems for lectures #4 and #5: Griffiths et al, 7th Edition: Ch. 11 Tier 1: #1,2,3,4,8 Tier 2: #9,14,15 Concepts: How are prokaryotic genes regulated? 1. In prokaryotes, operons are clusters of genes 2. The lac operon - a classic prokaryote operon 3. Transcriptional regulation involves the binding of proteins to the DNA. Central Dogma DNA ------> RNA -------> Protein 1) Transcription 2) Translation Gene Regulation - frequently at the transcription step Status: Gene on Gene off Gene Regulation involves the activity of proteins Transcription Translation Prokaryote Gene Regulation: Operons Lac operon: Figure 11-1 The I gene product is a repressor protein which acts by binding to the operator DNA sequence thereby preventing the RNA polymerase (which binds to the promoter-P sequence) from transcribing a polycistronic messager RNA of the cistrons Z, Y, A. (cistron = gene) -Z-Y- NOTE – Griffiths mistake on pg 336, mix up definitions of Z and Y -A-I- lac repressor I has 2 recognition sites Lecture #4 Page 1 Example of negative control Operon POZYA Transcription of mRNA- polycistronic - polycistronic mRNA Note: not all prokaryotic genes are in operons Genetics of the lac operon Jacob and Monod in the 1950's and 1960's Inducers - specific substrates that trigger the appearance of specific enzymes beta-galactosidase could be induced with several types of beta-galactosides (Figure 11-3 IPTG). The enzyme is not normally present in absence of an inducer. Constitutive mutants of the I locus Constitutive mutant Genetically mapped close to, but distinct from, the ZYA genes These mutants defined the I locus. + I cells I- mutants Table 11-2 & Figure 11-5 For further study, need "heterozygous diploid strains" F-factor - Ch. 7 pp 210-219 Fertility factor - Fig. 7-14 F-factor is an "episome" - capable of being either a free plasmid or a sequence integrated into bacterial chromosome If F factor is present then the strain is an F+ strain F factor can contain some E.coli genes therefore making the cell a partial diploid. lac- host with F factor with lac+ = lac-/F lac+ From this one can examine lac mutants in partial diploid strains Determine if mutations are cis or trans acting. cis trans - Lecture #4 Page 2 trans-acting locus A gene able to affect the activity of other genes on a different DNA molecule usually through a diffusible product (e.g. protein) I+ is dominant over I- in the trans - position Figure 11-6 Table 11-3 I+ Z -Y + / F I-Z + Y - Z inducible; Y is inducible IS is dominant to both I+ and I- in trans Figure 11-7 Table 11-3 IS Z + Y + is uninducible. Repressor protein: I gene product I- mutation Normal Inducer DNA Inducer Is mutation XXX XXXXXX Summary of I mutant classes: Dominant in trans therefore it is a diffusible product Lecture #4 Page 3 DNA