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BISC 202 (28)


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Simon Fraser University
Biological Sciences
BISC 202
Timothy Swartz

11Regulation of Gene Expression in Bacteria and Their Viruses WORKING WITH THE FIGURES1 Compare the structure of IPTG shown in Figure 117 with the structure of galactose shown in Figure 115 Why is IPTG bound by the lac repressor but not broken down by galactosidaseAnswer The sulfur atom in IPTG prevents hydrolysis by the betagalactosidase enzyme 2 Looking at Figure 119 why were partial diploids essential for establishing the transacting nature of the lac repressor Could one distinguish cisacting from transacting genes in haploidsAnswer Partial diploids were essential for distinguishing cisacting from transacting mutants because by definition one must introduce a second copy of the locus in trans to test this property of the mutants 3 Why do promoter mutations cluster at positions 10 and 35 as shown in Figure 1111Answer Promoter mutations cluster around the 10 and 35 positions because these are the DNA sequences recognized and bound by the sigma subunit of the polymeraseAlterations to these sequences will affect the ability of the RNA polymerase holoenzyme to recognize the promoter 4 Looking at Figure 1116 how large is the overlap between the operator and the lac transcription unitAnswer The operator sequence overlaps the lac operon transcription unit by 24 base pairsChapter Eleven 314 5 Examining Figure 1121 what effect do you predict trpA mutations will have on tryptophan levelsAnswer Mutations that inactivate the trpA gene will block the synthesis of tryptophan in the cells resulting in trp auxotrophs 6 Examining Figure 1121 what effect do you predict trpA mutations have on trp mRNA expressionAnswer Because the trpA gene is the last to be transcribed mutations in the trpA would not be predicted to affect transcription of the operon BASIC PROBLEMS7 Explain why I alleles in the lac system are normally recessive to I alleles and why I alleles are recessive to IS alleles Answer The I gene determines the synthesis of a repressor molecule which blocks expression of the lac operon and which is inactivated by the inducer The spresence of the repressor I will be dominant to the absence of a repressor I I mutants are unresponsive to an inducer For this reason the gene product cannot be stopped from interacting with the operator and blocking the lac operon sTherefore I is dominant to I C8 What do we mean when we say that O mutations in the lac system are cisactingc Answer O mutants are changes in the DNA sequence of the operator that impair the binding of the lac repressor Therefore the lac operon associated c operator cannot be turned off Because an operator controls only the with the Ogenes on the same DNA strand it is cis on the same strand and dominant cannot be turned off Unpacking the Problem 9 The genes shown in the following table are from the lac operon system of E coli The symbols a b and c represent the repressor I gene the operator O region and the structural gene Z for galactosidase although not necessarily in that order Furthermore the order in which the symbols are written in the genotypes is not necessarily the actual sequence in the lac operonActivityor inactivityof Z gene
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