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28 Cards in this Set
- Front
- Back
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What are -inducible- enzymes?
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They are enzymes that are only produced when certain chemical substrates are present. (Remember the 1900 lactose experiment)
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What are inducers?
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Inducers are certain chemical substrates which, when present, induce the production of certain enzymes.
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The system governing tryptophan expression is said to be repressible? What does this mean?
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The presence of sufficient amount of tryptophan in the cell environment causes the cell to stop producing the enzymes necessary for tryptophan production. It represses transcription of mRNA essential to the production of tryptophan.
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Explain Genetic Regulation under negative control.
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Under negative control, genetic expression occurs unless it is shut off by some form of a regulator molecule.
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Explain Genetic regulation under positive control.
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Under positive control, transcription only occurs if a regulator molecule directly stimulates RNA production.
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The regulatory unit of certain clusters of genes are almost always upstream to these genes. What do you call the site where the regulatory units are?
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cis-acting site
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What are trans-acting elements?
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binding molecules that control the transcription of gene clusters.
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What does it mean when a trans-acting element at a cis-acting site regulate a gene cluster negatively?
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It means that the trans-acting elements are there to turn the genes off.
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What does it mean when a trans-acting element at a cis-acting site regulate a gene cluster positively?
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It means that the trans-acting elements are there to turn the genes on.
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What is lac operon?
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The lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria. It consists of three adjacent structural genes, a promoter, a terminator, and an operator.
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Define: structural genes
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Genes that code for the primary structure of enzymes
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How many structural genes are present in the lac operon. What are their functions?
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3.
LacZ gene- converts the disaccharide lactose into monosacharide glucose and galactose. LacY gene- it facilitates the entry of lactose into the bacterial cell. LacA gene- believed to be responsible for the removal of toxic byproducts of lactose digestion. |
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Explain the difference between Monocistronic v. Polycistronic mRNA.
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An mRNA molecule is said to be monocistronic when it contains the genetic information to translate only a single protein. On the other hand, polycistronic mRNA carries the information of several genes, which are translated into several proteins
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Define: gratuitous inducers
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chemical analogues of lactose that behave like natural inducers, but they do not serve as substrates for the enzymes that are subsequently produced.
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What proves that the primary induction event does not depend on the interaction between the inducer and the enzyme?
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the existence of gratuitous analagoues
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What is constitutive mutation?
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In this type of mutation, enzymes are produced whether or not lactose is present.
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What happens if a cell is lacI- ?
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LacI is a repressor gene located near the structural genes constituting the lac operon. Without it, enzymes are produced even if lactose is present or not.
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What happens if a cell is lacO^c?
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The same effect happens as when a cell is lacI-. The enzymes coded by the structural genes of the lac operon are continuously produced whether lactose is present or not.
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What is the effect of constitutive mutation on the inducibility of lactose?
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It eliminates the inducibility of lactose. Even if its present or not, the enzymes are still produced no matter what.
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What is the operator region?
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The operator region is a sequence of DNA adjacent to the structural genes lacZ, lacY, and lacA.
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lacI gene produces a repressor molecule that inhibits the transcription of structural genes. this repressor is allosteric. What does allosteric mean?
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Allosteric compounds are compounds that bind at a protein's site other that the active site causing a conformational change in shape as well as chemical activity.
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What is the role of lactose in induction?
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The role of lactose in induction is to prevent the repressor gene, lacI, in repressing
the production of the related enzymes. The repressor gene is located upstream of the the lac operon (a regulatory unit comprised of three structural genes, and an operator region; it deals with the presence or absence of lactose in a cell). When lactose is not present, lacI prevents the expression of the structural genes by binding allosterically with the operator region adjacent to these cells. When bound, the operator region changes conformation and shape disabling the RNA polymerase to bind to the promoter right next to it. Because the RNA polymerase cannot bind to the DNA, the structural genes are not transcripted and therefore, their respective gene products will not be produced. However, if lactose is present it can stop the actions of the repressor genes by binding to it allosterically. By changing the conformational shape of the repressor, lactose prevents the repressor from being able to fit in the active site of the operator region. RNA polymerase can then bind freely to the promoter and transcribe the structural genes downstream. |
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How can lacZ, lacY, lacA be expressed despite the absence of lactose?
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constitutive mutation of lacI and lacO^c genes !! lacO^c mutation- alters the DNA sequence of the operator region preventing it to bind with the repressor gene.
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The presence of the catabolite-activating proteing (CAP) effectively repress the expression of _______ when ample amounts of _______ is present.
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lac operon; glucose
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What are two ways to prevent the production of the enzymes expressed by lacz lacy lac A
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Supererepressed LacI gene and excess amounts of glucose which lowers the amount of adenyl cyclase. Adenyl cyclase is a molecule that the catabolite-activating protein needs before it can bind the the CAP site. If its bound to the cap site, RNA polymerase cannot bind to the promoter region.
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Lac operon is under “+” control by CRP (aka CAP) gene. Explain.
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A regulatory unit (in this case, the lac operon) is said to be under positive control if transcription only occurs because a regulator molecule (CAP gene) is present to directly stimulate RNA production. In order for RNA polymerase to attach to the promoter region of lac operson, The protein produced by the cap gene must be first bound to the CAP site which is located adjacent to the promoter region. If it's not bound to that CAP site, then RNA polymerase also cannot bind with the promoter region and therefore, transcription will not occur. wow grammar police needed. Whatever.
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What role does glucose play in inhibiting CAP binding when it is present?
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Glucose binds with adenyl cyclase, a molecule that the CAP protein needs to bind with first before it attaches to the CAP site. Ample amounts of glucose decreases the concentration of adenyl cyclase. CRP gene is “inactive” until bound by cAMP so it cannot attach to the camp site and therefore, transcription cannot occur
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How can the tryp operon be repressed?
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To account for repression, Monod and colleagues suggested the presence of a normally inactive repressor that alone cannot interacct with the operator region of the operon. However, the repressor is an allosteric molecule that can bind to tryptophan. The resultant complex of repressor and tryptophan attains a new conformation that binds to the operator, repressing transcription. (negative control)
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