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36 Cards in this Set
- Front
- Back
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constitutive enzyme (gene)
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enzymes that are produced continuously, regardless of the chemical makeup of the environment
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inducible enzyme (gene)
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(a.k.a. adaptive/ facultative enzymes) enzymes that are produced only when specific chemical substrates are present
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repressible enzyme (gene)
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an enzyme whose synthesis is regulated by the intracellular concentration of certain metabolites. Ex: the system governing tryptophan expression is said to be repressible- if a sufficient supply of tryptophan is present in the environment or culture medium, then there is no reason for the organism to expend energy in synthesizing the enzymes necessary for tryptophan production, so tryptophan plays a role in repressing the transcription of mRNA needed for producing tryptophan-synthesizing enzymes
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repressor gene (protein)
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a protein that binds to a regulatory sequence adjacent to a gene and blocks transcription of the gene
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negative control
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genetic expression occurs unless if it shut off by some form of a regulator molecule. The enzymes involved in lactose digestion and tryptophan synthesis are under negative control. Lactose serves as the inducer
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positive control
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transcription occurs only if a regulator molecule directly stimulates RNA production
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cis-acting sites (cis-acting elements)
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a DNA sequence that regulates the expression of a gene located on the same chromosome. This contrasts with a trans-acting element where regulation is under the control of a sequence on the homologous chromosome. Cis-acting regulatory regions bind molecules that control transcription of the gene cluster
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trans-acting element
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a gene product (usually a diffusible protein or an RNA molecule) that acts to regulate the expression of a target gene. Binding of a trans-acting element at a cis-acting site can regulate the gene cluster either negatively (by turning off transcription) or positively (by turning on transcription of genes in the cluster).
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lac (lactose) operon
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made up of three structural genes and the adjacent regulatory site that function in an integrated fashion to provide a rapid response to the presence or absence of lactose.
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beta-galactosidase
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encoded by the lacZ gene; an enzyme whose primary role is to convert the dissacharide lactose to the monosaccharides glucose and galactose. This conversion is essential if lactose is to serve as the primary energy source in glycolysis
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lacZ+
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the gene that encodes beta-galactosidase, an enzyme whose primary role is to convert the disaccharide lactose to the monosaccharides glucose and galactose
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lacY+
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the gene that specifies the primary structure of permease, an enzyme that facilitates the entry of lactose into the bacterial cell
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lacZ-
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mutant cells that fail to produce active beta-galactosidase and are therefore unable to use lactose as an energy source
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lacY-
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mutant cells that fail to produce active permease and are therefore unable to use lactose as an energy source
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gratuitous inducer
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chemical analogs of lactose such as the sulfur-containing analog isopropylthiogalactoside (IPTG); they behave like natural inducers, but they do not serve as substrates for the enzymes that are subsequently synthesized. Their discovery provides strong evidence that the primary induction event does NoT depend on the interaction between the inducer and the enzyme
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constitutive mutations
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in cells bearing these types of mutations, enzymes are produced regardless of the presence or absence of lactose. This mutation led to the discovery of the lacI gene, which is appropriately called a repressor gene; in these mutants, the enzymes are produced continually, inducibility is eliminated, and gene regulation has been lost
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allosteric (repressor)
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the molecule reversibly interacts with another molecule, undergoing both a conformational change in 3D shape and a change in chemical activity. In the lac operon when lactose is absent, the repressor binds to the operator, blocking transcription. When lactose is present it binds to the repressor, altering the operator-binding region so that it cannot bind to the operon and therefore transcription proceeds (In other words, when lactose is present, it indirectly induces the activation of the genes by binding with the repressor. If all lactose is metabolize, non is available to bind to the repressor, which is again free to bind to operator DNA and to repress transcription)
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lacI-
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a constitutive mutation that acts as a repressor gene; the repressor protein is altered or absent and cannot bind to the operator region, so the structural genes are always turned on
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lacO^C
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another class of constitutive mutations producing effects identical to those of lacI- and is present in a region immediately adjacent to the structural genes in the operator region of the operon. In the case of this mutant, the nucleotide sequence of the operator DNA is altered and will not bind with a normal repressor molecule, so the results are the same as with the I- mutant: the structural genes are always transcribed
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merozygote
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a cell that is diploid for certain added genes (But not for the rest of the chromosome)
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lacI^S
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the operon is "superrepressed" and an additional I+ gene does not effectively relieve repression of gene activity
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catabolite-activating protein (CAP)
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is a dimer that inserts into adjacent regions of a specific nucleotide sequence of the DNA making up the lac promoter; involved in effectively repressing the expression of the lac operon when glucose is present (by inhibition called "catabolite repression"); Careful examination has revealed that polymerase binding is never very efficient unless CAP is also present to facilitate the process. In the absence of glucose and under inducible conditions, CAP exerts positive control by binding to the CAP site, facilitating RNA-polymerase binding at the promoter, and thus transcription (therefore, for maximal transcription of the structural genes, the repressor must be bound by lactose AnD CAP must be bound to the CAP-binding site). The presence of glucose inhibits CAP binding
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catabolite repression
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the selective inactivation of an operon by a metabolic product of the enzymes encoded by the operon; regulation of the lac operon b y catabolite repression results in efficient energy use, because the presence of glucose will override the need for the metabolism of lactose, should the lactose also be available to the cell
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adenyl cyclase
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an enzyme catalyzes the conversion of ATP to cAMP; so controls the levels of cAMP which in turn effects the binding of CAP to the promoter
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cAMP
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cyclic adenosine monophosphate; CAP must be bound to cAMP in order to bind to the promoter. The presence of glucose inhibits the activity of adenyl cyclase (which converts ATP to cAMP), causing a decline in the level of cAMP in the cell. Under this condition, CAP cannot form the CAP-cAMP complex essential to the positive control of transcription of the lac operon; the cAMP-CAP complex, when bound to DNA, bends it, causing it to assume a new conformation
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cooperative binding
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ex: Alone, neither cAMP-CAP nor RNA polymerase has a strong tendency to bind to the lac promoter DNA, nor does either molecule have a strong affinity for the other. however, when both are together in the presence of the lac promoter DNA, a tightly bound complex is formed. In the case of cAMP-CAP and the lac operon, this phenomenon illustrates the high degree of specificity that is involved in the genetic regulation of just one small group of genes
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trp operon
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a repressible operon; tryptophan acts a corepressor (when tryptophan is present, it binds to the repressor, causing an allosteric transition. The repressor-tryptophan complex can then bind to the operator and block transcription. Since the regulatory complex inhibits transcription of the operon, this repressible system is under negative control.
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trpR-
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a class of constitutive mutations located at a considerable distance from the structural genes that represents the gene coding for the repressor. The mutation inhibits either the repressor's interaction with tryptophan or repressor formation entirely. Whichever the case, no repression ever occurs in cells with the trpR- mutation
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trpR+
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encodes a functional repressor molecule; the presence of a copy of this gene will restore repressibility when there is a trpR- mutation
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trpE,D,C,B,A
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the five contiguous structural genes of the trp operon are transcribed as a polycistronic message directing translation of the enzymes that catalyze the biosynthesis of tryptophan.
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trpP
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as in the lac operon, trpP is a promoter region that represents the binding site for RNA polymerase (trpO binds the repressor)
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leader sequence
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that portion of an mRNA molecule from the 5' end to the initiating codon, often containing regulatory or ribosome binding sites. In the absence of binding, transcription is initiated within the trpP-trpO region and proceeds along a leader sequence 162 nucleotides prior to the first structural gene (trpE). Within the leader seq, still another regulatory site has been demonstarated, called an attenuator, which is an integral part of the control mechanism of this operon
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attenuation
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a form of repression in which the transcription of the operon is greatly reduced rather than prevented entirely; Yanofsky discovered that, following initiation of transcription in the presence of high concentrations of tryptophan, mRNA synthesis is usually terminated at a point about 140 nucleotides along the transcript. (in contrast, when tryptophan is absent, or present in very low concentrations, transcription is initiated and NOT subsequently terminated, instead continuing beyond the leader sequence along the DNA encoding the structural genes, starting with the trpE gene. As a result, a polycistronic mRNA is produced, and the enzymes essential to the biosynthesis of tryptophan are subsequently translated).
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attenuator
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a nucleotide sequence between the promoter and the structural gene of some bacterial operons that regulates the transit of RNA polymerase, reducing transcription of the related structural gene.
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antiterminator hairpin
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if tryptophan is scarce, the alternative structure of the mRNA molecule, referred to as the antiterminator hairpin, is formed. Transcription in this case proceeds past the antiterminator hairpin region, and the entire mRNA is subsequently produced
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terminator hairpin
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in the presence of excess tryptophan, the mRNA hairpin that is formed behaves as a terminator structure, almost always causing transcription to be terminated prematurely
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