Bio 240 Chapter 16 Notes!
Bio 240 Chapter 16 Notes! Bio 240
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This 3 page Class Notes was uploaded by Izabella Nill Gomez on Thursday November 26, 2015. The Class Notes belongs to Bio 240 at University of Tennessee - Knoxville taught by Dr. Hughes in Summer 2015. Since its upload, it has received 28 views. For similar materials see General Genetics (Bio 240) in Biology at University of Tennessee - Knoxville.
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Date Created: 11/26/15
Biology 240 Chapter 16 Notes! Inducible enzymes: bacteria adapt to their environment, producing certain enzymes only when specific chemical substances are present. Constitutive enzymes are enzymes produced continuously regardless of chemical makeup of the environment. Negative control: genetic expression occurs unless it is shut off by some form of regulatory molecules. Positive control: transcription occurs only if a regulatory molecule directly stimulates RNA production. -In prokarya, genes that code for enzymes with related functions (ex: lactose metabolism) tend to be organized into a cluster on the bacterial chromosome, and transcription of these genes is often under coordinated control of a singular regulatory region, almost always upstream (5’) of the gene cluster it controls (cis- acting site)--this binds to molecules that control transcription of the gene cluster (trans-acting elements). Binding of trans-acting elements on cis-acting sites can regulate the gene cluster negatively (turning off)/positively (turning on). Structural genes: genes coding for the primary structure of an enzyme. There are 3 in the lac operon: the lac Z gene codes for Beta galactosidase (which converts lactose to glucose+galactose). Lac Y specifies permease, an enzyme that allows lactose into the cell. Lac A codes for transacetylase (function not really clear). All 3 genes are transcribed as a single unit, resulting in a polycistronic mRNA--results in coordinated regulation of all three genes. Lac I gene is a repressor gene. -Jacques and Monod proposed the operon model, which consists of lac Z,Y and A structural genes + adjacent DNA sequences (operator region). Lac I gene regulates transcription of structural genes by producing the repressor molecule (allosteric)--it undergoes conformational and chemical change. Normally binds to the DNA sequence of the operator region, inhibiting RNA pol and repressing transcription.**All under negative control. When lactose is present, it binds to the repressor and casues change to pull it off of the operator region for transcription. I- and O- mutations interfere, allowing continuous transcription. In I-, the repressor is altered/absent, to the structural genes are always on. In O-, nucleotide sequence of the operator DNA is altered and cannot bind to the repressor: the genes are then always on. -Allolactose (inducer) binds to I repressor molecule when present to prevent binding to the DNA operator. The I gene produces a trans-acting product; O region is involved in regulation but does not produce a product (cis-acting). The O region is adjacent to structural genes in order to regulate transcription. Proven with partially diploid bacteria (merozygote). I(s) mutation super-represses the operon and lactose cannot be digested. Catabolite-activating protein (CAP) is involved in diminishing expression of the lac operon when glucose is present--catabolite expression. -When the lac operon is activated, RNA pol initiates transcription by binding to the promoter region upstream (5’) from the initial coding site. The promoter is found between the I gene and operator region (O). In the absence of glucose, CAP exerts positive control by binding to the CAP site, facilitating RNA pol binding at the promoter and thus transcription. So for maximum transcription, the repressor should be bound by lactose and CAP must bind to its site. cAMP is needed for CAP (must be bound to lac). The level of cAMP is dependent on adenyl cyclase, which catalyzes the conversion of ATP to cAMP. Glucose inhibits cAMP levels and CAP, leading to less transcription of the lac operon. Alone, neither cAMP-CAP or RNA pol has a strong tendency to bind to the lac promotor, nor a strong affinity. Cooperative binding occurs together through the forming of a tight complex. cAMP-CAP shows positive regulation. There is a combination of positive and negative regulation for the lac operon transcription. -Monod discovered that if tryptophan is present in sufficient quantity in the growth medium of a culture, the enzymes necessary for synthesis are not produced. 5 genes on the E. coli chromosome are involved in tryptophan synthesis. In the presence of tryptophan, transcription is repressed and no enzymes are produced. The system is analogous to the lac operon. The presence of a normally inactive repressor that alone can’t interact with the operon region of the operon. But the repressor is an allosteric molecule that can bind to tryptophan. When present, transcription is repressed (under negative control). Tryptophan is a corepressor. trpP (promoter region) is the binding site for RNA pol, trpO (operator region) binds the repressor. IN the absence of binding, transcription is initiated (between trpP and trpO) and proceeds along the leader sequence (62 nucleotides prior to the first structural gene (trpE)). -Prokaryotic DNA-binding regulatory proteins: ex: lac repressor and CAP protein. Act to decrease/increase transcription initiation from target promotors by affecting the binding of RNA pol to the promoter. Regulation can also occur through interaction of regulatory molecules with certain regions of nascent mRNA after transcription is initiated. Binding alters secondary structure of mRNA, leading to premature transcription termination or repression of translation. Attenuation: weakened/impairs expression of an operon when a certain amount of substance (like tryptophan) still present. If tryptophan source, mRNA secondary structure is an antiterminator hairpin formed. Transcription proceeds post aniterminator region, and an entire mRNA is produced. The presence of excess tryptophan creates a terminator hairpin; transcription is almost always terminated prematurely, just beyond attenuation. If tryptophan levels are low, ribosomes stall during translation. Possible exam Questions: 1. When referring to attenuation in regulation of the trp operon, it would be safe to say that when there are high levels of tryptophan available to the organism,: transcriptional termination is likely 2. The ara operon is controlled by a regulator protein that exerts: positive and negative control