Genetics Exam 3
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means it can be turned on, therefore it is off.
the gene can be tuned off, therefore it is normally on
a positively controlled operon
the genes in the operon are expressed only when an active regulator protein, (an activator) is present. thus the operon will be turned off when the positive regulatory protein is absent or inactivated.
a negatively controlled operon
the genes in the operon are expressed unless they are switched off by a repressor protein. thus the operon will be turned on constituively (the genes will be expressed) when the repressor is inactivated
an operon's 4 basic DNA components
- promoter (recognized by RNA polymerase)
- regulator (control the operator gene in cooperation with inducers an corepressors)
- operator (The segment of DNa that the repressor binds to.
- structural genes (the genes that are co-regulated by the operon)
Not included within the operon is the regulatory gene that codes for the repressor proteins.
catalyze the breakdown of nutrients (the substrate for the pathway) to generate energy, (ATP). In the absence of the substrate, there is no reason fo rte catabolic enzymes to be present, and the operon encoding them is repressed. In the presence of the substrate, when the enzymes are needed, the operon is induced or de-repressed.
Anabolic or biosynthetic pathways
use energy in the form of ATP and reducing equivalents in the form of NAD(P)H to catalyze the synthesis of cellular components (the product) from simpler materials, e.g. synthesis of amino acids from small dicarboxylic acids (components of the the citric acid cycle). If the cell has plenty of the product already (in the presence of the product), the the enzymes catalyzing its synthesis are not needed, and the operon encoding them is repressed. In the absence of the product, when the cell needs to make more, the biosynthetic operon is induced. E.g., the trpoperon encodes the enzymes that catalyze the conversion of chorismic acid to tryptophan. When the cellular concentration of Trp (or Trp-tRNAtrp) is high, the operon is not expressed, but when the levels are low, the operon is expressed.
Inducible versus repressible operons
a. Inducible operons are turned on in reponse to a metabolite (a small molecule undergoing metabolism) that regulates the operon. E.g. the lacoperon is induced in the presence of lactose (through the action of a metabolic by-product allolactose). b. Repressible operons are switched off in reponse to a small regulatory molecule. E.g., the trpoperon is repressed in the presence of tryptophan. Note that in this usage, the terms are defined by the reponse to a small molecule. Although lac is an inducible operon, we will see conditions under which it is repressed or induced (via derepression).
Lac Operon Structural genes:
lacZYA: lacZ encodes b-galactosidase, which cleaves the disccharide lactose into galactose and glucose.lacYencodes the lactose permease, a membrane protein that faciltitates uptake of lactose.lacAencodes b-galactoside transacetylase; the function of this enzymes in catabolism of lactose is not understood (at least by me).
affect the amount of all the enzymes encoded by an operon, whereas mutations in a structural gene affects only the activity of the encoded (single) polypeptide. 1. Repressor mutants a. Wild-type strains (lacI+) are inducible. b. Most strains with a defective repressor (lacI-) are constitutive, i.e. they make the enzymes encoded by thelacoperon even in the absence of the inducer. c. Strains with repressor that is not able to interact with the inducer (lacIS) are noninducible. Since the inducer cannot bind, the repressor stays on the operator and prevents expression of the operon even in the presence of inducer.
Mutations in the operator are
cis-acting; they only affect the expression of structural genes on the same chromosome.
what components are needed for this form of regulation?
Glucose leads to repression of expression of lacand some other catabolic operons. This phenomenon is called catabolite repression.
cAMP and CAP
the breakdown of complex molecules in living organisms to form simpler ones, together with the release of energy, destructive metabolism
a product of catabolism
CAP=catabolite activator protein
sometimes called 'jumping genes', segments of DNA that can move around in the genome
substitution mutation where you trade a purine to purine (A to G) or pyr to pyr (C to T)
purine to pyr, A to T or G to C
if not in a group of 3, leads to a frameshift
Expanding trinucleotide repeat
usually happens when repetition occurs and the Polymerase gets lazy (called strand slippage)... the extra repeats as a mismatched stem loop, then matches back up, then next time its made the other strand gets 5 more repeats in it...
Genetic anticipation-certain number of repeats can be normal, but too many can cause problems, (ie huntington's disease).
2 types -reciprical or non
Acetylation and de-acetylation
Acetylation removes the positive charge on histones and allows the structure to relax because it is not as attracted to the negative phosphate groups of DNA. typically catalyzed by enzymes with "histone acetyltransferase" (HAT) or "histone deacetylase" (HDAC) activity. The source of the acetyl group in histone acetylation is Acetyl-Coenzyme A, and in histone deacetylation the acetyl group is transferred to Coenzyme A.
A corepressor is a molecule that can bind to repressor and make it bind to the operator tightly, which decreases transcription.
Can bind to repressors or activators.
Inducers function by disabling repressors. The gene is expressed because an inducer binds to the repressor. The binding of the inducer to the repressor prevents the repressor from binding to the operator. RNA polymerase can then begin to transcribeoperon genes.Inducers also function by binding to activators. Activators generally bind poorly to activator DNA sequences unless an inducer is present. Activator binds to an inducer and the complex binds to the activation sequence and activates target gene. Removing the inducer stops transcription.
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