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Prokaryotic DNA, Regulation, and Operons

lac operon

MCAT Biochemistry

The lac operon is a set of genes located adjacent to each other and expressed together, allowing E. coli to metabolize lactose. It contains a catabolite activator protein (CAP) binding site, promoter, operator, and lacZ, lacY, and lacA genes. Associated with the lac operon are the CAP protein, repressor, and inducer. RNA polymerase binds to the promoter to initiate transcription. The lac operon can be turned on or off depending on the availability of glucose and lactose.

There are four possible scenarios for lac operon expression. When lactose is not available and glucose is present, the lac operon is not expressed. When lactose and glucose are both available, the lac operon is minimally expressed. In this case, the inducer allolactose binds to the repressor protein to move it away from the operator site, but the CAP binding site remains empty, resulting in low-level transcription. When lactose is available and glucose is not present, the lac operon is fully expressed, with adenylate cyclase producing cyclic AMP (cAMP) to activate CAP protein binding to the CAP binding site. Allolactose also moves the repressor protein away from the operator site in this scenario. Finally, when neither lactose nor glucose is available, the lac operon is not expressed, as CAP protein binds to the CAP binding site but allolactose is absent, keeping the repressor protein bound to the operator site preventing transcription.

Lesson Outline

<ul> <li>Lac operon overview <ul> <li>Set of genes in E. coli allowing lactose metabolism</li> <li>Contains CAP binding site, promoter, operator, lacZ, lacY, and lacA genes</li> <li>Associated with CAP protein, repressor, and inducer</li> </ul> </li> <li>Lac operon components and functions <ul> <li>Catabolite activator protein (CAP) binding site</li> <li>Promoter: binds RNA polymerase</li> <li>Operator: binds repressor protein</li> <li>lacZ: encodes for galactosidase</li> <li>lacY: encodes for permease</li> <li>lacA: encodes for transacetylase</li> <li>LacI gene: encodes for repressor protein</li> </ul> </li> <li>4 scenarios of lac operon expression <ul> <li>1. Lactose absent, glucose present: lac operon not expressed</li> <li>2. Lactose and glucose both present: lac operon minimally expressed</li> <li>3. Lactose present, glucose absent: lac operon fully expressed</li> <li>4. Lactose and glucose both absent: lac operon not expressed</li> </ul> </li> </ul>

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What is the main difference between prokaryotic DNA and eukaryotic DNA in terms of organization and regulation?

Prokaryotic DNA, typically found in bacteria such as E. coli, is circular and not enclosed in a nucleus, whereas eukaryotic DNA is linear and compartmentalized within a nucleus. In terms of regulation, prokaryotic genes are typically organized into operons, allowing for coordinated regulation of functionally related genes. Eukaryotic genes, on the other hand, tend to be individually regulated by specific enhancers and silencers.

What is the role of the lac operon in E. coli gene expression?

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. These genes (lacZ, lacY, and lacA) are under the control of a single promoter and operator, allowing for coordinated regulation of their expression. When lactose is available and glucose is scarce, the lac operon is activated to produce enzymes required for the metabolism of lactose. Conversely, when lactose is absent, the lac operon is repressed to conserve resources.

How does the catabolite activator protein (CAP) influence the transcription of the lac operon?

The catabolite activator protein (CAP) is a regulatory protein that plays a major role in regulating the transcription of the lac operon when glucose levels are low. When glucose is scarce, the intracellular concentration of cyclic AMP (cAMP) increases, binding to CAP and forming the CAP-cAMP complex. This complex binds to a specific site on the DNA near the lac operon promoter, facilitating the binding of RNA polymerase to the promoter and increasing transcription of the lac operon genes.

What are the functions of the lacZ, lacY, and lacA genes in the lac operon?

LacZ, lacY, and lacA genes code for proteins with different functions in lactose metabolism. The lacZ gene encodes for β-galactosidase, an enzyme responsible for hydrolyzing lactose into glucose and galactose. The lacY gene encodes for lactose permease, a membrane protein that facilitates the uptake of lactose into the cell. Lastly, the lacA gene codes for thiogalactoside transacetylase, which detoxifies lactose metabolites by transferring an acetyl group to them.

How do the promoter and operator sequences control gene expression in the lac operon?

The promoter and operator sequences play crucial roles in controlling gene expression in the lac operon. The promoter is a DNA sequence recognized and bound by RNA polymerase, initiating transcription. The operator is an adjacent sequence that acts as a binding site for the lac repressor protein. When lactose is absent and the repressor is bound to the operator, RNA polymerase cannot initiate transcription. In the presence of lactose, an inducer molecule (allolactose) binds to the repressor protein, releasing it from the operator. This allows RNA polymerase to bind to the promoter and initiate transcription of the lac operon genes.