Question 1

Mechanism of steroid (sex) hormone action: How do these hormones activate transcription of specific genes?

Accepted Answer

Introduction / Context:Steroid (sex) hormones are lipid-soluble ligands that enter cells and modulate gene expression. Their primary mode of action is via ligand-activated transcription factors known as nuclear hormone receptors.Given Data / Assumptions:Hormone molecules diffuse across membranes.Intracellular receptors (e.g., estrogen receptor, androgen receptor) bind hormone.Ligand binding converts receptors into active transcription factors.Concept / Approach:Hormones do not bind DNA or RNA polymerase directly. Instead, they bind their receptors, causing conformational changes, receptor dimerization, co-activator recruitment, and binding to specific DNA response elements (enhancers) to regulate transcription.Step-by-Step Solution:Identify the direct binding partner of the hormone: the receptor (a TF).Activated receptor binds DNA at hormone response elements.Transcription is modulated through co-activators and the basal transcriptional machinery.Verification / Alternative check:ChIP assays show receptor occupancy at response elements only in the presence of ligand; polymerase recruitment increases subsequently.Why Other Options Are Wrong:RNA polymerase binding: No direct hormone–polymerase interaction.Direct DNA binding by hormone: The receptor, not the small-molecule hormone, binds DNA.Promoter-only binding: Receptors often target enhancer elements as well.Histone phosphorylation without receptor: Not the canonical mechanism.Common Pitfalls:Confusing ligand (hormone) with its receptor; the receptor is the DNA-binding TF.Final Answer:By binding to a transcription factor (nuclear hormone receptor), activating it.

Question 2

How many proteins are translated from the E. coli lac operon structural genes (lacZ, lacY, lacA)?

Accepted Answer

Introduction / Context:The lac operon is a textbook example of polycistronic gene organization in bacteria. A single mRNA encodes multiple proteins that collaborate in lactose uptake and metabolism.Given Data / Assumptions:Structural genes: lacZ, lacY, lacA.Prokaryotic mRNAs can be polycistronic (multiple open reading frames).Each cistron is translated to yield a distinct protein product.Concept / Approach:lacZ encodes beta-galactosidase, lacY encodes lactose permease, and lacA encodes thiogalactoside transacetylase. Together they amount to three translated proteins from one operon transcript.Step-by-Step Solution:Count the structural genes in the lac operon (Z, Y, A).Recognize polycistronic translation in bacteria.Select the total number of protein products: 3.Verification / Alternative check:Mutational analyses and biochemical assays confirm three distinct enzymatic activities corresponding to the three gene products.Why Other Options Are Wrong:1 or 2: Underestimates the number of structural genes.4 or 5: Overestimates beyond known structural genes of the classical lac operon.Common Pitfalls:Confusing regulation components (lacI repressor) with the structural genes on the polycistronic mRNA.Final Answer:3.

Question 3

State of the trp repressor in the presence of intracellular tryptophan: what is it bound to?

Accepted Answer

Introduction / Context:The trp operon is a repressible system. Its repressor protein requires tryptophan as a corepressor to bind DNA and block transcription initiation at the promoter.Given Data / Assumptions:Tryptophan acts as a corepressor.Repressor–corepressor complex has high affinity for the trp operator.Binding to the operator prevents RNA polymerase from initiating transcription.Concept / Approach:In high-tryptophan conditions, tryptophan binds the aporepressor, converting it into an active DNA-binding repressor. This complex occupies the operator and reduces transcription of the trp biosynthetic genes.Step-by-Step Solution:High Trp → corepressor binds aporepressor.Holo-repressor binds to operator DNA.Transcription initiation is blocked; attenuation provides additional fine control.Verification / Alternative check:DNA-binding assays show minimal operator binding without Trp; robust binding occurs upon Trp addition.Why Other Options Are Wrong:Tryp only / DNA only: Both are required for stable operator occupancy under physiological conditions.Neither / polymerase binding: Incorrect interactions for repressor function.Common Pitfalls:Overlooking that tryptophan is not just a metabolic product but also a regulatory corepressor.Final Answer:Bound to both tryptophan (as corepressor) and operator DNA.

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