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.

Genetic Regulation Prokaryotes Questions