Cell Surface Receptors
What are the three main classes of cell surface receptors?
Ion channel-linked receptors, G-protein-coupled receptors (GPCRs), and enzyme-linked receptors (e.g., receptor tyrosine kinases).
What two major enzymes are most commonly the targets of activated G proteins?
Adenylyl cyclase and phospholipase C (PLC).
What is the fast block to polyspermy in sea urchins?
A rapid depolarization of the egg membrane preventing additional sperm fusion.
At which steps can gene expression be regulated in eukaryotic cells?
Transcription, RNA processing, translation, and post-translational modifications.
What is a promoter’s main function?
To provide a binding site for RNA polymerase to start transcription.
Which type of receptor directly allows ions to pass through the plasma membrane when activated by a signal?
Ion channel-linked receptors.
What is the primary second messenger produced by adenylyl cyclase activation?
Cyclic AMP (cAMP).
Which receptor type triggers the fast block to polyspermy?
Ion channel-linked receptors activated by the fertilizing sperm.
What is the most common control point for long-term gene expression regulation?
Transcriptional control, because it prevents unnecessary RNA and protein synthesis.
How do activators and repressors regulate gene expression?
Activators enhance transcription by facilitating RNA polymerase binding; repressors block transcription by preventing RNA polymerase access or function.
GPCRs activate trimeric G proteins. What structural similarity do all GPCRs share?
They all have seven transmembrane alpha helices.
Which molecules act as second messengers in the PLC pathway after GPCR activation?
Inositol trisphosphate (IP3), diacylglycerol (DAG), and Ca²⁺.
Describe the signaling events leading to the slow block to polyspermy.
Fertilization activates phospholipase C, producing IP3, which releases Ca²⁺ from intracellular stores triggering cortical granule exocytosis and zona pellucida modification.
What is the functional consequence of a strong promoter on gene expression?
Increased frequency of transcription initiation leading to higher gene expression levels.
Under what condition is the lac operon fully on in E. coli?
When lactose is present and glucose is absent.
How does ligand binding to a GPCR ultimately lead to activation of a trimeric G protein?
Ligand binding causes a conformational change in the GPCR, which facilitates GDP-GTP exchange on the alpha subunit of the G protein, activating it.
Explain how IP3 and DAG lead to activation of protein kinase C (PKC).
IP3 triggers Ca²⁺ release from the ER, and DAG, together with Ca²⁺, activates PKC at the membrane.
Which ion triggers the slow block, and what is its developmental effect?
Ca²⁺; it modifies the extracellular matrix to prevent sperm binding, ensuring monospermy.
Which gene expression regulation step determines if RNA polymerase can initiate transcription?
Transcriptional initiation regulated by promoters and transcription factors.
How do lactose and glucose levels affect the Lac repressor and CAP activator?
Lactose binds and inactivates the Lac repressor; low glucose increases cAMP, activating CAP which enhances transcription; both together turn operon ON.
Provide a diagram or verbal sequence showing how RTKs activate downstream signaling molecules such as Ras.
Ligand binding induces RTK dimerization and autophosphorylation; adaptor proteins bind phosphorylated RTKs; these recruit and activate Ras via a GDP-GTP exchange, triggering downstream pathways.
What would be the consequence if phospholipase C function was inhibited after fertilization?
The slow block to polyspermy would fail, leading to polyspermy and abnormal zygote development.
Why is the fast block alone insufficient for monospermy?
Because the membrane depolarization is temporary and does not modify the egg’s extracellular layers permanently.
Place these control mechanisms at the correct gene expression steps: RNA splicing, protein degradation, translation control, RNA degradation, transcriptional control.
Transcriptional control–transcription; RNA splicing and RNA degradation–post-transcriptional; translation control–translation; protein degradation–post-translational.
How do enhancers increase transcription from distant locations?
By DNA looping to bring activator proteins bound at enhancers into contact with promoter regions.