Question 1

Which of the following best explains why neuronal signaling molecules can act over longer distances compared to contact-dependent signaling molecules?

Accepted Answer

Neuronal signaling molecules are transported along axons and released at synapses, enabling them to reach distant target cells.

Question 2

A cell receives an extracellular signal and responds within seconds. Which molecular change is most likely responsible for this rapid response?

Accepted Answer

Modification of existing proteins, such as phosphorylation.

Question 3

Which molecular change is most likely to cause a slow change in cell behavior in response to an extracellular signal?

Accepted Answer

Activation of gene expression and synthesis of new proteins.

Question 4

Which of the following correctly matches a Gα protein type with its primary signaling pathway?

Accepted Answer

Gαs activates adenylyl cyclase, increasing cAMP production.

Question 5

Acetylcholine slows heart rate, while epinephrine increases it. Which trimeric G proteins are activated by their respective GPCRs, and what are the key enzymes involved?

Accepted Answer

Acetylcholine activates Gαi, inhibiting adenylyl cyclase; epinephrine activates Gαs, stimulating adenylyl cyclase.

Question 6

Can signaling via a steroid hormone receptor lead to amplification of the original signal? If so, how?

Accepted Answer

Yes, because a single activated receptor can induce transcription of many copies of target genes, amplifying the effect.

Question 7

Succinylcholine binds to the acetylcholine receptor on skeletal muscle cells but causes muscle relaxation instead of contraction. Which model best explains this effect?

Accepted Answer

Succinylcholine acts as an agonist but causes prolonged receptor activation, leading to desensitization and muscle relaxation.

Question 8

Which of the following is NOT one of the three main classes of cell-surface receptor?

Accepted Answer

Steroid hormone receptors

Question 9

A mutation prevents a CaM-kinase from binding ATP. What is the most likely effect on spatial learning in mice?

Accepted Answer

Spatial learning is impaired because the kinase cannot phosphorylate substrates without ATP.

Question 10

A mutation deletes the calmodulin-binding part of CaM-kinase. What is the most likely effect on the kinase's activity?

Accepted Answer

The kinase will not be activated by Ca2+, leading to impaired spatial learning.

Question 11

A mutation destroys the site of autophosphorylation in CaM-kinase. What is the most likely effect on spatial learning?

Accepted Answer

Spatial learning is impaired because the kinase cannot remain active after Ca2+ levels drop.

Question 12

If a mutation reduces CaM-kinase's interaction with the protein phosphatase responsible for inactivating the kinase, what is the most likely effect?

Accepted Answer

The kinase will remain active longer, potentially leading to abnormal signaling.

Question 13

A mutation reduces the affinity of the G protein α subunit for GDP but does not affect its affinity for GTP. What is the most likely effect on G protein activity?

Accepted Answer

The G protein will be more active because GDP will dissociate more easily, allowing GTP to bind and activate the protein.

Question 14

An antibody specific for the extracellular domain of an RTK brings together two RTK molecules. What is the most likely effect on the kinase activity?

Accepted Answer

The kinase will be activated because dimerization is required for activation.

Question 15

How can the same nuclear receptor, which binds to a specific DNA sequence, regulate different genes in different cell types?

Accepted Answer

Different cell types express different co-activators and co-repressors, allowing the nuclear receptor to regulate distinct genes.

Question 16

In a mouse fur color signaling pathway, mice lacking cAMP phosphodiesterase have dark brown fur. What is the most likely explanation for this phenotype?

Accepted Answer

cAMP phosphodiesterase breaks down cAMP; without it, cAMP levels remain high, leading to increased pigment production.

Question 17

A mouse is genetically engineered so that the α subunit of the G protein in the fur color pathway cannot hydrolyze GTP to GDP. What color will the mouse's fur be, and why?

Accepted Answer

The mouse will have dark brown fur because the G protein remains active, keeping cAMP levels high and increasing pigment production.

Question 18

Which equation best describes the relationship between cAMP concentration and phosphodiesterase activity in a cell?

Accepted Answer

$$[cAMP] = \frac{	ext{rate of synthesis}}{	ext{rate of degradation by phosphodiesterase}}$$

Question 19

A cell exposed to a signal that activates a GPCR linked to Gαq will most likely experience which downstream effect?

Accepted Answer

Increased IP3 and DAG production via phospholipase C activation.

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