Question 1

Which of the following post-translational modifications (PTMs) most commonly targets serine, threonine, or tyrosine residues and is crucial for regulating protein activity in signal transduction pathways?

Accepted Answer

Phosphorylation

Question 2

The chemical reaction for phosphorylation can be represented as follows. Which molecule donates the phosphate group in this reaction?

$$\mathrm{Protein{-}OH} + \mathrm{ATP} \rightarrow \mathrm{Protein{-}O{-}PO_3$^{2-}} + \mathrm{ADP}$$

Accepted Answer

ATP

Question 3

Which of the following best describes the role of E3 ligase in the ubiquitylation process?

Accepted Answer

It transfers ubiquitin from E2 to the substrate protein, conferring substrate specificity.

Question 4

A protein is tagged with a polyubiquitin chain linked through lysine-48 of ubiquitin. What is the most likely fate of this protein?

Accepted Answer

It will be targeted for proteasomal degradation.

Question 5

Which of the following statements about the proteasome is CORRECT?

Accepted Answer

It is a processive machine with multiple protease active sites, allowing degradation of nearly all polypeptides.

Question 6

During the cell cycle, which of the following events is regulated by the Anaphase Promoting Complex/Cyclosome (APC/C) in association with Cdc20?

Accepted Answer

Transition from metaphase to anaphase

Question 7

Cyclin-dependent kinases (Cdks) are regulated by multiple mechanisms. Which of the following is NOT a direct mechanism of Cdk inactivation?

Accepted Answer

Binding of cyclins

Question 8

A mutation causes the loss of function of the Cdc25 phosphatase. What is the most likely effect on M-Cdk activity and cell cycle progression?

Accepted Answer

M-Cdk remains inactive, and the cell is arrested before mitosis.

Question 9

Which of the following best describes the function of the signal recognition particle (SRP) during protein targeting to the endoplasmic reticulum (ER)?

Accepted Answer

It binds the ER signal sequence and pauses translation until the ribosome docks on the ER membrane.

Question 10

A protein has an N-terminal ER signal sequence followed by a stop-transfer sequence. After insertion into the ER membrane, what is the orientation of this protein?

Accepted Answer

N-terminus in the ER lumen, C-terminus in the cytosol

Question 11

Which of the following is a key function of ER-resident chaperones?

Accepted Answer

Assisting in the proper folding of newly synthesized proteins

Question 12

Which of the following statements about translation is CORRECT?

Accepted Answer

Elongation factors facilitate the movement of tRNAs and mRNA during translation.

Question 13

In a ribosome profiling experiment, which of the following data would indicate a region of mRNA with high translation efficiency?

Accepted Answer

Many ribosome-protected fragments mapping to the region

Question 14

Which of the following is an example of a ligand-gated ion channel discussed in class?

Accepted Answer

NMDAR (N-methyl-D-aspartate receptor)

Question 15

Which of the following best describes a second messenger in cell signaling?

Accepted Answer

A small intracellular molecule that relays signals from receptors to target molecules

Question 16

Which of the following is an example of a molecular switch in cell signaling?

Accepted Answer

GTP-bound G protein

Question 17

A signaling pathway is activated by a GPCR that stimulates adenylyl cyclase. Which G protein is most likely involved?

Accepted Answer

Gs

Question 18

Which of the following best illustrates negative feedback in a signaling pathway?

Accepted Answer

A kinase activates a phosphatase that dephosphorylates the kinase

Question 19

Which of the following is a plausible outcome if a mutation prevents the hydrolysis of GTP by a G protein in a GPCR pathway?

Accepted Answer

The G protein remains constitutively active, leading to prolonged signaling.

Unit 2 Study Guide: Post-Translational Modifications, Proteasome, Cell Cycle, Translation, Protein Targeting, and Cell Signaling

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