Question 1

Which of the following statements best describes the role of the Nuclear Localization Signal (NLS) in protein targeting within eukaryotic cells?

Accepted Answer

NLS is a short, positively charged amino acid sequence necessary and sufficient for nuclear import.

Question 2

A researcher mutates the lysine residues in the NLS of a nuclear protein to polar, uncharged amino acids. What is the most likely outcome?

Accepted Answer

The protein will remain in the cytoplasm due to loss of nuclear import signal.

Question 3

Which organelle is defined in large part by the specific proteins associated with it and is bounded by a double membrane continuous with the rough ER?

Accepted Answer

Nucleus

Question 4

What is the primary function of importins in nuclear transport?

Accepted Answer

They recognize NLS and mediate the import of proteins into the nucleus.

Question 5

A protein with a molecular weight of 70,000 Da is synthesized in the cytoplasm. How will it most likely enter the nucleus?

Accepted Answer

By active, gated transport mediated by importins recognizing its NLS.

Question 6

Which of the following best explains why nuclear transport is considered bidirectional?

Accepted Answer

Both import and export of proteins and RNAs occur through the nuclear pore complex.

Question 7

During T-cell activation, which molecular event exposes the NLS of NF-AT, allowing its nuclear import?

Accepted Answer

Dephosphorylation of NF-AT by calcineurin in response to increased $$[Ca$$^{2+}$$].$$

Question 8

Which of the following is TRUE regarding the Ran GTPase cycle in nuclear transport?

Accepted Answer

Ran-GTP in the nucleus provides directionality and energy for import and export processes.

Question 9

A cytosolic protein is engineered to contain a nuclear export signal (NES). What is the expected result?

Accepted Answer

The protein will be exported from the nucleus if it first enters the nucleus.

Question 10

Which of the following statements about nuclear pore complexes is CORRECT?

Accepted Answer

They mediate the transport of fully folded proteins and ribonucleoprotein particles.

Question 11

What is the consequence of mutating the NLS of a protein so that it is buried inside the folded structure?

Accepted Answer

The protein will remain in the cytoplasm due to lack of recognition by importins.

Question 12

Which of the following best describes the process by which mRNA is exported from the nucleus?

Accepted Answer

mRNA is exported as part of a ribonucleoprotein complex (mRNP) via nuclear pores.

Question 13

Which of the following is a key difference between nuclear import and mitochondrial import of proteins?

Accepted Answer

Nuclear import occurs posttranslationally with folded proteins, while mitochondrial import often requires proteins to be unfolded.

Question 14

Which of the following is NOT a possible reason for a protein with an NLS to fail to enter the nucleus?

Accepted Answer

The protein lacks a NES.

Question 15

Steroid hormones activate gene expression by which of the following mechanisms?

Accepted Answer

Diffusing through the membrane, binding to cytoplasmic receptors, and exposing an import signal for nuclear entry.

Question 16

Which of the following equations best represents the energy requirement for nuclear transport mediated by Ran-GTP hydrolysis?

Accepted Answer

$$GTP ightarrow GDP + P_i + 	ext{energy}$$

Question 17

Which of the following statements about the nuclear envelope is TRUE?

Accepted Answer

It is a double membrane structure, with the outer membrane continuous with the rough ER.

Question 18

Which of the following is a function of the nuclear lamina?

Accepted Answer

It provides structural support to the nuclear envelope.

Question 19

Which of the following best describes the fate of the NLS after a protein enters the nucleus?

Accepted Answer

It remains on the protein and is reused after each cell division.

Protein Targeting and Nuclear Localization in Eukaryotic Cells

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