Transport into the Nucleus

Part A

Which molecules do not normally cross the nuclear membrane?

	Proteins
	Nucleotide triphosphates
	mRNA
	DNA

Correct

All processes involving DNA take place in the nucleus.

Part B

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

	Molecules pass into and out of the nucleus through nuclear pores.
	The nuclear envelope is continuous with the Golgi apparatus.
	The nuclear envelope is composed of two lipid bilayers.
	Nuclear pores are made up of a group of proteins that are collectively called the nuclear pore complex.

Correct

This statement is false; the nuclear envelope is continuous with the endoplasmic reticulum.

Part C

True or false? Large proteins containing a nuclear localization signal (NLS) bind to the nuclear pore and enter the nucleus without any expenditure of energy.

	True
	False

Correct

Cytoplasmic proteins called importins bind to large proteins containing an NLS and mediate their transport across the nuclear membrane through an active transport (energy-requiring) process.

Part D

A small protein (molecular weight = 25,000 daltons) is injected into a cell and observed in the nucleus a short time later. What type of transport has taken place?

	Active transport
	Osmosis
	Passive transport

Correct

A 25,000-dalton protein is small enough to diffuse through nuclear pores without any expenditure of energy.

Part E

In experiments to test whether a protein can enter the nucleus, why would proteins be labeled with fluorescent molecules?

	To give the protein molecules energy
	To target the proteins to the nucleus
	To make the proteins bigger
	To make the proteins easy to see

Correct

Fluorescent tags make the location of the proteins easy to visualize.

Part F

Nucleoplasmin is a nuclear protein. This protein was divided into two segments and linked to the same large cytoplasmic protein, generating two fusion proteins. After injecting these fusion proteins into a cell, one of the proteins was found in the nucleus and the other in the cytoplasm. Which of the following conclusions can be drawn from these results?

	The cytoplasmic protein contains a nuclear localization signal.
	Nucleoplasmin does not have a nuclear localization signal.
	One of the fusion proteins entered the nucleus by passive transport.
	Only one of the two fusion proteins possesses a nuclear localization signal.

Correct

The nuclear localization signal is only present in the fusion protein that enters the nucleus.