Molecules move into and out of the nucleus through the _____.
nuclear pores
The nuclear pore complex directs movement of molecules into and out of the nucleus.
Which of the following macromolecules leaves the nucleus of a eukaryotic cell through pores in the nuclear membrane?
mRNA
What can you infer about a high molecular weight protein that cannot be transported into the nucleus?
It lacks a nuclear localization signal (NLS).
Which statement most accurately describes what happens to proteins that lack an ER signal sequence?
They are released into the cytosol.
What scientific hypotheses can be tested by a pulse-chase experiment?
Movement of molecules through a cell over time
True or false? Proteins produced during the “chase” phase of a pulse-chase experiment are labeled with radioactive material.
False
What is the first step in a pulse-chase experiment?
Incubating cells with a labeled molecule
Cells are first “pulsed” with a large amount of labeled material for a short time.
What three distinct elements make up the cytoskeleton in eukaryotic cells?
actin filaments, intermediate filaments, and microtubules
Actin filaments are also known as microfilaments.
Which answer most accurately summarizes the (1) structural and (2) functional properties of actin filaments, intermediate filaments, or microtubules?
Intermediate filaments: (1) many subunit types; (2) one cellular role
Although intermediate filaments can be built from a variety of subunit types (keratins, lamins, or others), they serve purely a structural role
How are these signals read?
They bind to receptor proteins.
Which of the following results provided evidence of a nuclear localization signal in the nucleoplasmin protein?
After cleavage of the protein, only the tail segments appeared in the nucleus.
Using concepts from Chapter 3, which statement best explains why proteins—and not RNA, DNA, carbohydrates, or lipids—are the molecules responsible for “reading” the array of molecular zip codes in cells?
Because proteins can have diverse structures and chemical properties, different proteins can “read” different zip codes by interacting specifically with them.
Zip codes consist of short amino acid sequences that are “read” through “lock and key” interactions with specific shapes of other (receptor) proteins.
Which molecules do not normally cross the nuclear membrane?
DNA
All processes involving DNA take place in the nucleus.
Which of the following statements about the nuclear envelope is false?
The nuclear envelope is continuous with the Golgi apparatus.
the nuclear envelope is continuous with the endoplasmic reticulum.
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.
False
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.
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?
Passive transport
A 25,000-dalton protein is small enough to diffuse through nuclear pores without any expenditure of energy.
In experiments to test whether a protein can enter the nucleus, why would proteins be labeled with fluorescent molecules?
To make the proteins easy to see
Fluorescent tags make the location of the proteins easy to visualize.
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?
Only one of the two fusion proteins possesses a nuclear localization signal.
The nuclear localization signal is only present in the fusion protein that enters the nucleus.
Which motor protein(s) can move cargo-containing vesicles along microtubule tracks?
Kinesin and dynein
Kinesin and dynein motors move along microtubules toward the plus and minus ends, respectively.
