A Pulse Chase Experiment

Part A

Pancreatic cells, which secrete a large amount of digestive enzymes, are labeled with radioactive leucine and then chased for several hours with nonradioactive leucine. Photographic emulsions are prepared at different times during the chase. Where would the black spots appear on an emulsion prepared 3 hours after the pulse?

Secretory vesicles

Rough ER

Exterior of the cell

Golgi apparatus

Correct

Black spots would appear on the exterior of the cell after 3 hours, indicating that the proteins had been secreted from the cell.

Part B

What path does a protein in the secretory pathway take, starting from its site of synthesis?

Rough ER, secretory vesicles, Golgi apparatus, plasma membrane

Plasma membrane, secretory vesicles, Golgi apparatus, rough ER

Golgi apparatus, rough ER, secretory vesicles, plasma membrane

Rough ER, Golgi apparatus, secretory vesicles, plasma membrane

Correct

Proteins are synthesized in the rough ER, modified in the Golgi apparatus, and carried in secretory vesicles to the plasma membrane, where they are secreted.

Part C

During a pulse-chase experiment, photographic emulsions were prepared at different times during the chase, and radioactive spots were detected at the following times and locations: 5 minutes: rough ER; 10 minutes: Golgi apparatus; 40 minutes: endosomes; 70 minutes: lysosomes; 140 minutes: lysosomes. Which of the following conclusions can be drawn from these results?

The proteins were secreted.

The proteins traveled from lysosomes to endosomes.

The proteins did not travel through the Golgi apparatus.

The final destination of the proteins was the lysosome.

Correct

This conclusion can be drawn from the results, since the radioactive spots were last detected at the lysosome, which is not a secretory vesicle.

Part D

What scientific hypotheses can be tested by a pulse-chase experiment?

Solubility of a molecule

The steady-state amount of protein in a cell

Molecular zip code of a protein

Movement of molecules through a cell over time

Correct

Pulse–chase experiments allow for the tracking of molecules in a cell over time.

Part E

True or false? Proteins produced during the “chase” phase of a pulse–chase experiment are labeled with radioactive material.

True

False

Correct

During the chase phase, cells are provided with a large amount of nonradioactive material, so they are notlabeled.

Part F

What is the first step in a pulse-chase experiment?

Preparing cells for electron microscopy

Incubating cells with a labeled molecule

Incubating cells with an unlabeled molecule

Examining cells for the location of the labeled molecules

Correct

Cells are first “pulsed” with a large amount of labeled material for a short time.

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