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Text Sections
Section 4.3 A Tour of the Animal Cell: The Protein Pathway, p. 59
Introduction
One way to think of a cell is as a living factory. In this tutorial, we'll begin learn- ing about the animal cell by tracing the way it manufactures a product—a pro- tein—for export outside itself. Just as a new employee might tour an assembly line as a means of learning about factory equipment, so you are going to follow the path of protein production as a means of learning about cell equipment.
Learning Objectives
- Understand the role of mRNA in protein synthesis.
- Describe the cellular structures involved in protein synthesis.
- Know what exocytosis is.
W E B T U T O R I A L 4. 2
A Protein Pathway
Narration
Protein Synthesis
Proteins are critical working molecules in living things, and DNA contains the information for producing these proteins. In the eukaryotic cell, DNA is largely confined within a nucleus bound by a membrane. The process of protein synthesis requires that the DNA's instructions first get copied onto another long-chain mol- ecule called messenger RNA (mRNA). Given that mRNA goes to the cytoplasm, it must, of course, have some way of getting out of the nucleus; its exit routes turn out to be thousands of channels that stud the surface of the nuclear envelope—the nuclear pores.
As the messenger RNA leaves the nucleus, it attaches to a small structure in the cytoplasm called a ribosome. For now, we can define the ribosome as an organelle that serves as the site of protein synthesis in the cell, acting as a kind of playback head on a cassette deck. The ribosome reads the instructions provided by the mRNA molecule, and produces a chain of amino acids that folds up into the mol- ecule we call a protein.
For proteins destined for export out of the cell, after a very short sequence of the amino acid chain has grown from the ribosome, the ribosome, bound up with its mRNA and amino acid chain, will migrate to the rough endoplasmic reticulum (RER) and dock on its outside face, thus joining many other ribosomes that have done the same thing.
The endoplasmic reticulum is a network of membranes that aids in the processing of proteins in eukaryotic cells. The rough endoplasmic reticulum is “rough” because it is studded with ribosomes. The amino acid (or polypeptide) chain that is being output from the ribosome is, in essence, an unfinished protein that needs to go through more processing before it can be exported. The first step in this process- ing leads only to the other side of the ER wall the ribosome is embedded in. As the ribosome goes on with its work, the amino acid chain it is producing drops into chambers inside the rough ER.
As amino acid chains drop into the sacs of the ER, they first fold up into their pro- tein shapes. Beyond this, most proteins that are exported from cells have sugar side-chains added to them here.
The proteins that have been processed within the rough ER need to move out of the ER to the next station of the assembly line before being exported. Part of the ER membrane containing the newly synthesized protein buds off and enters the cytosol. These membrane spheres, carrying proteins and other molecules, are called transport vesicles.
Once a transport vesicle has budded off from the rough ER, it then moves through the cytosol to fuse with the membrane of another organelle called the Golgi com-
