There are many organelles in a cell. Most of them are compartmentalized by membrane. I like to say that organelles that are surrounded like membrane like that are "membrane-bounded." Sometimes you might see the term "membrane-bound," but that sounds like the organelle is attached to membrane, even if that is not what they meant. So I will try to keep using the term "membrane-bounded."
There are four main organelles that are important in making proteins. Three of these are membrane-bounded, while one is not. The four organelles, in the order in which they work to make protein, are:
Of these four, only one of them is actually the one that physically assembles the polypeptides needed for the protein-- that is the ribosome. The other four are all involved in helping to create the protein that is needed.
What's involved in making protein?
There are millions of proteins. And the specific types of proteins that you have make you who you are. If you have brown hair, you have the brown pigment protein. If you are a plant, you have the chlorophyll protein. We have to have the right proteins in the right cells. How is that possible? We have to use our genetic code to do all this. That is our DNA.
So, you have to have a plan for making the protein, and that plan comes from your DNA, the hereditary material in your cells that makes you who you are. We have to be able to take the DNA and use its information to make protein. Here's how it is done: . In other words:
DNA makes RNA which makes protein.
Got it? Now memorize it!
The DNA is housed in your nucleus. It is the main part of your chromosomes. DNA never leaves your nucleus during normal cell activities! The proteins, however, are made on other organelles in the cytoplasm called ribosomes. So, how do our cells deal with the fact that the DNA (the plan) is inside the nucleus and the protein synthesis machinery is outside of the nucleus? Ah-hah! Now you should see why we need that item called RNA! RNA is made in the nucleus off of the DNA. Then, RNA is small enough to slip out of the nucleus into the cytoplasm where it brings the information for making the protein. Keep in mind that you do not need to know how DNA makes RNA, just that it does.
Finally, think about what it means to make a protein. You have to take amino acids and stick them together (in the right order). Then you have a polypeptide. This polypeptide folds up into a certain shape, possibly with other polypeptides, too. When the final shape is achieved, only then is the molecule functional and given a name as a protein.
Ready to navigate through these four protein-making organelles? I hope so. Just click here to get reading about the nucleus.
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007