The Theme of Levels of Organization in Gene to Protein

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Up until this point we have talked about transcription and translation as though all the players magically came together. But in reality, it is a lot more complicated. DNA is in the nucleus in a eukaryotic cell. Where do you think transcription must occur? Where do you think proteins are made? What might happen if the proteins or RNA didn't make it to the correct spot?

Let's start at the beginning. DNA is in the nucleus in a eukaryotic cell. Transcription occurs in the nucleus. Therefore, all the transcription stars, such as polymerase, transcription factors, regulatory proteins, need to be there too.

Once an mRNA is made, the RNA immediately undergoes processing. Splicing and capping occur. These covalent modifications are important, because they indicate that an mRNA is complete and ready to transport. The completion of processing is the green flag for the RNA to begin its transport out of the nucleus.

You may remember that the nucleus is a membrane bound organelle. Think of this barrier as the bouncer of nucleus. Not anyone will get by this dude. In fact, the mRNA needs to have its own connection to get by him. This connection is called the nuclear pore complex.

The nuclear pore complex is pretty much what it sounds like: a pore where proteins and RNA can be released into the rest of the cell. A specific group of proteins, called poly-A binding proteins, bind to the mature mRNA and guide it to the nuclear pore complex so it can "sneak" out of the nucleus into the cytoplasm. It is here in the cytoplasm where translation takes place: on the rough endoplasmic reticulum, or on free-floating ribosomes. The proteins produced by translation then are transported to the correct regions of cell such that they can fulfill their functions.

What about the RNAs that don't have a poly-A tail and aren't mRNAs? Great question. The transport of rRNAs and tRNAs is closely linked to their transport as well. rRNA are produced and processed in a special region of the nucleus called the nucleolus. This region isn't membrane bound, but it is visible under the microscope. It contains the many copies of ribosomal RNA genes needed to produce the mass amount of rRNA needed by the cell.

What we have told you is that transcription and translation, and the components involved in both processes, are localized to specific regions of the cell. Pretty important stuff. Now though, we'll leave you with three important questions to ponder.

1. How do you think that these levels of organization influence the production of a protein? Hint: Think regulation.
     
2. How do you think the situation might work in prokaryotes? Hint: Think about the structure of a prokaryotic cell. If you need a refresher, click here.
     
3. Can you explain how the disruption of this organized system might result in a disease state? Hint: Think regulation again. What happens if an mRNA can't make it past the big bad nuclear pore complex?