Promoters How does RNA polymerase know where to start and stop making a strand of RNA? The answer is that RNA polymerase doesn't bind to DNA just anywhere. The enzyme binds only to promoters, regions of DNA that have specific base sequences. Promoters are signals in the DNA molecule that show RNA polymerase exactly where to begin making RNA. Similar signals in DNA cause transcription to stop when a new RNA molecule is completed.

RNA Editing Like a writer's first draft, RNA molecules sometimes require a bit of editing before they are ready to be read. These pre-mRNA molecules have bits and pieces cut out of them before they can go into action. The portions that are cut out and discarded are called introns. In eukaryotes, introns are taken out of pre-mRNA molecules while they are still in the nucleus. The remaining pieces, known as exons, are then spliced back together to form the final mRNA, as shown in Figure 13–4.

Why do cells use energy to make a large RNA molecule and then throw parts of that molecule away? That's a good question, and biologists still don't have a complete answer. Some pre-mRNA molecules may be cut and spliced in different ways in different tissues, making it possible for a single gene to produce several different forms of RNA. Introns and exons may also play a role in evolution, making it possible for very small changes in DNA sequences to have dramatic effects on how genes affect cellular function.

FIGURE 13–4 Introns and Exons Before many mRNA molecules can be read, sections called introns are “edited out.” The remaining pieces, called exons, are spliced together. Then, an RNA cap and tail are added to form the final mRNA molecule.

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13.1 Assessment

Review Key Concepts

1. 1. Review Describe three main differences between RNA and DNA.

   2. Explain List the three main types of RNA, and explain what they do.

   3. Infer Why is it important for a single gene to be able to produce hundreds or thousands of the same RNA molecules?

2. 1. Review Describe what happens during transcription.

   2. Predict What do you think would happen if introns were not removed from pre-mRNA?

WRITE ABOUT SCIENCE

Creative Writing

3. An RNA molecule is looking for a job in a protein synthesis factory. It asks you to write its résumé. This RNA molecule is not yet specialized and could, with some structural changes, function as mRNA, rRNA, or tRNA. Write a résumé for this molecule that reflects the capabilities of each type of RNA.

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