Cracking the AP Biology Exam

6

Molecular Genetics

RNA

Now that we’ve seen how DNA is replicated, let’s take a look at how the genetic code is expressed as proteins. As we mentioned earlier, genetic information is first passed to an intermediate molecule called RNA. Proteins called transcription factors control the transfer of genetic information from DNA to RNA by binding to specific DNA sequences. Here’s a “roadmap” of how information is transferred from DNA to proteins:

Before we discuss what RNA does, let’s talk about its structure. Although RNA is also made up of nucleotides, it differs from DNA in three ways:

1. RNA is single-stranded, not double-stranded.

2. The five-carbon sugar in RNA is ribose instead of deoxyribose.

3. The RNA nitrogenous bases are adenine, guanine, cytosine, and a different base called uracil. Uracil replaces thymine.

Here’s a table to compare DNA and RNA. Keep these differences in mind—ETS loves to test you on them.

There are three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). All three types of RNA are key players in the synthesis of proteins.

  • Messenger RNA (mRNA) copies the information stored in the strand of DNA.
  • Ribosomal RNA (rRNA), which is produced in the nucleolus, makes up part of the ribosomes. You’ll recall from our discussion of the cell in Chapter 3 that the ribosomes are the sites of protein synthesis. We’ll see how they function a little later on.
  • Transfer RNA (tRNA) shuttles amino acids to the ribosomes. It is responsible for bringing the appropriate amino acids into place at the appropriate time. It does this by reading the message carried by the mRNA.

Now that we know about the different types of RNA, let’s see how they direct the synthesis of proteins.