5.6–5.8: Information in DNA directs the production of the molecules that make up an organism.

A display of the expression levels of many genes simultaneously, based on DNA microarray technology.

If DNA is like a cookbook filled with recipes, transcription and translation are like cooking. In cooking, you use information about how to make chocolate chip cookies to actually produce the cookies. In an organism, the information about putting together proteins is used to build the proteins that the organism needs to function. In this section, we examine transcription, the first step in the two-step process by which DNA regulates a cell’s activity and its synthesis of proteins (see Figure 5-11). In transcription, a copy is made of one specific gene within the DNA. Continuing our cookbook analogy, transcription is like copying a single recipe from the cookbook onto an index card. Transcription happens in four steps (FIGURE 5-12).

Figure 5.12: Transcription: copying the base sequence of a gene. The first step in a two-step process by which DNA regulates a cell’s activity and synthesis of proteins. (Shown here is eukaryotic transcription.)

Step 1. Recognize and bind. To start the transcription process, the enzyme RNA polymerase recognizes a promoter site, a sequence in a gene that indicates the start of the gene and, in effect, tells the RNA polymerase, “Start here.” RNA polymerase binds to the DNA molecule at the promoter site and unwinds it just a bit, so that only one strand of the DNA can be read.

Step 2. Transcribe. As the DNA strand is processed through the RNA polymerase, the RNA polymerase builds a copy—called a “transcript”—of the gene from the DNA molecule, just as (to use another analogy) a court reporter transcribes and creates a record of everything that is said in a courtroom. This copy is called messenger RNA (mRNA) because, once this copy of the gene is created, it can move elsewhere in the cell and its message can be translated into a protein. Throughout transcription, DNA is unwound ahead of the RNA polymerase so that a single strand of the DNA can be read, and it is rewound after the polymerase passes.

The mRNA transcript is constructed from four different nucleotides, which have a structure similar to that of DNA nucleotides but with a different kind of sugar. Each pairs up with an exposed base on the unwound and separated DNA strand, following these rules:

If the DNA strand has a thymine (T), an adenine (A) is added to the mRNA.

If the DNA strand has an adenine (A), a uracil (U) is added to the mRNA.

If the DNA strand has a guanine (G), a cytosine (C) is added to the mRNA.

If the DNA strand has a cytosine (C), a guanine (G) is added to the mRNA.

Step 3. Terminate. When the RNA polymerase encounters a sequence of bases on the DNA at the end of the gene (called a termination sequence), it stops creating the transcript and detaches from the DNA molecule. After termination, the mRNA molecule is released as a free-floating, single-strand copy of the gene.

Step 4. Capping and editing. In prokaryotic cells, once an mRNA transcript is produced and begins to separate from the DNA it is ready to be translated into a protein (it doesn’t have a nuclear membrane to cross). In eukaryotes, mRNAs receive extra processing before they can be translated into a protein. First, a cap and a tail may be added at the beginning and end of the transcript. Like the front and back covers of a book, these serve to protect the mRNA from damage and help the protein-making machinery recognize the mRNA. Second, because (as we saw earlier in the chapter) there may be some non-coding bits of the DNA that are transcribed into mRNA, those sections—the introns—are snipped out. Once the mRNA transcript has been edited, it is ready to leave the nucleus for the cytoplasm, where it will be translated into a polypeptide that will fold into a protein.