5.7: In translation, the mRNA copy of the information from DNA is used to build functional molecules.

Once the mRNA molecule has moved out of the cell’s nucleus and into the cytoplasm, the translation process begins. In translation, the information carried by the mRNA is read, and ingredients present in the cytoplasm are used to produce a protein (see Figure 5-11). The process of translation is like combining and baking the ingredients listed in our chocolate chip cookie recipe to produce a cookie.

Several ingredients must be present in the cytoplasm for translation to occur. First, there must be large numbers of free amino acids floating around. Recall from Chapter 2 that amino acids are the raw materials for building proteins and an essential component of our diet. Second, there must be ribosomal subunits, which are components of ribosomes, the protein-production factories where amino acids are linked together in the proper order to produce the protein. And there must also be molecules that can read the mRNA code and translate that message from a sequence of bases into a protein.

To read an encrypted message, you need to know the secret code by which to translate each of the encrypted characters into a readable character in the message. Similarly, to understand which sequence of amino acids in a polypeptide is specified by a sequence of nucleotides in a DNA molecule—and in the mRNA molecule transcribed from the DNA—you need to know a secret code of sorts.

A special type of RNA molecules holds the secret code. These molecules, called transfer RNA (tRNA), interpret the mRNA code, translating the language of DNA—coded in the linear sequence of bases—into the language of proteins, coded in the linear sequences of amino acids.

Picture a molecule with two distinct ends (FIGURE 5-13). On one end of the tRNA molecule is an attachment site consisting of a three-base sequence that matches up with a three-base sequence on the mRNA transcript. This matchup enables the tRNA molecule to attach to the mRNA. Attached to the other end of the tRNA molecule is an amino acid. Each three-base sequence in mRNA—called a codon—matches with a tRNA molecule that carries a particular amino acid. The codon ACG, for example, is recognized by the tRNA molecule that carries the amino acid threonine. And the codon CAG is recognized by the tRNA molecule that carries glutamine. For every possible codon, there is one type of tRNA molecule that will recognize and bind to the mRNA at that point, and it will always carry the same amino acid.

The codon table (FIGURE 5-14), also called the “genetic code,” describes which tRNA, and therefore which amino acid, is specified by each codon. With four possible bases in each of the three positions of a codon, 64 different codons are possible. Sixty-one of these codons specify amino acids, and 3 are “stop” sequences, indicating the end of translation. Because there are only 20 amino acids, many amino acids are specified by multiple codons. UGU and UGC, for example, both specify cysteine. With just a few small exceptions, the genetic code is the same in every organism on earth.

The translation of an mRNA molecule into a sequence of amino acids (that will then fold into the complex three-dimensional shape of a protein) occurs in three steps (FIGURE 5-15).

Step 1. Recognize and initiate protein building. Translation begins in the cell’s cytoplasm when the subunits of a ribosome, essentially a two-piece protein-building factory, recognize and assemble around a codon on the mRNA transcript called the start sequence. This start sequence is always the codon AUG. As the ribosomal subunits assemble themselves into a ribosome, the attachment site of a particular tRNA molecule also recognizes the start sequence on the mRNA and binds to it. This initiator tRNA carries the amino acid methionine (met). Thus, methionine is always the first amino acid in any protein that is produced. (Occasionally, in eukaryotes, this initial methionine is “edited out” later in the protein-building process.)

Step 2. Elongate. After the mRNA start sequence (AUG), the next three bases on the mRNA specify which amino acid–carrying tRNA molecule should bind to the mRNA. If the next three bases on the mRNA transcript are GUU, for example, a tRNA molecule that recognizes this sequence, and carrying its particular amino acid (valine, or val), will attach to the mRNA at that point. The ribosome then facilitates the connection of this second amino acid to the first. After the amino acid carried by a tRNA molecule is attached to the new amino acid, the tRNA molecule detaches from the mRNA and floats away.

As this process continues, the amino acid chain grows. The next three bases on the mRNA specify the next amino acid to be added to the first two. And the three bases after that specify the fourth amino acid, and so on. This process of progressively linking together the amino acids specified by an mRNA strand is called protein synthesis, because all proteins are chains of amino acids, like beads on a string. Precision in this process is essential. If the tRNAs were to misread or skip or double-read any bases, they could alter the amino acid sequence (and possibly the normal functioning) of the protein specified by the mRNA.

Step 3. Terminate. Eventually, the ribosome arrives at the codon on the mRNA that signals the end of translation. Once the ribosome encounters this stop sequence, the assembly of the amino acid chain is complete. Translation ends, and the amino acid chain and mRNA molecule are released from the ribosome. As it is being produced, the amino acid chain folds and bends, based on the chemical features in the amino acid side chains (as we saw in Chapter 2). Through the folding and bending, the protein acquires its three-dimensional structure. The completed protein—such as a membrane protein or insulin or a digestive enzyme—may be used within the cell or packaged for delivery via the bloodstream to somewhere else in the body where it is needed.

Following the completion of translation, the mRNA strand may remain in the cytoplasm to serve as the template for producing another molecule of the same protein. In bacteria, an mRNA strand may last from a few seconds to more than an hour; in mammals, an mRNA may last several days. Depending on how long it lasts, the same mRNA strand may be translated hundreds of times. Eventually, it is broken down by enzymes in the cytoplasm.