acid. The charged tRNA molecule recognizes the codon through complementary base
pairing with a region of it called an anticodon (Fig. 6.13).
6.4.2 Translation Elongation
Now polypeptide synthesis takes place with amino acids joining together as successive
codons are read in the elongation phase of translation. Before elongation can occur, the
large ribosomal subunit joins to create a complete ribosome. The ribosome now has
three sites that can accommodate a tRNA molecule: a peptidyl (P), an aminoacyl (A),
and an exit (E) site. The initiator tRNA occupies the P site of the ribosome, which is posi-
tioned over the initiator AUG codon and is adjacent to the A site, which at this stage is
available and is over the next codon to be read. Then the appropriately charged tRNA
for this next codon in the A site enters it, and its anticodon pairs with the codon. A
peptide bond then forms between the amino acids that are attached to the tRNAs in the
P and A sites. Now the initiator amino acid is released from its tRNA and the ribosome
moves down the mRNA or translocates to position the growing polypeptide in the P site
and free the A site, which once again positions over the next codon to be translated. The
initiator tRNA that no longer is charged is in the E site and it is then free to leave the ribo-
some and become charged again. This elongation cycle is repeated until the entire polypep-
tide chain is made.
6.4.3 Translation Termination
Polypeptide synthesis is over when the ribosome encounters a stop codon in its A site. Since
no tRNAs can base pair with these stop codons, proteins called “release factors” bind to the
ribosome instead. These release factors allow the polypetide chain to be released from the P
site as well as the mRNA to no longer bind to the ribosome. The ribosome also splits into its
two subunits.
6.5 Protein Postranslational Modification
Following translation, polypeptides can be modified in a number of ways before they are
fully functional, and in fact, different organisms modify proteins in different ways that
can have biological significance. The initiator amino acid, methionine, can be changed
or removed. More amino acids can be added, or the polypeptide can be “trimmed” by
removing amino acids. Also, amino acids can be modified by the addition of carbohydrate
sidechains, phosphates, methyl groups, or conjugated with metals. These modifications can
significantly alter the function of proteins, and subsequently control cellular function. For
example, phosphorylation is an important mechanism for controlling intracellular signaling.
In order to be a functional protein, polypeptides also must be appropriately folded into a
three-dimensional conformation, which can occur either spontaneously or under the direc-
tion of molecular “chaperones.” As mentioned earlier, some proteins are composed of a
single polypeptide, whereas others are multimeric, composed of one or more additional
polypeptides that form the complete protein. Posttranslational modifications can fundamen-
tally alter gene expression by changing protein function, allowing the cell to rapidly
respond to variable internal and external stimuli. Understanding how to control
152 MOLECULAR GENETICS OF GENE EXPRESSION