2.5 The Genetic Code for Protein Synthesis
Scientists puzzled for some time on how four nitrogenous bases could provide the code for the 20 amino acids used in protein synthesis. The puzzle was solved once it was realized that three bases were used to code for one amino acid. Now the number of possible combinations could be calculated as 43 = 64, or more than enough!
In general, each amino acid can be coded for by several different three-base codons (the code is redundant; see table). For example, leucine is coded for by CUU, CUC, CUA, CUG, UUA, and UUG. Notice that in the first four examples, only the last base is different. Three codons, TAA, TAG, and TGA are used as stop codons to tell ribosomes to stop reading the code and stop making protein. Methionine has only one codon (AUG) and it is used as the first amino acid to start a new protein (it sometimes is removed later, as all proteins do not have an N-terminal methionine).
Figure 10 The genetic code
We now may think back concerning the necessary sequence of events. One strand of the DNA must be copied into an RNA sequence beginning at the promoter region. Either DNA strand may be used, depending on the location of the promoter region. Each three-base code of the RNA specifies a specific amino acid that will be polymerized together on ribosomes to give the final amino acid sequence which, in turn, will determine the properties of the new protein.
We will not go into the details of protein synthesis, but
the general features are shown in the figure. Ribosomes assemble on the
mRNA in the region containing a ribosome binding site and the initiation
codon (AUG for methionine). Amino acids are brought to the proper site on
the ribosome in the form of complexes with special small RNA molecules known
as transfer RNAs. Each transfer RNA is specific for its own amino acid and
has an "anti-codon" on one end of the loop shaped molecule and
the amino acid at the other. In the case of methionine, the anti-codon would
be UAC which will associate with AUG of the bound RNA. After a second transfer
RNA with its amino acid enters the site, a peptide bond is synthesized between
the two amino acids. The ribosome then shuttles three bases along the mRNA
so that the next transfer RNA and its amino acid may enter and undergo polymerization.
The process continues until a stop codon is reached and the ribosome falls
off the mRNA.
Figure 11. The synthesis of proteins on ribosomes
Some proteins have short stretches of amino acids that serve to direct the protein into specific cell compartments as it is being synthesized. These "signal" sequences are then removed by specific enzymes. In this way, proteins may be captured into vesicles for excretion out of the cell (insulin) or, in bacteria, may be directed to the space between the inner and outer membranes or excreted.