Molecular Genetics ❮ 123
Translation of RNA
Now that the mRNA has escaped from the nucleus, it is ready to help direct the construction
of proteins. This process occurs in the cytoplasm, and the site of protein synthesis is the
ribosome. As mentioned in Chapter 5, proteins are made of amino acids. Each protein has
a distinct and particular amino acid order. Therefore, there must be some system used by
the cell to convert the sequences of nucleotides that make up an mRNA molecule into the
sequence of amino acids that make up a particular protein. The cell carries out this conver-
sion from nucleotides to amino acids through the use of the genetic code.An mRNA mol-
ecule is divided into a series of codons that make up the code. Each codonis a triplet of
nucleotides that codes for a particular amino acid. There are 20 different amino acids, and
64 different combinations of codons. This means that some amino acids are coded for by
more than one codon. For example, the codons GCU, GCC, GCA, and GCG all call for
the addition of the amino acid alanine during protein creation. Of these 64 possibilities, one
is a start codon,AUG, which establishes the reading frame for protein formation. Also
among these 64 codons are three stop codons:UGA, UAA, and UAG. When the protein
formation machinery hits these codons, the production of a protein stops.
Before we go through the steps of protein synthesis, we would like to introduce to you
the other players involved in the process. We have already spoken about mRNA, but we
should meet the host of the entire shindig, the ribosomes,which are made up of a large
and a small subunit. A huge percentage of a ribosome is built out of the second type of
RNA mentioned earlier, rRNA. Two other important parts of a ribosome that we will dis-
cuss in more detail later are the A siteand the P site,which are tRNA attachment sites.
The job of tRNA is to carry amino acids to the ribosomes. The mRNA molecule that is
involved in the formation of a protein consists of a series of codons. Each tRNA has, at its
attachment site, a region called the anticodon,which is a three-nucleotide sequence that is
perfectly complementary to a particular codon. For example, a codon that is AUU has an
anticodonthat reads UAA in the same direction. Each tRNA molecule carries an amino
acid that is coded for by the codon that its anticodonmatches up with. Once the tRNA’s
amino acid has been incorporated into the growing protein, the tRNA leaves the site to pick
up another amino acid just in case its services are needed again at the ribosome. An enzyme
known as aminoacyl tRNA synthetasemakes sure that each tRNA molecule picks up the
appropriate amino acid for its anticodon.
Uh-oh... there is a potential problem here. There are fewer than 50 different types of
tRNA molecules. But there are more codons than that. Oh, dear... but wait! This is not a
problem because some tRNA are able to match with more than one codon. How can this
be? This works thanks to a phenomenon known as wobble,where a uracil in the third posi-
tion of an anticodon can pair with A or G instead of just A. There are some tRNA molecules
that have an altered form of adenine, called inosine (I), in the third position of the anticodon.
This nitrogenous base is able to bind with U, C, orA. Wobble allows the 45 tRNA mole-
cules to service all the different types of codons seen in mRNA molecules.
We have met all the important players in the translation process (see also Figure 11.6),
which begins when an mRNA attaches to a small ribosomal subunit. The first codon for
this process is always AUG. This attracts a tRNA molecule carrying methionine to attach
to the AUG codon. When this occurs, the large subunit of the ribosome, containing the
A site and the P site, binds to the complex. The elongation of the protein is ready to begin.
The P site is the host for the tRNA carrying the growing protein, while the A site is where
the tRNA carrying the next amino acid sits. Think of the A site as the on-deck circle of a
baseball field, and P site as the batter’s box. So, AUG is the first codon bound, and in the
P site is the tRNA carrying the methionine. The next codon in the sequence determines
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