The G e n e t i c Code
The genetic code is defined based on the codewords (codons) found in
mRNA.
You will have a copy of the genetic code to use on any exam where it is
needed. You can see a copy of the code in the exam study guide at:
http://plantpathology.tamu.edu/PLPA/Projects/4/index.html
The genetic code was deciphered rapidly once chemists discovered ways
to make artificial messages. These messages, even if only one codon long
could be stuck to ribosomes in a test-tube system (in vitro) to see what
amino acid (via tRNA) was attached. Longer messages could be used to
make short proteins. The first message used was polyU and gave a
protein that was only phenylalanine, leading to the correct conclusion
that UUU codes for phe. (Note that in vitro translation can be made to
occur without a start codon, but adding a start greatly improves
efficiency).
The genetic code is:
a triplet code, meaning that 3 adjacent bases (U, C, A, or G) in a mRNA
code for one amino acid in the translated protein.
Note that all 64 possible codons have meaning. All 64 also occur in
natural messages. The three codons that do not code far any amino
acid are UAA, UAG and UGA. These codons are all signals for the end
of translation so are called stop codons. When the meaning of each
codon was being deciphered through the use of artificial messages,
none of the 20 amino acids would be incorporated when one of these
codons appeared, so they are also called nonsense codons. The other
61 codons call for an aminoacid so are sense codons.
a degenerate code, meaning that that more than one codon (up to 6 in the
case of arginine, serine and leucine) code for the same amino acid.
Much of the degeneracy is in the third base position. In many cases
such as the codons for glycine, the third base can be any of the 4.
Thus GGA, GGC, GGG and GGU are all glycine codons. This is often
written as GCX or GCN where X or N can be any base. In all cases if the
third base is a pyrimidine (U or C) the same amino acid is coded.
However UGG is the only tryptophan codon and AUG and AUA code for
different amino acids. Crick explained degeneracy on the basis that
the tRNA on the ribosome tended to curve away from the codon, so
that less precise pairing is permitted between the anticodon(on the
tRNA ) and the codons. This is called the "Wobble Hypothesis".
unambiguous, meaning that the same codon always codes for the same
amino acid. The only exception is that the first AUG at the 5' beginning
of a message is the start codon and calls for a special tRNA carrying either
a modified methionine (formylmethionine of F-met) or a blank, but any
internal AUG codes for methionine.
commaless and nonoverlapping, meaning that it is read 3 bases at a time
with no extra bases between codons and no base being used in adjacent
codons.
universal, meaning that the same code works for all living organisms,
from prokaryotes to eukaryotes. There are a few known exceptions to this
rule. For quite a few cases the code inside mitochondria of higher
organisms including man uses UGA for another Tryptophan codon rather
that acting as a stop codon. Other examples are given at
http://www.web-books.com/MoBio/Free/Ch3E1.htm