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THE GENETIC
CODE
and how we got to the
decipherment
In the early sixties,
molecular biologists
were able to
"decrypt" the
genetic code.
There was a
problem that left
them perplexed:
how can someone
write 20 "words"
with an alphabet of
only 4"letters"?
That is:
is :
how can the four
nitrogenous bases (A
U C G) specify 20
different amino
acids?
acids
The only possibility
was a triplet code,
based on the codons
codons,,
each one has three
nitrogenous bases.
Having only four letters (A U C G) clearly, a
codon of a single letter could encode only four
amino acids, not 20; and a codon of two letters
would have coded 4x 4 = 16, still too little.
But a codon of three letters could encode
4x4x4 = 64 codons
codons,, more than enough for 20
amino acids.
acids
But how did they
arrive to the
deciphering of the
genetic code?
The first step in the decoding of
the genetic code was made in
1961, ten years after the
"discovery" of the structure of
DNA by Watson and Crick.
The scientists who carried out the
first experiments to decipher the
genetic code were the
biochemical Niremberg Marshall,
winner of the Nobel Prize for
medicine, and his German
colleague Heinrich Matthaei
Matthaei..
They realized that they could
use a simple artificial
polynucleotide as a
messenger, rather than a
natural messenger RNA,
much more complex, in order
to identify the polypeptide
encoded by this artificial
messenger RNA.
The two researchers prepared
an artificial messenger RNA,
previously discovered by
biochemist Severo Ochoa,
containing only a nitrogenous
base, uracil
uracil,, repeated over and
over again. This RNA molecule
was called poly
poly-- U .
Adding a poly - U in a test tube
containing the ingredients
necessary for protein synthesis
( ribosomes
ribosomes,, enzymes, amino acids,
ATP), could compose a
polypeptide chain composed
entirely of a single type of amino
acid: phenylalaninel . Thus, a poly u encoded the phenylalanine;
consequently, UUU was the codon
(the "code word") to specify the
phenylalanine.
With a similar method were decrypted
all possible codons for the 20 amino
acids. Similar experiments were carried
out later, using artificial RNA in which
two or three nucleotides were repeated
over and over according to a certain
sequence. Of the 64 codons encoded, 61
were amino acids and the remaining 3
were signs of stopping.
A s c a n b e s e e n f rom t h e t a b l e b e l ow, t h e re
a re g e n e ra l ly d i f fe re n t c o d o n s for t h e
s a m e a m i n o a c i d : t h e g e n e t i c c od e i s
re d u n d a n t . Fu r t h e r m ore , for a l m os t a l l
orga n i s ms i s a lways t h e s a m e : i t i s s a i d,
t h e re fore , t h a t t h e g e n e t i c c od e i s a n
universal
code