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Genetic Code Information Resources
Genetic Code
Genetic code refers to the means through which information within the
genetic material is translated into proteins by cells. The genome of an
organism (its encoded heredity sequence) is emblazoned on the DNA or RNA.
The portion of the genome, referred to as genes, code for proteins that are
made of triple nucleotide units called codons.
Each codon will code for
particular amino acids. The nucleotide subunits are composed of deoxyribose,
phosphate and one of the following four nitrogenous nucleotide bases:
adenine (A), guanine (G), cytosine (C) and thymine (T). In RNA (viruses)
uracil (U) replaces thymine and ribose substitutes deoxyribose. Therefore
the ‘code’ of the sequence is shown by an arrangement of four letters;
either A, G, C & T in DNA or A, G, C, U in RNA. The three-letter codon was
proposed by George Gamov to represent the 20 different amino acids used by
living organisms to encode proteins.
In the 1950s, scientists from across the globe were in competition with one
another to unlock the method by which DNA is translated into proteins. By
1961, Marshall Nirenberg claimed to have unlocked the code, and for his
achievements, he was awarded the Nobel Prize. One of Nirenberg’s main
challenges was to discover how many bases would be in each codon.
Scientists
knew there were the four nucleotide bases of adenine (A), guanine (G),
cytosine (C) and thymine (T), and also knew there were twenty known amino
acids. This led them to the conclusion that there must be at least three
bases in each codon (4 x 4 x 4) providing 64 possibilities.
Nirenberg pioneered a technique for testing different amino acids on cell
materials. He created the ‘cell-free system’, which required him to rupture
the walls of the cell so that the cytoplasm within could be released. This
cytoplasm was then placed into twenty different test tubes. The cytoplasm
can synthesise protein outside of the cell, but only when the correct kind
of RNA is added, which allowed him to control the experiment.
Each of the
twenty test tubes were filled with a different amino acid and the cytoplasm
from E. Coli bacteria cells were added to these acids. One of the twenty
test tubes was radioactively tagged to enable the scientists to watch the
reaction. On the 27th May 1961, Nirenberg combined the cytoplasm with a
synthetic RNA called poly-U (made from uracil), with the radioactively
tagged tube containing phenylalanine.
The control tubes were showing a level
of 70 counts per milligram of protein, whereas the phenylalanine tube was
showing 38,000 counts. This experiment showed that a repeating uracil chain
of bases could force a protein chain of phenylalanine (the repeating amino
acid).
Even after this breakthrough experiment, there were still several tasks to
be completed before the code could truly said to have been solved. The
researchers still had to establish which bases make up each codon and
determine the sequence of these bases in the codons.
By 1965, Nirenberg had completed the code sequencing. The letters of DNA and
RNA could now be expressed in the form of a four-letter code, with groups of
three letters representing individual amino acids. If mutations exist within
the DNA’s code, incorrect protein formation can result. |
