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Decoding the Gene
The Genetic Code is contained in a threeletter sequence called a codon.
• A codon consists of three consecutive
nucleotides, which specify a single amino
acid.
• RNA codons are modeled after the DNA
codons.
• Given a sequence of DNA with the letters:
• The corresponding mRNA sequence
would be:
This sequence represents three codons:
What amino acids are represented by
these three codons? (You may use a
codon table or codon wheel.)
UCG
Serine
CAC
Histidine
GGU
Glycine
• Therefore the protein coded by the DNA
gene segment
would have the amino acids:
Serine, Histidine & Glycine.
Notice AUG specifies methionine, or the
“Start” codon & there are three “Stop”
codons to specify the end of a protein.
How did they figure this out?
George Gamow postulated that a
three-letter code must be
employed to encode the 20
standard amino acids used by
living cells.
The fact that codons consist of
three DNA bases was first
demonstrated in the Crick,
Brenner et al. experiment.
Crick
Brenner
Marshall Nirenberg and Heinrich J.
Matthaei Experiment
1) They set up twenty test tubes,
each with a different (radioactively
labeled) amino acid.
2) E.coli bacterial cytoplasm was
added to each test tube.
(Ribosomes etc. from bacteria)
3) Artificially synthesized mRNA with the codon UUU only,
was place in each test tube.
4) The then analyzed each test tube to find out which one
contains amino acids bonded in a chain.
-
Which amino acids was linked into chain?
UUU
Phenylalanine
They looked for the protein produced with the
radiation. It was indeed made up of just
phenylalanine.
Therefore Nirenberg and
Matthaei knew the
codon UUU represented
the amino acid
phenylalanine.
vial of poly U
used in the
experiment
• They did a similar experiment with AAA which
represents Lysine. They then published their
results.
• Nirenberg assembled a team of scientists to
crack the rest of the genetic code.