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Analyze the Data 5-2: Martian Genetic Code
NASA has identified a new microbe present on Mars and requests that you determine the
genetic code of this organism. To accomplish this goal, you isolate an extract from this
microbe that contains all the components necessary for protein synthesis except mRNA.
Synthetic mRNAs are added to this extract and the resulting polypeptides are analyzed:
Synthetic mRNA
Resulting Polypeptides
AAAAAAAAAAAAAAAA
CACACACACACACACA
AACAACAACAACAACA
Lysine-Lysine-Lysine etc.
Threonine-Histidine-Threonine-Histidine etc.
Threonine-Threonine-Threonine etc.
Glutamine-Glutamine-Glutamine etc.
Asparagine-Asparagine-Asparagine etc.
a.
b.
c.
From these data, what specifics can you conclude about the microbe’s genetic
code?
What is the sequence of the anticodon loop of a tRNA carrying a threonine?
If you found that this microbe contained 61 different tRNAs, what could you
speculate about the fidelity of translation in this organism?
Solution
a.
From translation of the first synthetic mRNA, you can conclude that the amino acid
lysine is specified by a group of A residues. By examining the results using the
second and third synthetic mRNAs, you can conclude that a triplet codon is
required, rather than a mono, bi, or quadruplet, etc. codon. Specifically, using the
second mRNA, an alternating threonine-histidine peptide is derived. This could
result if an odd number of bases specified an amino acid, but not if an even number
of bases specified the code. The third mRNA can be translated in three different
frames resulting in three different polypeptides composed of a single type of amino
acid. From this, you can conclude that a triplet codon is required. Therefore, AAA
must specify lysine, ACA must specify threonine, and CAC must specify histidine.
b.
The anticodon loop of a tRNA specifying threonine would have the sequence UGU.
c.
Since this microbe contains four bases and a triplet codon genetic code, one could
conclude that the genetic code of this organism is degenerate in that more than one
codon probably specifies the same amino acid.
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