Download Encoding Amino Acids • mRNA codes for amino acids

The Genetic Code | OAT
Encoding Amino Acids
• mRNA codes for amino acids,
which combine to form proteins
o But in what way does
RNA encode amino
acids?
• There are 4 RNA nucleotides
• Clearly, each nucleotide cannot
encode a different amino acid
o After all, there are only 4
RNA nucleotides and 20
amino acids
• Similarly, suppose we tried using
combinations of two nucleotides
to encode each amino acid
o There would be 42, or 16,
possible combinations
o This is still not enough to
unambiguously represent
all 20 amino acids
• Finally, suppose combinations of
3 RNA nucleotides represented
amino acids
o That would be 43, or 64,
possible combinations
o This is more than enough
to represent all 20 amino
acids
The Genetic Code
This chart shows every possible RNA codon and
the amino acids they encode. Suppose we want
to know the amino acid coded by the codon
ACU. To read the chart, find the first nucleotide
(A) in the red cells. Then, match it up with the
second nucleotide (C) in the yellow cells.
Finally, match those up with the 3rd nucleotide
(U) in the green cells. The combination of those
3 nucleotides points to one cell, which indicates
that the codon encodes threonine.
•
•
The Genetic Code
• So, we’ve established that there
are 64 possible 3-nucleotide
combinations
o These 3-nucleotide
combinations are known
as codons
• To represent our 20 amino acids,
we only need to use 20 of those
64 codons
• The remaining 44 are used as:
o Alternate codes for those
20 amino acids
o Start codons
o Stop codons
•
•
Multiple codons can code for the
same amino acid
o In other words, the code
is degenerative
o i.e. ACU and ACC both
code for threonine
However, each codon can only
code for one amino acid
o In other words, the code
is unambiguous
o i.e. ACU codes for
threonine and only
threonine
Notice that three codons don’t
encode amino acids:
o UAA, UAG, and UGA
o They are stop codons;
they tell a cell where to
stop translating the RNA
There is also a start codon: AUG
o Unlike the stop codons,
AUG also happens to
code for an amino acid:
methionine
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© 2017 J Co Review, Inc., Accessed by Guest on 06-18-2017