How to read a codon table

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How to read a codon table
Use in protein synthesis for
translating the mRNA code into
amino acid sequence
You need a sequence of
mRNA
• Where does this come from?
From the transcription
of DNA --> mRNA
Example:
DNA sequence: TAC GGA CAT AAC ACC TGC ATC
mRNA sequence: AUG CCU GUA UUG UGG ACG UAG
Transcription
• mRNA sequence leaves the nucleus and travels to
the cytoplasm to a free floating ribosome or to
the rough ER.
• It will attach to the ribosome and begin the
second step of protein synthesis, translation.
Translation
• mRNA is read as a series of codons (three letters)
within the ribosome.
• tRNA molecules have an anticodon sequence of
letters that are complements to the mRNA
ex: mRNA CGA UCC (codon)
tRNA GCU AGG (anticodon)
So now we get to the codon table!
• Locate the first letter
of your codon using
the left side of the
table.
• Ex. AUG
• look for the A
• Now move to the
second letter of your
codon which is ‘U’
• Look at the top of the
table where you see the
title ‘2nd letter’
• Find the letter ‘U’ and
follow it down until it
intersects with the letter
‘A’ from the left side.
• You should see four
amino acids (isoleucine,
isoleucine, isoleucine,
and (start) methionine.
• Down to the last letter
of the codon!
• Look to the right hand
side for the third letter.
Find the letter ‘G’ which
will intersect with the
box that had our four
choices.
• Move your finger from
the ‘G’ on the left over
to the left and you
should land on …..
Methionine (start)
• Yes you did it!!!
• Now try another codon
Try the codon CAC
Don’t peek until you
come up with
your answer!
Did you get the
amino acid
‘histidine’?
What do these codons have to do
with proteins?
• Each codon represents
an amino acid that will
eventually form a
protein that is used
within a cell.
• Proteins are made up of
hundreds of amino
acids in a specific
sequence.
• When they get “out of
order’ a mutation
occurs.
Long string
of amino
acids will
form

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