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Ah, codons.
If you chose to watch this short video, you are perhaps searching for a little additional
help to read a codon chart.
And you’ve come to the right place.
We’re going to assume you already have the background from our DNA versus RNA video and
our Protein Synthesis video.
If not…you might want to check those out first or otherwise jumping right into this
isn’t going to make much sense.
Oh, and in addition, we have a video companion handout to this video which can be helpful.
Expand this video’s details for the link as you may want to download it first.
So if you had this DNA template strand here, you’d have this mRNA built.
I know this because of how RNA bases pairs with DNA bases.
Notice how these mRNA bases are written here in groups of 3?
Each is a codon.
Bases are read in threes so a codon is how these bases will be “read.”
In this example, there are a total of 5 codons.
15 individual RNA bases.
By reading a codon chart, you can determine the amino acids that will be brought in by
the tRNAs.
You can determine the sequence of amino acids that make up a protein.
And if you’re wondering why this matters---understanding how amino acids are coded for helps us understand
human traits and how changes in amino acids have the potential to alter those traits.
Not just for humans either but all living organisms!
The codons and their corresponding amino acids have been discovered by scientists.
There are many ways you can represent the information in a chart.
The most common ways are the rectangular codon chart and the circular codon chart.
Let’s get started.
The first codon---remember codons have three bases and codons are on the mRNA---is AUG.
That means the tRNA anticodon will be UAC, and we know that because of the base pairing
rules.
Now, which amino acid will that tRNA carry?
Time to use the codon chart!
We’ll use a rectangular one, and remember, it’s the mRNA codon that we look at.
Not the tRNA anticodon.
So notice on the left side of the chart, it is for the first base.
The top is for the second base.
And the right side is for the third base.
We’re looking at the codon AUG.
We go in order.
First base letter: A. We isolate this row here and only this row.
Second base letter: U.
We isolate this column here and only this column.
We’ve narrowed it down to this area here where they intersect.
Third base letter: G.
Here!
The codon AUG codes for the amino acid methionine.
Fun fact: methionine is commonly the starting amino acid for many polypeptide chains as
AUG is a start codon.
Next codon is CCA.
That means the anticodon for the tRNA will be GGU.
Now, what amino acid will that tRNA carry?
Time to use the codon chart!
This time, we’re going to ask you to pause the video while you try to determine which
amino acid this tRNA would be carrying.
Again, don’t forget to use the mRNA codon.
Not the tRNA anticodon.
So, in slow motion, let’s go through this chart.
First base letter: C. That narrows down this row.
Second letter C. That narrows down this column, and therefore this area here where they meet.
Third letter A. The amino acid proline!
Aright, the third codon.
GUC.
This time, we need you to determine the tRNA anticodon, and also, the amino acid that tRNA
would be carrying.
Pause the video to determine those.
The anticodon, based on the base pairing rules, would be CAG.
But we use the mRNA codon for this codon chart, and when you used it---showing this in slow
motion here--- you should have gotten the amino acid valine.
Now, with those three codons, we used a rectangular codon chart.
We mentioned that there are other ways to represent the information, and the circular
chart is common to find as well.
The circular codon chart typically works by starting on the inside and moving outward.
So, this fourth codon we have here is UUC.
That means the tRNA anticodon would be AAG, but again we’re going to use the mRNA codon
in the chart.
So, we start with the center.
First letter U, which isolates to this area.
Then we move out one to the second letter which isolates further: U.
Finally, we move one more time to the C.
This is the amino acid phenylalanine.
Your turn now to use the circular codon chart with this last codon.
UAA.
What does the mRNA codon UAA stand for?
[PAUSE] In slow motion, you can see that we get “stop.”
This does not code for an amino acid; instead, it is generally at the end of a sequence for
a polypeptide.
And, well, this signals the end of reading the codon chart…for this little chain of
amino acids anyways.
In reality, polypeptides tend to have many more amino acids than this and proteins tend
to be made of one or more of these polypeptide chains.
We hope this was helpful, but before we go, we want to mention just a few pitfalls with
reading codon charts.
Pitfall #1: Not using the mRNA codon.
Unless it specifies otherwise, the mRNA codon is what you want to use for the codon chart.
Some students will accidentally try to use the tRNA anticodon.
Some students will try to use the original DNA template.
Now while there are charts that exist for the tRNA anticodon or DNA, unless it specifically
says otherwise, you should assume it’s for the mRNA codon.
Pitfall #2: Getting stuck on the standard rectangular chart.
Pinky has noticed students tend to have more challenges with the rectangular one.
We suggest going slowly, and even using a highlighter if you’re able to---that way
you can see where the first base isolates the row, the second base isolates the column,
and third base finishes in isolating the amino acid.
Remember to go carefully in order.
Pitfall #3: Not practicing.
You got to practice; that’s why this video has a video companion.
So let’s do some more practice by going backwards!
So using this circular codon chart, can you tell me the two codons that code for lysine?
Go ahead and pause the video for a moment.
So, using this circular codon chart, we can see the codons AAA and AAG code for lysine.
Let’s use this rectangular codon chart here now.
Which six mRNA codons could code for the amino acid serine?
The mRNA codons that code for serine are UCU, UCC, UCA, UCG, AGU, and AGC.
Phew that’s a lot of codons!
Well, that’s it for the Amoeba Sisters, and we remind you to stay curious!