What Determines a Genetic Code?
Build Your Own Genetic Code
The relationship between three-nucleotide codons in the mRNA and amino acids in polypeptides
is called the genetic code. You've seen the human genetic code on the preceding handout. But
why does, for instance, CCC code for proline? In other words, in your own cells, what causes a
proline to be inserted in the protein being made whenever a CCC is found in the mRNA being
translated (why not a Tryptophan, a Methionine, a Glycine, an Alanine, etc...)? Let's review the
steps that lead up to this (it may help to review the previous assignments).
First, what will be the anticodon of the tRNA that binds to CCC in the mRNA?
Why must this particular anticodon pair with CCC? Explain.
The tRNA that has the anticodon above must have proline attached to it. How did the proline get
there?
There's a synthetase enzyme for that specific tRNA. It has an active site that fits the tRNA and the
amino acid proline.
Notice that the proline has an OH in its acid group; remember (from the proteins page) that every
amino acid has an acid group like this. The tRNA has, at its top end, an -OH group. Every
nucleotide has one of these, and every tRNA would have an OH at the end like this. What kind
of reaction will be catalyzed by the synthetase to join them?
Suppose you could construct a different synthetase enzyme (it could be done, actually, by
altering the gene that codes for that enzyme). If you constructed a synthetase enzyme with an
active site that fit this tRNA and the amino acid leucine, could it then put leucine on this tRNA?
Explain.
What's different about different amino acids?
Note that this group, the variable one in an amino acid, isn't involved in the bonding to the tRNA.
So any amino acid could be put on any tRNA. The thing that ensures that the right one is put
onto a given tRNA is the synthetase enzyme specific to that tRNA. In other words, there's no
functional necessity for the tRNA with the anticodon GGG to have a proline on it; the bond that
joins proline to the tRNA is a bond between the acid group on the amino acid, and the OH at one
end of the tRNA, and these groups are constant -- every amino acid and every tRNA would have
the same groups in these spots.
So, if you did have the altered synthetase mentioned above (that puts leucine on the tRNA with
the anticodon GGG), what would happen during translation of an mRNA like this one?
AUGCCCGGGCAG
Explain.
Now, this would be bad if it happened to you; your mRNAs are set up so that CCC codes for
proline, and if the wrong amino acid were put in when every time this codon showed up, your
proteins probably wouldn't work.
But suppose you were going to construct an organism in your lab (cue
the creepy music and thunderstorm). If you constructed the mRNAs
from scratch, and also made all the synthetases, you could set up the
genetic code any way you wanted. Suppose I wanted, ACG to code for
Alanine. I'd just have to make a synthetase that would bind alanine and
the tRNA with the anticodon UGC, and then ACG would code for
alanine in the organism I was making. Of course, when I made the
mRNAs for my organism, I'd have to remember that when I wanted an
alanine in a protein, I should use ACG as the codon in the mRNA.
Again, there's no functional necessity for a given codon to go with any particular amino
acid; it's just that once you've set up the mRNAs with particular code, changing it later would
mess up all the proteins the mRNAs code for.
So, here's your chance. In the blanks below in the genetic code table, put in the three-letter
amino acid abbreviations to construct your very own genetic code. Note that you must have
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at least one start codon (which codes for an amino acid as well), that signals the start
of translation (at bottom of table), and you must have
at least one stop codon, that signals the end of translation, and you must have
all 20 amino acids represented somewhere, and
every codon must code for something.
Otherwise, you're free to arrange them any way you want, but don't do it according to the table
you've already seen (or any you see in the text, etc.). Just stick all the amino acids (plus stop) in
wherever you like. Some amino acids have to be used more than once, of course, but you can
choose which ones and how often.
Tips about this assignment:
1.
Yes, it's kind of silly. There are lots of right ways to do it -- just follow the instructions and bear with me,
ok? I've got a point that I'll be making.
2.
Are the codon assignments really wide open? Are there no constraints about what codon can go with what
amino acid? Well, chemically, any amino acid really can be joined to any tRNA, and in fact some
experiments have been done in which "wrong" amino acids have been put on tRNAs. It works, and when
the tRNAs are used in translation, the "wrong" amino acid is incorporated into the protein.
3.
You may have noticed that codons for the same amino acid often have the same first two nucleotides.
There's actually some slop in the complementary pairing of codon and anticodon, so that a tRNA with, say,
the anticodon AAG can actually pair with UUC or UUU -- so it really only takes one phenylalanine (phe)
tRNA to bind to either of those codons. This is called wobble, and it reduces the actual number of possible
codes. Don't worry about it for the purpose of this assignment, though if you want to group codons for the
same amino acid this way, that's fine.
Here are the abbreviations for the 20 amino acids. Use them to fill in the codon table below.
TRP, MET, GLY, ALA, ASN, ASP, GLU, GLN, PHE, HIS, LYS, VAL, TYR, THR, ARG,
LEU, ILE, CYS, PRO, SER
U
SECOND NUCLEOTIDE
C
A
G
UUU
UCU
UAU
UGU
U
UUC
UCC
UAC
UGC
C
UUA
UCA
UAA
UGA
A
UUG
UCG
UAG
UGG
G
FIRST
CUU
CCU
CAU
CGU
U
THIRD
NUCLE-
CUC
CCC
CAC
CGC
C
NUCLE-
CUA
CCA
CAA
CGA
A
O
CUG
CCG
CAG
CGG
G
TIDE
AUU
ACU
AAU
AGU
U
AUC
ACC
AAC
AGC
C
AUA
ACA
AAA
AGA
A
AUG
ACG
AAG
AGG
G
GUU
GCU
GAU
GGU
U
GUC
GCC
GAC
GGC
C
GUA
GCA
GAA
GGA
A
GUG
GCG
GAG
GGG
G
U
O
C
TIDE
A
G
Now, you've made your very own genetic code. Now look at the mRNA below. What would be
the amino acid sequence of the polypeptide that would be translated from this mRNA, using your
very own genetic code from the table above?
AUGCCCGUAGCCAGUGCCCAGUCGGAAUGGUGA