Genetic Code

As mentioned before, the way that the mRNA specifies a sequence of amino acids is by means of three-nucleotide codons. Each codon is complementary to the three-nucleotide anticodon on one of the tRNAs. That tRNA has an amino acid attached to the other end of it. This was attached by a synthesis reaction catalyzed by a synthetase enzyme -- a specific enzyme that has an active site that fits that particular tRNA and the particular amino acid that goes on it.

Consider the tRNA shown below. What codon will it bind to? (Remember the codon is in the mRNA.)

There isn't any choice about which codon it can bind to -- it has to be complementary, and the complementary bases are determined by patterns of opposite charges that attract each other. Now, consider this tRNA:

What codon on the mRNA will it bind to?

Can you tell what amino acid goes on this tRNA just by looking at it?

Actually, you would have to know what the synthetase enzymes bind to. If you knew which synthetase enzyme binds to this tRNA, and which amino acid also bound in the active site of that enzyme, then you would know which amino acid will be found on this tRNA. And then you'd know what amino acid would go into the polypeptide when the mRNA had the codon UGG, which is complementary to this tRNA's anticodon. To make this easier for you, here's a table that shows you which mRNA codon goes with each amino acid in your own (or any human's) cells. This is called the GENETIC CODE. The amino acids are given here by their three-letter abbreviations.. / FIRST LETTER / SECOND LETTER / THIRD LETTER
U / C / A / G
U / UUU phe / UCU ser / UAU tyr / UGU cys / U
UUC phe / UCC Ser / UAC tyr / UGC cys / C
UUA leu / UCA ser / UAA (stop) / UGA (stop) / A
UUG leu / UCG ser / UAG (stop) / UGG trp / G
C / CUU leu / CCU pro / CAU his / CGU arg / U
CUC
leu / CCC
pro / CAC
his / CGC
arg / C
CUA leu / CCA pro / CAA glu / CGA arg / A
CUG leu / CCG pro / CAG glu / CGG arg / G
A / AUU ile / ACU thr / AAU asn / AGU ser / U
AUC ile / ACC thr / AAC asn / AGC ser / C
AUA ile / ACA thr / AAA lys / AGA arg / A
AUG
(start)
met / ACG
thr / AAG
lys / AGG
arg / G
G / GUU val / GCU ala / GAU asp / GGU gly / U
GUC val / GCC ala / GAC asp / GGC gly / C
GUA val / GCA ala / GAA glu / GGA gly / A
GUG val / GCG ala / GAG glu / GGG gly / G

Ok. So, suppose there is a mRNA in one of your cells with the sequence

AUGCCCGUGUCGGAAUGGUGA.

What is the sequence of the polypeptide that would be formed from this mRNA by your ribosomes, using your tRNAs?