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Transcript
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 threenucleotide 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? FIRST LETTER U 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.. SECOND LETTER U C A G THIRD LETTER UUU UCU UAU UGU phe ser tyr cys U UUC UCC UAC UGC phe Ser tyr cys C UUA UCA UAA UGA leu ser (stop) (stop) A UUG UCG UAG UGG leu ser (stop) trp G CUU CCU CAU CGU leu pro his arg U CUC CCC CAC CGC C C A G leu pro his arg CUA CCA CAA CGA leu pro glu arg A CUG CCG CAG CGG leu pro glu arg G AUU ACU AAU AGU ile thr asn ser U AUC ACC AAC AGC ile thr asn ser C AUA ACA AAA AGA ile thr lys arg A AUG ACG AAG AGG (start) met thr lys arg G GUU GCU GAU GGU val ala asp gly U GUC GCC GAC GGC val ala asp gly C GUA GCA GAA GGA val ala glu gly A GUG GCG GAG GGG val ala glu 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?
