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Even though there are only 20 amino acids that exist, there are actually 64 possible tRNA molecules:
4 X 4 X 4 = 64 possible combinations
There are four choices of bases for the first space (A, U, G, or C), the same four choices for the second space (you can repeat the same bases), and the same four bases as a choice for the third spot. So, 4 x 4 x 4 is 64! 61 of the tRNAs code for specific amino acids and 3 code for chain termination as a result of pairing up with "stop codons", signaling the end of the mRNA message. The table shows which codons code for which amino acids:
AMINO ACID |
RNA CODON |
ALANINE |
GCC, GCA, GCG, GCU |
ARGININE |
AGA, AGG, CGU, CGA, CGC, CGG |
ASPARAGINE |
AAC, AAU |
ASPARTIC ACID |
GAC, GAU |
CYSTEINE |
UGC, UGU |
GLUTAMIC ACID |
GAA, GAG |
GLUTAMINE |
CAA, CAG |
GLYCINE |
GGA, GGC, GGG, GGU |
HISTIDINE |
CAC, CAU |
ISOLEUCINE |
AUA, AUC, AUU |
LEUCINE |
UUA, UUG, CUA, CUC, CUG, CUU |
LYCINE |
AAA, AAG |
METHIONINE (INITIATION) |
AUG |
PHENYLALANINE |
UUC, UUU |
PROLINE |
CCA, CCC, CCG, CCU |
SERINE |
UCA, UCC, UCG, UCU, AGC, AGU |
THREONINE |
ACA, ACC, ACG, ACU |
TRYPTOPHAN |
UGG |
TYROSINE |
UAC, UAU |
VALINE |
GUA, GUC, GUG, GUU |
STOP |
UAA, UAG, UGA |
After looking at this chart, something should strike you...why does each amino acid have more than one codon? Isn't one codon sufficient for each amino acid? In theory, yes, this would be correct. But cellular processes do not occur in a perfect world! What if the coding sequence in a particular codon should be GUA, but, due to a mutation, the coding sequence became GUC? What would happen? Check the chart to find out!
OK, so your next question might be...
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