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The m-RNA carries a specific sequence of nucleotides in "triplets" called "codons", responsible for the synthesis of a specific protein molecule.On the other hand t-RNA has Anticodon arm.
This is anoher unpaired and non bonded loop carrying specific sequences of three bases constituting the anticodon. The bases of anticodon are hydrogen bonded with three complementary bases of condon of m-RNA. The base pair stem leading to anticodon loop is called the 'anticodon stem'. For example codon of glycine amino acid is GGG on mRNa so anticodon on tRNa will be CCC.
Every Codon is triplet and universal.
In all the living organisms the genetic code is the same. This phenomenon is called as universality of the code. The exception to universality is found in mitochondrial genome where AUA codes for methionine and UGA for tryptohan instead of isoleucine and termination or stop respectively. AGA and AGG code for Arginine in normal condition but in mitochondria it terminates protein synthesis. Studies of mutations in viruses, bacteria and higher organisms have established the universality of the genetic code. Most of the aminoacid substitutions in proteins can be accounted for by a change of a single DNA base.
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