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Molecular Basis of Heredity: Part 1. Nucleic Acids

Author(s): Raye L. Alford, PhD

The Genetic Code

In mRNA, each three-nucleotide codon specifies a particular amino acid. As such, an mRNA has at least three times as many nucleotides as the protein it encodes has amino acids. mRNAs also often contain non-translated 5' and 3' sequences that typically serve regulatory or other purposes.

Note in the diagram on the left how the anti-codon loop of the tRNA molecule is complementary to the three-nucleotide codon sequence in the mRNA. This tRNA carries a methionine, MET, amino acid (typically the start, or initial, amino acid of a nascent eukaryotic protein). The complementarity of codon and anti-codon insures proper amino acid incorporation into growing protein molecules.

The table demonstrates the genetic code. The 20 different amino acids are shown in the center of the table, while the nucleotides of the various codons are shown around the outside edges, with the first position of the triplet codon on the left side, the second position across the top, and the third position down the right side of the table.

As shown, there are 64 possible codons but only 20 amino acids. This makes the genetic code redundant; that is, some amino acids are coded for by more than one codon. Note in the table how the codons UCU, UCC, UCA, UCG, AGU, and AGC all code for the amino acid serine. The redundancy of the genetic code means that not every change, or mutation, of the nucleotide sequence within a gene will result in a change in the encoded protein.

At the end of each mRNA is a final codon, called a stop codon. This codon tells the ribosome that construction of the protein is complete. The ribosome then falls off the mRNA strand and the newly completed protein is released for further processing and migration to its final intra- or extra-cellular destination.