MoBio The Genetic Code Chapter 3

Protein synthesis is based on the sequence of mRNA, which is made up of nucleotides while proteins are made up of amino acids. There must be a specific relationship between the nucleotide sequence and amino acid sequence. This relationship is the so called genetic code, which was deciphered by Marshall Nirenberg and his colleagues in early 1960s. One of their approaches is illustrated in Figure 3-E-2. It turns out that three nucleotides (a codon) code for one amino acid, as shown in the following figure.

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Figure 3-E-1. The standard genetic code. Synthesis of a peptide always starts from methionine (Met), coded by AUG. The stop codon (UAA, UAG or UGA) signals the end of a peptide. This table applies to mRNA sequences. For DNA, U (uracil) should be replaced by T (thymine). In a DNA molecule, the sequence from an initiating codon (ATG) to a stop codon (TAA, TAG or TGA) is called an open reading frame (ORF), which is likely (but not always) to encode a protein or polypeptide.