The Genetic Code

During the next process of gene expression, amino acids are assembled based on instructions encoded in the sequence of nucleotides in the mRNA. The genetic code is the term for the rules that relate how a sequence of nitrogenous bases in nucleotides corresponds to a particular amino acid. In the genetic code, three adjacent nucleotides ("letters") in mRNA specify an amino acid ("word") in a polypeptide. Each three-nucleotide sequence in mRNA that encodes an amino acid or signifies a start or stop signal is called a codon.

Table 10-1 lists the 64 mRNA codons and the amino acids they encode in most organisms. For example, the codon GCU specifies the amino acid alanine in the genetic code. The genetic code is nearly universal to all life on Earth and supports the idea that all organisms share an ancient common ancestor.

Some amino acids are encoded by two, three, or more different codons, as shown in Table 10-1. These codons often differ from one another by only one nucleotide. No codon encodes more than one amino acid. One special codon, AUG, acts as a start codon. A start codon is a specific sequence of nucleotides in mRNA that indicates where translation should begin. The start codon encodes the amino acid methionine. Certain sequences of nucleotides in mRNA (UAA, UAG, or UGA), called stop codons, do not code for amino acids, but instead signal for translation to end.

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TABLE 10-1 Codons in mRNA

First base

Second base

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