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The Genetic Code

To communicate the message, the “words” of the mRNA must be read and understood. These words specify the amino acid that must be placed into the polypeptide chain. If we think of the nucleotides as the “letters” in the alphabet of the genetic code, then groups of those letters make up the words. As it turns out, each word in the mRNA is three letters, or nucleotides, long and is called a codon. Since there are only four letters in the alphabet, and each word contains three letters, there are only 4 X 4 X 4, or 64, possible words. The deciphering of the words is called the genetic code and is shown in Figure 6.3. By using this chart, an mRNA sequence can be translated. To use this chart, you must begin with the first nucleotide in the codon and find it’s symbol on the left hand side of the chart (the first, or 5’, position). Then find the second position from the top of the chart, and the third nucleotide from the right hand side (or 3’ end). Now find where all three intersect in the interior of the chart. This is the amino acid the codon specifies.


Example : the codon UUG encodes the amino acid leucine.


Only 20 amino acids are specified by the genetic code, yet 64 different codons are possible. How, exactly, is this code used by the cell? There are three interesting answers to this question: 
  • Three codons, UAA, UAG, and UGA do not specify any amino acids, but rather signal the end of the message. These are often referred to as stop or termination codons.
  • Many amino acids are specified by more than one codon. We call this redundancy.
Example: ACA, ACG, ACU, and ACC all code for the amino acid threonine. You should also note that, in each of these codons, the first two nucleotides are A and C. It doesn’t matter what the nucleotide in the third position is. We call this the wobble effect.
  • One codon, AUG, specifies the amino acid methionine. However, it also signals the start of translation for all mRNA molecules. Therefore, all polypeptide chains begin with methionine. 

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