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Each possible set of three nucleotides (i.e. a codon) in the DNA is what specifies one particular amino acid, which in combination with other amino acids forms a protein. For example, AAA specifies the amino acid lysine while GCT specifies alanine. A codon is defined as a triplet of three nucleotides in the mRNA that codes for a specific amino acid during the synthesis of a protein molecule. Itis a sequence of nucleotides formed by triplet of bases (e.g. TAC is the codon for the amino acid tyrosine). Codon provides the genetic information which causes a specific amino acid to be produced in a cell. There are various types of codons, and each performs a specific function during protein synthesis in the cell of an organism.

A start codon is the codon that signals translation. It tells the mRNA when to initiate the peptide chain formation or elongation. It is the first codon of an mRNA transcript that is translated by the ribosomes during protein synthesis. Start codons are also known as initiator codons because they signal and initiate the translation process. AUG is a start codon, and it also codes for the amino acid, methionine. Apart from signaling the incorporation of the amino acid methionine into the growing polypeptide chain, the start codon AUG also signals the start of translation during protein synthesis in the cell. Stop codons are triplet of nucleotide sequence that signals the end of translation. They are also known as termination or nonsense codons because they bring protein synthesis in the cell to an end. Examples of stop codons include UAG, UAA, and UGA.

In the genetic code, there are 64 possible codons. This comprises 3 nucleotides in each codon with 4 possible bases which are adenine (Figure 1), guanine (Figure 2), cytosine (Figure 3) and thymine (Figure 4) or uracil (Figure 5).

Figure 1. Structure of adenine (A). Photo courtesy: https://www.microbiologyclass.com
Figure 2. Structure of guanine (G). Photo courtesy: https://www.microbiologyclass.com
Figure 3. Structure of cytosine (C).
Photo courtesy: https://www.microbiologyclass.com
Figure 4. Structure of thymine (T). Photo courtesy: https://www.microbiologyclass.com
Figure 5. Structure of uracil (U). Photo courtesy: https://www.microbiologyclass.com

The language of the gene is created when these bases (i.e. A, G, C, and T) are genetically arranged or set in the DNA just the same way the 26 English alphabets are ordered to create meaningful words. These four bases which includes adenine (A), guanine (G), cytosine (C), and thymine (T), and which are usually found in the mRNA of the cell can generate 64 possible triplet combinations (e.g. UAG). Out of this 64 possible triplet combinations or codons, 61 triplet combinations encode the 20 essential amino acids (Table 1) while the remaining three triplet combinations (UAG, UGA & UAA) are generally known as stop or nonsense codons because they signify the termination of the polypeptide chain formation during protein synthesis in the ribosome. Stop codons generally signal the end of translation during protein synthesis or gene expression in the cell.    

Mathematically, the codons are expressed as follows:

43 = 64 possible codons; where “4” refer to the bases (i.e. A, G, C & T)and “3” refer to the triplet combinations or codons.

Table 1: The Twenty Essential Amino Acids

Amino acidAbbreviation
Aspartic acidASP
Glutamic acidGLU

Further reading

Cooper G.M and Hausman R.E (2004). The cell: A Molecular Approach. Third edition. ASM Press.

Das H.K (2010). Textbook of Biotechnology. Fourth edition. Wiley edition. Wiley India Pvt, Ltd, New Delhi, India.

Davis J.M (2002). Basic Cell Culture, A Practical Approach. Oxford University Press, Oxford, UK. 

Mather J and Barnes D (1998). Animal cell culture methods, Methods in cell biology. 2rd eds, Academic press, San Diego.

Noguchi P (2003).  Risks and benefits of gene therapy.  N  Engl J Med, 348:193-194.

Sambrook, J., Russell, D.W. (2001). Molecular Cloning: a Laboratory Manual, 3rd edn. Cold Spring Harbor Laboratory Press, New York.

Tamarin Robert H (2002). Principles of Genetics. Seventh edition. Tata McGraw-Hill Publishing Co Ltd, Delhi.     

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