The triplet nucleotide base sequences in mRNA or DNA that act as code words for synthesis of amino acids in proteins, constitute the genetic code or simply codons.

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The genetic code may be regarded as a dictionary of nucleotide bases (A,G,C and U) that determines the sequence of amino acids in proteins.

The translation of such information in the mRNA or DNA into proteins is done by the ribosomes, which link amino acids in the polypeptide chain in an order specified by the mRNA. The mRNA uses tRNA to carry amino acids and to read the three nucleotides bases(codon). The genetic code simply specify which amino acid will be added next during the protein synthesis.

Fig. Molecular Cell Biology (8th Edition)

The codons are composed of four nucleotide bases namely the Purines- Adenine(A) and Guanine(G) & the Pyrimidines- Cytosine(C) and Uracil(U). These four bases produce 64 different combinations of three base codons. The triplet nucleotide code in the nucleic acid sequence code for a single amino acid. However, there are exceptions.

The codons are composed of four nucleotide bases namely the Purines- Adenine(A) and Guanine(G) & the Pyrimidines- Cytosine(C) and Uracil(U). These four bases produce 64 different combinations of three base codons. The triplet nucleotide code in the nucleic acid sequence code for a single amino acid. However, there are exceptions.

In DNA, the codons are similar to that of RNA except for Thymidine (T) is found in place of Uridine(U).

(Uridine is a glycosylated pyrimidine-analog containing uracil attached to a ribose ring (or more specifically, a ribofuranose) via a β-N1-glycosidic bond.)

Exceptions:  In human mitochondria, UGA is read as tryptophan rather than stop, AUA is read as methionine rather than isoleucine, and AGA and AGG are read as stop rather than arginine.

The three codons UAA, UAG, and UGA do not code for amino acids. They act as stop signals in protein synthesis. These three codons are collectively called termination codons or nonsense codons. The codons AUG and sometimes GUG are chain-initiating codons.

The nucleotide sequence of codon on mRNA is written from 5′ end to 3′ end.  Sixty-one codons code for 20 amino acids found in proteins.

Properties of a Genetic Code

1. Universal

The same codons are used to code for the same amino acids in all living organisms. Thus, genetic code has been preserved during the course of evolution and is universal. However, there are a few exceptions like AUA code for methionine in Mitochondria and Isoleucine in Cytoplasm.

2. Specificity

A particular codon always codes for same amino acid which makes the genetic code highly specific. For eg. UGG codes for Tryptophan, GGC codes for Glycine (as given in the table above).

3. Non-Overlapping

The genetic code is read from a fixed point as a continuous base sequence. It is non-overlapping, commaless, and without punctuation. Eg. UAAGUGUGA

 is read as UAA

/GUG

/UGA

. Deletion of one or two bases causes mutations and the protein synthesized will be completely different (Frameshift Mutation).

4.Degenerate

Most amino acids are coded by more than one codon, which is the degeneracy of the code. Thus, the codon is degenerate or redundant, since there are 61 codons for only 20 amino acids. Eg. Glycine is coded by four codons. They are also called synonyms.

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