Messenger RNA encodes a genetic message for protein synthesis which is decided by the DNA, as mRNA is transcribed by the DNA. The genetic message in mRNA is in the form of a sequence of nucleotides (bases). A specific sequence of nucleotides in mRNA specify one amino acid which is called as codon. The base sequence in tRNA that combines with codon is called as anticodon. The sequence of the nucleotides (bases) in mRNA that code 20 amino acids is called as genetic code. There are four types of nucleotides (bases) in mRNA and their arrangement code 20 amino acids.
Initially, singlet codon concept was considered which means one base (mucleotide) codes one amino acid. It gives only 4 genetic codes [4 x 1⇒ 4] which are very less to code 20 amino acids, hence discarded. Lateron, doublet codon concept was considered in which two nucleotides code one amino acid. This gives 16 genetic codes [4 x 4 ⇒ 16] which are not sufficient to code 20 amino acids, hence discarded.
Triplet Genetic Code:
Crick, Brenner et al. first demostrated that codons consist of three bases. Marshall Nirenberg and Heinrich J. Mathaei (1961) where the first to reveal the nature of a codon. According to triplet codon concept, three nucleotides (base) code one amino acid. It gives 64 genetic codes [4 x 4 x 4 = 64] which are more than enough to code 20 amino acids.
Holley, Khorana & Nirenberg studied all the 64 genetic codes, for this work they were awarded Nobel prize of 1968 in medicine/physiology. Nirenberg worked with Leder & determined 54 codons but Leder was not given the Noble prize.
Salient Features of Triplet Genetic Code:
1.The genetic code is collinear i.e. all the nucleotides (genetic codes) in mRNA are arranged in one line (It is called as colinearity).
2. The genetic code is universal i.e. it is common for all organisms and even in viruses also. However, genetic code of mammalian mitochondria is slightly different.
3. The genetic code is non-overlapping i.e. after three nucleotides, the next three nucleotides form a new or next genetic code.
4. The genetic code is non-ambiguous i.e. one genetic code express only one amino acid.
5. The genetic code is commaless i.e. there is no code which denotes comma (,). The successive codons are contiguous and not separated by non coding bases or group of bases.
6. The genetic code always begins with AUG, hence AUG is called as starting codon or Initiating codon. The AUG code stands for metheonine is eukaryotes and for formyl methionine is prokaryotes. The mitochondrial genetic code in human beings use alternate start codons viz. – AUA and AUU and in some prokaryotes the alternate start condons are GUG&UUG .
7. The standard genetic code has three stop codons viz. – UAA (Ochre), UAG (Amber) and UGA (Opal). The protein synthesis stops when any one comes in the sequence. Hence, they are also called as terminating codons. They do not code (express) any amino acid, hence they are also called as non-sense codons. No tRNAs exist for these codons.
8. The genetic code exhibits degeneracy, as it has many codons which have same meaning i.e. different codons specify the same amino acid. Hence, the common genetic code is a degenerate code. Eighteen amino acids out of 20 are expressed by more than one genetic codes.
