Genetic code is the relationship between the sequence of amino acids in a polypeptide and nucleotide sequence of DNA or mRNA .
DNA or RNA carries all the genetic information. It is expressed in the form of proteins. Proteins are made of 20 different types of essential amino acids. The information about the number and sequence of these amino acids forming protein is present in DNA. It is then passed on to mRNA during transcription.
Sugar and phosphate of DNA cannot form the message for amino acids because sugar is only of one type and so also the phosphate. This leaves only nucleotides to form the message for 20 amino acids but these too also are of four types only, which are too few for 20 amino acids.
The discovery that a codon i.e., hereditary unit of a gene containing coded information for one amino acid consists of three nucleotides (triplet). This was helpful in solving the above problem. Thus, for 20 amino acids 64, possible permutations are available. This important discovery was the result of experiments by Marshall W. Nirenberg and J. Heinrich and later by H.G. Khorana.
Deciphering or cracking the triplet genetic code:
Following are the different researches which were helpful in deciphering the triplet genetic code:
- According to Crick et al (1961) deletion or addition of one or two base pairs in DNA of T4 bacteriophage causes disturbance in the normal functioning of DNA. However, on addition or deletion of the 3 base pairs, the disturbance was minimum.
- According to Nirenberg and Mathaei (1961) singlet code (one amino acid specified by one nitrogen base) can specify only 4 acids, a doublet code only 16 while a triplet code can specify up to 64 amino acids. As there are 20 amino acids, a triplet code (three nitrogen bases for one amino acid) can be operative.
- Nirenberg prepared polymers of the four nucleotides-UUUUUU….(polyuridylic acid), CCCCCC…(polycytidylic acid), AAAAAA….(polyadenylic acid) and GGGGGG…(polyguanylic acid). According to his observation, poly-U stimulated the formation of polyphenylalanine, poly C of polypyroline while poly-A was helpful in the formation of polylysine. However, poly-G did not function. Later on, GGG was found to code for amino acid glycine.
- Khorana (1964) synthesized copolymers of nucleotides such as-UGUGUGUG….. According to his observation, these were helpful in stimulating the formation of polypeptides having alternately similar amino acids as cysteine-valine-cysteine. This is possible only if three adjacent nucleotides specify one amino acid (e.g., UGU and other have the second amino acid (e.g., GUG).
- Some amino acids are specified by more than one codon. The code languages of DNA and mRNA are complementary. Thus, the two codons for phenylalanine are UUU and UUC in case of mRNA while they are AAA and AAG for DNA.
Properties of the genetic code:
Three adjacent nitrogen bases constitute a codon which specifies the placement of one amino acid in a polypeptide.
The two initiation codons- AUG or methionine codon and GUG codon or valine codon signals the polypeptide synthesis.
The three termination codons- UAA (ochre), UAA (amber) and UGA (opal) signals the termination of polypeptide chain. They do not specify any amino acid and are hence also termed as nonsense codons.
Universal code :
The genetic code is applicable universally i.e., a codon specifies the same amino acid from a virus to a tree or human being. Thus, mRNA from chick oviduct introduced in Escherichia coli produces ovalbumen in the bacterium exactly similar to one formed in a chick.
Non ambiguous codons:
One codon specifies only one amino acid and not any other.
Amino acids with similar properties have related codons, e.g., aromatic amino acids tryptophan (UGG), phenylalanine (UUC, UUU), tyrosine (UAC, UAU).
The genetic code is continuous and does not possess pauses after the triplets. Deletion or addition of a nucleotide reads the whole genetic code differently. Thus, a polypeptide having 50 amino acid shall be specified by a linear sequence of 150 nucleotides. If a nucleotide is added or deleted in the middle of this sequence, the first 25 amino acids of polypeptide will be same but next 25 amino acids will be quite different.
Non overlapping code :
A nitrogen base is a constituent of only one codon.
Degeneracy of code :
Since there are 64 triplet codons and only 20 amino acids, the incorporation of some amino acids must be influenced by more than one codon. Only tryptophan (UGG) and methionine (AUG) are specified by single codons. All other amino are specified by 2-6 codons. The latter are termed as degenerate codons. In degenerate codons the first two nitrogen bases are similar while the third one is different. As the third nitrogen base has no effect on coding, the same is termed as wobble position.
Both polypeptide and DNA or mRNA have a linear arrangement of their components. Further, the sequence of triplet nucleotide bases in DNA or mRNA corresponds to the sequence of amino acids in polypeptide manufactured under the guidance of the former. Change in codon sequence also produces a similar change in amino acid sequence of polypeptide.
Cistron-Polypeptide Parity :
Portion of DNA termed as cistron specifies the formation of a particular polypeptide. It means that the genetic system should have as many cistrons as the types of polypeptides occurring in the organism.