- DNA contains a code which dictates the sequence in which amino acids are to be linked together to make a protein.
- The sequence of bases in a gene is a code for the sequence of amino acids in a protein.
- This relationship of nucleotide bases of a gene and the amino acids is known as the genetic code.
- The genetic code is a set of rules defining how the four-letter code of DNA is translated into the 20-letter code of amino acids, which are the building blocks of proteins.
- It can also be defined as a set of three-letter combinations of nucleotides called codons, each of which corresponds to a specific amino acid or stop signal.
- The smallest unit of 3 nitrogenous bases that codes for one amino acid of polypeptide chain is called a codon or code word.
- The code in a DNA molecule is carried in the sequence of the four bases, adenine (A), guanine (G), thymine (T), and cytosine (C).
- This base sequence is always ‘read’ in the same direction.
- There are 20 amino acids available in the cells which are coded by AUGC letter.
- The genetic information coded in these letters are passed into mRNA and then into proteins.
- The concept of codons was first described by Francis Crick and his colleagues in
- Marshall Nirenberg and Heinrich J. Matthaei were the first to reveal the nature of a codon in 1961.
- Experimental evidence shows that the code is a triplet one and 61 of the 64 codons code for individual amino acids during protein synthesis.
- The remaining three codons are called terminating codons or stop signals.
- These codons (UAA UAG UGA) are used by the cell to signal the ending point of protein synthesis.
Properties of genetic code
Some of the properties of genetic code have been studied and proved experimentally. They are as follows:
1.The genetic code is triplet
- A genetic code consists of three nitrogenous bases.
- There are 64 possible different codons which code for 20 different amino acids.
- Examples: UAA UGC CCU, etc.
2.The genetic code is commaless
- There is no punctuation (comma) between the adjacent codons.
- It means that if one aminoacid is coded the another amino acid will be automatically coded by the next three letters.
- Examples: UUU CCC ACG GGA, etc.
3.The genetic code is universal
- The same genetic code is used to code the same aminoacid.in all organisms including virus.
- Example: If CAU and CAC codes for histidine in one organism the same triplet is used in another organism to code histidine.
4.The genetic code is non-overlapping
- The same letter is not used for two different codons.
5.The genetic code is degenerate
- It means that more than one code may be used for one amino acid.
- The multiple system of coding is called degenerate system or degenerate code.
6.The genetic code is non-ambiguous
- It means that the particular codon will always code for the same amino acid.
- However, there is some exception like GGA codes for two amino acids glycine and glutamic acid.
- AUG codon is called starting codon as it initiates polypeptide chain formation.
8.Non sense codon
- Certain codons like UAA, UAG and UGA do not code for any amino acid and give an indication for the termination of the polypeptide chain.
- Therefore, they are called termination or non-sense codons.
9.The genetic code has polarity
- The code always read in a fixed direction, i.e. in the 5’→ 3’ direction.