Synthesizing a protein with the help of the information in RNA is similar to translating a language to another. A four letter language is translated to 20 letter language during protein synthesis. These should be a specific relationship among the four bases of DNA and sequence of 20 amino acids in the protein.
Genetic Code TableThe complete set of relationships among amino acids and codons is said to be a genetic code which is often summarized in a table.
As you can see, that many amino acids are shown in the table by more than one codon. For example, there are six ways to write leucine in mRNA language.
A key point of the genetic code is its universal nature. This indicates that virtually all species with minor exceptions use the genetic code for protein synthesis.
A genetic code is defined as the nucleotide sequence of the base on DNA which is translated into a sequence of amino acids of the protein to be synthesized.
Properties of Genetic Code
- Triplet code
- Non-ambiguous code
- Degenerate code
- Nonoverlapping code
- Comma less
- Chain termination code
Triplet code – A codon or a code word is defined as a group of bases that specify an amino acid. The codons are developed by using the bases available in mRNA. There is a strong evidence which proves that a sequence of three nucleotides codes for an amino acid in the protein, i.w, the code is a triplet.
The four bases of nucleotide i.e, (A, G, C, and U) are used to produce three base codons. The 64 codons involve sense codons( that specify amino acids). Hence there are 64 codons for 20 amino acids since every codon for one amino acid means that there exist more than code for the same amino acid.
Commaless code – No room for punctuation in between which indicates that every codon is adjacent to the previous one without any nucleotides between them.
Nonoverlapping code – The code is read sequentially in a group of three and a nucleotide which becomes a part of triplet never becomes part of the next triplet.
Each triplet is read from 5’ → 3’ direction and the beginning base is 5’ followed by the base in the middle then the last base which is 3’.
5’-UCU-3’ codes for Serine
5’-AUG-3’ codes for methionine
Degenerate code – Every amino acid except tryptophan (UGG) and methionine (AUG) are coded by various codons, i.e, a few codons are synonyms and this aspect is known as the degeneracy of genetic code. For instance, UGA codes for tryptophan in yeast mitochondria.
Exceptions to the Code
The genetic code is universal since similar codons are assigned to identical amino acids along with similar START and STOP signals in majority of genes in microorganisms and plants. However, a few exceptions have been discovered and most of these include assigning one or two of the STOP codons to an amino acid.