The genetic code is the sequence of nucleotides, in ribonucleic acid and deoxyribonucleic acid that determines the amino acid sequence of proteins. It is discussed using codons found in mRNA (the messenger RNA) that carries information from the DNA to the site of protein synthesis. The genetic code is largely invariant throughout the extent of all species, therefore, it is referred to as the canonical or universal genetic code. There are deviations in both cell organelle and nuclear genomes and they are known as non-canonical or deviant codes. These canonical codes are studies to find the origin and the evolution of the genetic code and the connection between certain mitochondrial diseases with mitochondrial code deviations and translational errors.
The properties or the characteristics of the genetic code are stated below:
The genetic code is the set of rules which dictates the linear sequence of nucleotides in the linear sequence of a polypeptide.
They specify how a nucleotide sequence of an mRNA is translated into the amino acid sequence of a polypeptide
Thus it explains the relationship between nucleotide sequences is of the mRNA and the amino acid sequence of the polypeptide.
The genetic code consists of 64 different codons and each code for 1 of the 20 amino acids.
Codons can be defined as a group of 3 nucleotides which is read by a cell to decode an mRNA
Most codons specify an amino acid
The start codon AUG marks the beginning of a protein
The stop codon marks the end of a protein
The codons are read during translation, beginning at the start codon till the stop codon. These mRNA codons are read from 5’ to 3’ and they specify the order of amino acids in proteins from the N-terminus to the C-terminus.
Triplet nature: The triplet nature of the genetic code explains that singlet and doublet codes are not adequate to code for 20 different amino acids.
Degeneracy of genetic code:
The genetic code is degenerate, this means that same amino acid is coded by more than one base triplet.
It does not imply a lack of specificity in protein synthesis, it just describes that one amino acid can be directed to its place in the amino acid sequence by more than one base triplets.
The amino acids, arginine, alanine and leucine have 6 same codons.
There are two types of degeneracy observed in the genetic code: partial and complete.
In partial degeneracy, the first 2 nucleotides are identical by the 3rd nucleotide differs. Example: CUU and the CUC codon for leucine.
Complete degeneracy is observed when any of the bases can take the 3rd position but still code for the same amino acid. Example: UCU, UCC, UCG and UCA all code for serine.
The non-overlapping nature of genetic code: The genetic code is non-overlapping, which means, two adjacent codons do not overlap each other
A non-overlapping code refers to the same letter not being used for two different codons. In other terms, a single base cannot take part in the formation of more than one codon.
The comma less feature of genetic code: The entire code is comma less and there is no signal to indicate the beginning of the end of a codon.
Also, there are no intermediary nucleotides between the codons.
Non-ambiguity: The genetic code is non-ambiguous which means a particular codon will always code for the same amino acid.
The same amino acid can be coded by more than one codon but the same codon cannot code for two or more different amino acids.
The universality of the genetic code: This means the same sequences of 3 bases encode the amino acids in all life forms from simple to complex organisms.
The entire code is based on a study conducted on E. coli, however, it is valid for organisms.
Only minor exceptions are yeast, mitochondria and the Mycoplasma.
Polarity: The genetic code has polarity and the code is always red in a fixed direction. It is read from 5’ to 3’
If the code is read in the opposite direction (i.e., 3′ → 5′), it would specify 2 different proteins, since the codon would have reversed base sequence.
The genetic code table shows the entire set of codons which code for particular amino acids.
[Image will be updated soon]
1. What is meant by genetic code?
The genetic code can be explained as a collection of rules used by all living cells in all organisms to translate information encoded within the genetic material into proteins. Translation or protein synthesis is accomplished by the ribosome, which links amino acids in an order which is specified by the mRNA, using tRNA molecules to carry amino acids and to read the mRNA codons at a time. The genetic code definition shows how codons or sequences of three nucleotides specify which amino acid will be added next during translation.
2. What are the important features of genetic code?
Some of the important features of the genetic code are:
The genetic code is comma less, there is no signal which indicates the beginning or the end of a codon.
The genetic code is non-overlapping, a single base cannot take part in the formation of more than one codon.
The code is degenerate, any amino acid can be coded by more than one triplet codon.
It is read from 5’ to 3’
What are the following codons?
Chain initiation codons
Chain termination codons
Chain initiation codons: AUG and GUG codons are translation chain initiation codons in E.coli. They code for valine and methionine and occur immediately after the terminator codons.
Chain termination codons: UAA, UAG, and UGA are termination codon as they do not code for any amino acid. They are also called stop codons
Sense codons: 61 codons of the genetic code table are known as the sense codons. All of them code for particular amino acids.
Non-sense codons: As UAA, UAG, and UGA do not code for any amino acid, they are also known as non-sense codons.