The translation is the process of the formation of protein from the information contained in mRNA with the help of ribosomes. Translation refers to the process of polymerization of amino acids to form a polypeptide. The order and sequence of amino acids are defined by the sequence of bases in the mRNA. The amino acids are joined by a bond which is known as a peptide bond. It involves the transport of amino acids from the intercellular pool to the ribosomes where they are assembled into proteins. The transfer of amino acids to the ribosome surface is accomplished by tRNA.
Requirements of Protein synthesis
The different molecules required for protein synthesis are:
DNA determines the type of protein to be synthesized. The protein synthesis is initiated, guided and regulated by DNA molecules.
2. Messenger RNA (mRNA)
It is a single stranded molecule that carries information for protein synthesis from DNA to cytoplasm. The information stored in the form of base sequence of mRNA is complementary to the base sequence present on template DNA.
3. Transfer RNA (tRNA)
It picks up and transports the activated amino acids from the amino acid pool to the ribosome where it recognizes a specific triplet codon of mRNA. Each amino acid is carried by a specific tRNA as the lower most segment of tRNA has three base sequences anti codon loop which are complementary to the triplet codon of mRNA.
These are the sites of protein synthesis and are found in cytoplasm. They contain a number of enzymes responsible for the formation of polypeptide chain. Each ribosome has two sub units- a larger sub unit and a smaller sub unit.
Larger sub unit has three sites:
i) A-site (amino acyl), which is the first binding site for tRNA in the ribosome.
ii)P-site (peptidyl) is the second binding site for tRNA in the ribosome.
iii) E-site (exit) is the third binding site for tRNA in the ribosome.
5. Amino acids
These are the building blocks of a polypeptide chain or protein. There are 20 types of amino acids which occur in cytoplasm forming an amino acid pool. These amino acids are assembled in polypeptide chain to form a protein.
A number of enzymes responsible for the formation of polypeptide chain are amino acyl tRNA synthetase.
Steps of Protein Synthesis
The steps involved in protein synthesis (Translation) are:
1. Activation of amino acids
The reaction is brought about by the binding of amino acid with ATP. This step is mediated by specific activating enzyme called amino acyl RNA synthetase. As a result of this reaction between amino acid (AA) and adenosine triphosphate (ATP), mediated by specific enzymes, amino acyl-AMP-enzyme complex is formed.
AA + ATP + Amino acyl synthetase ------------------------> Amino acyl AMP-enzyme complex + Pyrophosphate (PP)
It should be noticed that amino acyl RNA synthetases are specific with various amino acids. Therefore, for different amino acids, different enzymes would be required.
2. Transfer of amino acid to tRNA
Amino acyl-AMP-enzyme complex formed during above step reacts with particular tRNA. As a result the AMP and enzyme are liberated.
Amino acyl-AMP-enzymes complex + tRNA---------------------> Amino acyl-tRNA + AMP + Enzymes
3. Initiation of polypeptide chain
Aminoacyl tRNA with activated AA moves towards the mRNA in the smaller unit of ribosome and attaches to the first triplet AUG of the mRNA to form initiation complex with the help of GTP. Immediately after this, the larger sub unit comes and attaches with the smaller sub unit forming 70s/80s ribosome.
Fig: Initiation of polypeptide chain
4. Elongation of polypeptide chain
After formation of 70s/80s ribosomes the elongation of polypeptide chain begins with the regular adding of amino acid and relative movement of ribosomes and mRNA in presence of GTP at A site of ribosome. There is formation of peptide bond (between -COOH of AA at P site and -NH2 of AA of A site) by peptidyl transferase. Then there is the translocation of peptidyl tRNA from A site to P site and finally to E site by translocase.
5. Termination of polypeptide chain
The chain elongation continues until a terminal codon (UAA, UAG or UGA) on the mRNA is reached. There is no tRNA with the anticodons complementary to these termination codons. So, when these codons expose to A site, no tRNA is attached and thus protein synthesis stops. Finally, the tRNA release from E site, protein releases from tRNA, two subunits of ribosome dissociates and the mRNA disintegrates.
Fig: Termination of polypeptide chain
Thus, protein synthesis is completed.