04 Molecular Basis of Inheritance - part 09 - Translation - protein synthesis

04 Molecular Basis of Inheritance - part 09 - Translation - protein synthesis


Translation - protein synthesis : 
  • Translation is the mechanism in which codons of mRNA are translated and specific amino acids in a sequence form a polypeptide on ribosomes. 
  • All types of proteins are synthesised by the cell, within itself (i.e. intracellularly). 
  • Process of translation requires amino acids, mRNA, tRNA, ribosomes, ATP, Mg++ ions, enzymes, elongation, translocation and release factors.
  •  Amino acids form raw material for protein synthesis. 
  • About 20 different types of amino acids are known to form proteins. These are available in the cytoplasm. 
  • DNA controls synthesis of proteins having amino acids in specific sequence. This control is possible through transcription of m-RNA. 
  • Genetic code is specific for particular amino acid. 
  • RNAs serve as intermediate molecules between DNA and protein.  
  • Ribosomes serve as site for protein synthesis. 
  • Each ribosome consists of large and small subunits. These subunits occur separately in cytoplasm.
  • Only during protein synthesis, these two subunits get associated together due to Mg++ ions. 
  • A ribosome has one binding site for m-RNA and 3 binding sites for t-RNA. They are - 
  1. P site (peptidy t-RNA site)
  2. A site (aminoacyl ­ t-RNA site) and 
  3. E site (exit site). 
  • Only first t- RNA- amino acid complex, directly enters P site of ribosome. 
  • In Eukaryotes, a groove is present between two subunits of ribosomes. 
  • It protects the Polypeptide chain from the action of cellular enzymes and also protects mRNA from the action of nucleases. 
Mechanism of translation (i.e. synthesis of polypeptide chain) : 

It involves three steps :
  1. Initiation
  2. Elongation
  3. Termination

1. Initiation of Polypeptide chain :  

  • Activation of amino acids is essential before translation initiates for which ATP is essential. 
  • Small subunit of ribosome binds (attaches) to the m-RNA at 5' end. 
  • Initiator codon, AUG is present on m-RNA which initiates the process of protein synthesis (translation). 
  • Initiator t- RNA binds with initiation codon (AUG) by its anticodon (UAC) through hydrogen bonds. 
  • It carries activated amino acid methionine (in Eukaryotes) or formyl methionine (in prokaryotes). 
  • Now the large subunit of ribosome joins with the smaller subunit, that requires Mg++ ions. 
  • Initiator charged t-RNA (with activated amino acid methionine) occupies the P site of ribosome and A- site is vacant. 

2. Elongations of polypeptide chain : 

  • During this process, activated amino acids are added one by one to first amino acid (methionine). 
  • Amino acid is activated by utilising energy form ATP molecule. 
  • This amino acid binds with amino acid binding site of t-RNA- This results in formation of       t-RNA- amino acid complex. 
  • Addition of Amino acid occurs in 3 Step cycle 
a. Condon recognition- 

  • Amino acyl t- RNA molecule enters the ribosome at A-site. 
  • Anticodon binds with the codon by hydrogen bonds. 
b. Amino acid on the first initiator t-RNA at P-site and amino acid on t-RNA at A-site join by peptide bond. 

  • Here enzyme Ribozyme acts as a catalyst. 
  • At this time first tRNA at `P' site is kicked off. 
c. Translocation- 
  • The t- RNA at A-site carrying a dipeptide at A-site moves to the P-site. This process is called translocation. 
  • In translocation, both the subunits of ribosome move along in relation to tRNA and mRNA. Hence,tRNA carrying dipeptide now gets positioned at `P' site of ribosome, making `A' site vacant. 
  • At this site, then next charged tRNA molecule carrying amino acid will be received. 
  • During this process, first uncharged tRNA is discharged from E-site. This process is repeated as amino acids are added to Polypeptide. 
  • It takes less than 0.1 second for formation of peptide bond. 
  • Third charged t-RNA with its amino acid, arrives at A-site of ribosome. 
  • Anticodon and codon bind by hydrogen bond. Polypeptide bond is formed. 
  • Second t-RNA is discharged from P-site to E-site and leaves the ribosome. 
  • So the events like arrival of t-RNA- amino acid complex, formation of peptide bond, ribosomal translocation and removal of previous tRNA, are repeated. 
  • As ribosome move over the mRNA, all the codons on mRNA are exposed one by one for translation.

3. Termination and release of polypeptide: 

  • At the end of m-RNA, there is a stop codon (UAA/ UAG/ UGA). It is exposed at the A-site. 
  • It is not read and joined by anticodon of any t-RNA. The release factor binds to the stop codon, thereby terminating the translation process. 
  • The Polypeptide is now released in the cytoplasm. Two subunits of Ribosome dissociate and last tRNA is set free in the cytoplasm. 
  • m-RNA also has some additional sequences that are not translated and are referred as untranslated regions (UTR). 
  • The UTRs are present at both 5'-end (before start codon) and at 3'-end (after stop codon). 
  • They are required for efficient translation process. 
  • Finally mRNA is also released in the cytoplasm. 
  • It gets denatured by nucleases immidiately. Hence mRNA is short -lived.

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