Sunday, September 27, 2009

Translation in Protein synthesis


The Mechanism of Protein synthesis: Translation is the final stage of gene expression during which mRNA molecules are translated into Polypeptides. The process is complex and occurs only in association with ribosomes. Three major types of RNA molecules produced during transcription, messenger, ribosomal and transfer RNA work together to synthesize proteins by the process of translation. However the main fact about translation is that sequence of amino acids in the polypeptides being synthesized in specified by the sequence of nucleotides in mRNA molecule being translated. The genetic code determines the sequence of nucleotides in mRNA molecule that specified the sequence of amino acids in a polypeptide. The process of translation can be subdivided into the following four phases.

(1) t RNA charging (2) Chain initiation

(3) Chain Elongation (4) Chain termination

(1) Charging t RNA:

Role of t RNA: Each cell contains a number of different kinds of t RNAs. Each t RNA is distinguished in functional terms by its specificity for one of 20 amino acids involved in Protein synthesis for example t RNA tyr is specific tyrosine and t RNA 2Ly for glycerine.

A t RNA forms a covalent linkage with its amino acid and can recognize and attach to a Codon specifying that amino acid. There may be more than one type of t RNA molecule for single amino acid as most amino acids are Coded by more than one Codon.


Before translation starts t RNA molecules must be chemically linked to their respective amino acids. This is called charging or amino acylation. The charging is controlled by a group enzymes called amino acyl synthetases. There is specific amino acyl synthetase enzyme for each amino acid.

(2) Initiation of Polypeptide chain:

Initiation of translation involves ribosomal subunits, mRNA molecule, a specific initiater t RNA, GTP, Mg++ and at least three non ribosomal protein initiation factors which enhance binding between various components. Ribosomes do not exist free in the cytoplasm, rather they are dissociated into their large and small subunits. Small ribosomal subunit binds to mRNA molecule. The attachment takes place at specific point just upstream of the initiation Condon of the gene. The translation process itself starts when a charge t RNA base pair with small subunit. The resultant structure comprising of mRNA, small ribosomal subunit and charged t RNA is called initiation complex.

(3) Elongation of Polypeptide chain:

Once the initiation Complex has been formed, the large subunit of the ribosome attaches to the complex. The energy is provided by hydrolysis of GTP molecule. Large subunit contains two binding sites for charge t RNA molecules, P-site or Peptidyl and A site or amino acyl sites. The initiation t RNA binds to P-site provided AUG triplet is in corresponding position of the small subunit. The sequence of second triplet (anticodon) in m RNA dictates which charge t RNA molecule will become attached to A-site. If correct t RNA molecule is present, Peptidyl transferase catalyze the formation of Peptide bond which links two amino acid together. This enzyme is present in large ribosomal subunit. At the same time the covalent bond between amino acid and t RNA occupying P-site is broken. The product of this reaction is dipeptide which is attached to t RNA at A-site. This phase during which growing polypeptide chain increase in length by one amino acid is called elongation.

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