Monday, July 27, 2020

Molecular Basis of Inheritance - Notes | Class 12 | Part 7: Translation (Protein Synthesis)

6. MOLECULAR BASIS OF INHERITANCE

TRANSLATION (PROTEIN SYNTHESIS)

-  It is the process of polymerisation of amino acids to form a polypeptide based on the sequence of codons in mRNA. 

-    It takes place in ribosomes. Ribosome consists of structural RNAs and about 80 types of proteins.

-    Ribosome also acts as a catalyst (23S rRNA in bacteria is the enzyme- ribozyme) for the formation of peptide bond (peptidyl transferase enzyme in large subunit of ribosome).

-    Translation includes 4 steps:

  1. Charging of tRNA
  2. Initiation
  3. Elongation
  4. Termination

1. Charging (aminoacylation) of tRNA


·   Formation of peptide bond needs energy obtained from ATP.

·   For this, amino acids are activated (amino acid + ATP) and linked to their cognate tRNA in presence of aminoacyl tRNA synthetase. Thus, the tRNA becomes charged.


2. Initiation


·   In this, small subunit of ribosome binds to mRNA at the start codon (AUG).

·   Now large subunit binds to small subunit to form initiation complex.

·   Large subunit consists of aminoacyl tRNA binding site (A site) and peptidyl site (P site).

·   The initiator tRNA (which carries methionine) binds on P site. Its anticodon (UAC) recognises start codon AUG.



3. Elongation


·   Second aminoacyl tRNA binds to the A site of ribosome. Its anticodon binds to the second codon on the mRNA and a peptide bond is formed between first and second amino acids in presence of peptidyl transferase.

·    First amino acid and its tRNA are broken. This tRNA is removed from P site and second tRNA from A site is pulled to P site along with mRNA. This is called translocation.

·   These processes are repeated for other codons in mRNA.

·   During translation, ribosome moves from codon to codon.


4. Termination


·   When a release factor binds to stop codon, the translation terminates.

·    The polypeptide and tRNA are released from the ribosomes.

·   The ribosome dissociates into large and small subunits.

A group of ribosomes associated with a single mRNA for translation is called a polyribosome (polysomes).

An mRNA has additional sequences that are not translated (untranslated regions or UTR). UTRs are present at both 5’-end (before start codon) and 3’-end (after stop codon). They are required for efficient translation process.

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