TRANSLATION - EASY NOTES

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-It is a process of protein synthesis

-Information carried by m-RNA in the form of genetic code is translated by ribosomes to amino acids and protein is formed.
-Occurs in the cytoplasm
-Translation of m-RNA takes place from 5' end to 3' end

Requirements for Translation

  1. m-RNA
  2. t-RNA
  3. Ribosomes
  4.  Energy in the form of ATP & GTP
  5. Specific protein factors initiation factor, elongation factor
Steps:

Step I. ACTIVATION OF AMINO ACID


  • activation is the process of attachment of amino acid to t-RNA.
  • it takes place in the cytoplasm
  • the CCA 3' end of t-RNA carries a codon that is specific for each of 20 amino acids.
  • the 'D' arm of t-RNA has a site for recognition enzymes
  • as the t-RNA recognizes the amino acid, it gets attached to the amino acid with the help of enzyme aminoacyl t-RNA synthase.
  • in this process, 2 energy bonds are utilized.

The activation is a two step process - 

1.     formation of AMP amino acid enzyme complex

2.     attachment of amino acid to t-RNA.

Step II. INITIATION

Initiation is divided into four steps:

1. Ribosomal Dissociation

  • In eukaryotes: The ribosomes are 80S particles made up of 40S and 60S 
  • The ribosome has two binding sites for t-RNA - 
    • A(aminoacyl site) &
    • P(peptidyl) site 
  • The 80S subunit of the ribosome is dissociated into the 40S and 60S units and the 40s subunit binds to two initiation factors eIF3 and eIF-1A (eIF - eukaryotic initiation factor)

2. Formation Of 43S Pre-Initiation Complex

  • The first step is binding of GTP to initiation factor eIF2
  • This complex of GTP + eIF2 binds to Met - t-RNA forming a ternary complex (GTP+eIF2+Met t-RNA)
  • (the initiating codon 'AUG' in eukaryotes codes for methionine and the t-RNA is Met tRNA. in prokaryotes codes for N-formyl methionine and the tRNA is fmet-t-RNA.)
  • This ternary complex (GTP+eIF2+Met t-RNA) binds to the 40s subunit of ribosome stabilized by eIF3 and eIF-1A to form 43s pre-initiation complex. 

3. Formation of 48S Pre-Initiation Complex

  • mRNA binds to 43s pre-initiation complex to form a 48S preinitiation complex.
  • The 5'end is capped by methylguanosyl triphosphate which helps in binding of m-RNA to 43s pre-initiation complex to form a 48S preinitiation complex.
  • m-RNA binds to 43s pre-initiation complex with the help of cap-binding protein (CBP) or eIF4 and ATP.
  • The start codon AUG on mRNA is recognized with the help of marker sequence on m-RNA that is present at 5' end.
  • The marker sequence in 
    • prokaryotes is Shine Dalgarno sequence &
    • in eukaryotes, it is Kozak sequence. 

4. Foration of 80S InitiationComplex

48S preinitiation complex combines with the 60S unit of the ribosome to form 80s initiation complex.



This step involves

a) hydrolysis f GTP bound to eIF2 by eIF5

b) release of initiation factors eIF1A, eIF3

  • Now, the Met t-RNA is at 'P' site of ribosome pairing with the start codon AUG on the mRNA.

III.ELONGATION:

Elongation is divided into three steps:

1. Binding of amino acyl t-RNA to A site

  • New t-RNA with new amino acid gets attached to 'A' site.
  • The amino acid is decided by the codon present on mRNA.
  • The attachment of new t-RNA to a site requires eEF 1 and GTP
  • This complex of aminoacyl t-RNA +eEF1+GTP enters 'A' site on the ribosome and releases eEF1,GDP and phosphate.

2. Peptide Bond Formation

  • This step involves peptide bond formation between the amino acids present at 'A' site and 'P' site.
  • The amino group of an amino acid at 'A' sit forms peptide bond with the carboxyl group of an amino acid at 'P' site.
  • The reaction is catalysed by Peptidyl transferase.
  • After the peptide bond formation, the amino acid i.e. initiating methionine is transferred to A site, resulting in the t-RNA at 'A' site having two amino acids (dipeptidyl t-RNA) and the Met t-RNA at 'P' site empty which goes back into the cytoplasm for a new amino acid.

3.Translocation:


  • This involves shifting the whole ribosome to the next codon resulting in the movement of dipeptidyl t-RNA to initially present on 'A' site to 'P' site and setting the t-RNA at 'P' site free.
  • Thus a shift of ribosome along the mRNA is called translocation.
  • because of shifting the third codon of m-RNA is on 'A' site and the second codon is on 'P' site.
  • This process occurs with the help of eEF2 and GTP.

This whole process of elongation is repeated until the termination codon is reached.

IV.TERMINATION:



  • the elongation steps are repeated until any of the termination codons (UAA, UAG, UGA) appear on the 'A' site of mRNA.
  • since none of the t-RNA has the corresponding anticodon sequence the 'A' site remains free
  • the releasing factor eRF recognizes the termination signal on 'A' site.

in prokaryotes - releasing factors are RF1, RF2, RF3

in eukaryotes - releasing factor is eRF.

  • releasing factors along with both GTP hydrolyses peptide bond of t-RNA and releases mRNA and dissociates the 80s of ribosome into 40s and 60s.
  • the dissociated units of ribosomes are recycled.

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