Protein Translation
Molecular Components
Process Description
Initaition
Small ribosomal subunit binds to mRNA and initiator t-RNA (met).
Attachment of Large Ribosomal subunit completes the translation initiation complex.
Proteins called initiation factors bring all components together.
One molecule of GTP is expended by cell to bring all components together.
mRNA
tRNA, GTP
Ribosomes
initiator tRNA-met
initiation factors
Elongation
rRNA
Elongation Factors
GTP x 2
Termination
Release Factor
GTP x 2
Chapereronin
Post Process
Modification/
Other Events
SRP- Signal
Recognition Protein
Aminoacyl-tRNA
Synthetase, ATP
tRNA, animo acids
Prokaryotes vs.
Eukaryotes
Anticodon of tRNA basepairs with complementary mRNA at codon in A site of the Large Ribosome, with
the hydrolysis of one GTP.
rRNA molecule of Large Ribosomal subunit catalyzes formation of a peptide bond.
Ribosome translocates tRNA in A site to P site, and uncharged tRNA from P site to the E site, which gets
released. One molecule of GTP is expended.
Elongation Factors are required, and it is the mRNA that moves.
Stop Codon in mRNA reaches A site in Ribosome.
Release factor binds to stop in A site with addition of a water molecule to break the bond between
completed polypeptide and tRNA in P site and releasing protein through exit tunnel of Large Ribosomal
subunit.
Translation machinery breaks down with the expenditure of two GTP molecules.
The polypeptide chain begins to coil and fold spontaneously as a consequence of its amino acid sequence
forming a protein with a specific shape: a three-dimensional molecule with secondary and tertiary
structure. Certain amino acids may be chemically modified by the attachment of sugars, lipids, phosphate
groups, etc. Enzymes may remove one or more amino acids from the leading (amino) end of the
polypeptide chain. In some cases, a polypeptide chain may be enzymatically cleaved into two or more
pieces.
SRP recognizes signal peptide (20 aa) at amino end of protein while synthesis is happening and transports
protein to ER or Golgi if protein is destined for an endomembrane system or secretion out of cell.
The translocation complex in the membrane cleaves the signal peptide and holds the protein and signal.
Protein is released to the ER lumen or stays partially embedded in the ER membrane.
tRNA is used repeatedly
Aminoacyl-tRNA Synthetase binds amino acid and ATP molecule, appropriate tRNA covalently binds to
amino acid displacing AMP (3’end tRNA)
In Prokaryotes, the small ribosomal subunit can bind mRNA and tRNA in either order; it binds the mRNA at a specific RNA
sequence, just upstream of the start codon, AUG.
In Eukaryotes, the small ribosomal subunit, with the initiator tRNA already bound, binds to the 5' cap of the mRNA and then
moves, or scans, downstream along the mRNA until it reaches the start codon, and the initiator tRNA hydrogen bonds to itestablishes codon reading frame of mRNA. Large Ribosomal subunit binds which completes the translation initiation complex.
Bacteria can simultaneously transcribe and translate the same gene because it has no compartmental organization.
Eukaryotes segregate transcription from translation and provides a compartment for extensive RNA processing. This
processing stage includes additional steps whose regulation can help coordinate the eukaryotic cell's elaborate activities.
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