BIO 104
Instructor: Avis Gibons
Stages of Translation (Polypeptide Synthesis)
Initiation
1) Proteins called initiation factors bind to an aminoacyl-tRNA carrying methionine. This
initiation complex facilitates the binding of mRNA to the small ribosomal subunit. When
the subunit and the complex recognize the mRNA start codon (AUG), the aminoacyl-
tRNA anticodon pairs with that codon.
2) The aminoacyl-tRNA binds to the “P” (for peptide) site of the large ribosomal subunit,
uniting all the components needed for translation.
Polypeptide Elongation
1) Proteins called elongation factors facilitate binding of the next aminoacyl-tRNA to the
“A” (for amino acid) site of the ribosome. Only an aminoacyl-tRNA carrying the
anticodon for the next mRNA codon, and the amino acid specified by that codon, can bind
there.
2) A ribozyme catalyzes formation of a bond between the amino acid at the “P” site and the
amino acid at the “A” site. This results in a chain of amino acids attached to the tRNA at
the “A” site, and leaves the tRNA at the “P” site “unloaded.”
3) The ribosome moves three nucleotides along the mRNA in the 5’ to 3’ direction, to the
next codon. This releases the unloaded tRNA from the “P” site of the ribosome and
translocates the tRNA from the “A” site to the “P” site. Accordingly, this movement is
referred to as translocation. The “A” site is now available for another aminoacyl-tRNA.
4) Elongation repeats as specified by the mRNA instructions.
Termination
1) Elongation ends when the ribosome reaches a stop codon.
2) An enzyme called a release factor binds there.
3) This catalyzes cleavage of the bond between the last tRNA and the polypeptide, releasing
them from the ribosome.
4) The ribosome dissociates into its subunits and the mRNA molecule is released.
5) All components are free to begin the process again.