PROTEIN SYNTHESIS

Translation

Conversion of mRNA to protein

Codon

Group of three mRNA nucleotides that encode one amino acid

Ribosomal Structure

• 2 subunits each made of protein and RNA:

1. Small ribosomal subunit – has mRNA binding site
2. Large ribosomal subunit

• E-site (Exit site)
• P-site (Peptidyl-tRNA binding site)
• A-site (Aminoacyl-tRNA binding site)

PROTEIN SYNTHESIS STEPS

Initiation

• Initiator t-RNA (methionine) binds to small ribosomal subunit (with help from initiation factors)
• Small ribosomal subunit/initiator t-RNA complex binds mRNA (using 5′ cap to bind the 5′ end)
• Complex scans towards the 3′ end until it locates the start codon (AUG which sets the appropriate reading frame) and the large ribosomal subunit arrives

Elongation

• Current tRNA in the P-site
• New tRNA added to A-site
• Growing peptide chain added to amino acid bound to A-site tRNA
• Large ribosomal subunit shifts 3′ one codon
• Small ribosomal subunit follows – tRNA that was in P-site now in E-site and tRNA that was in A-site now in P-site
• E-site tRNA ejected and new A-site tRNA added

Termination

• When codon in A-site is stop codon (UAG, UAA, or UGA) a release factor is added to the A-site
• Release factor binding releases the peptide chain and causes ribosome to dissociate