Protein Synthesis

Basics of Translation & Protein Synthesis

• DNA is a sequence of bases: acts as a chemical code for the amino acid sequence.

  • Every 3 bases encode 1 amino acid in the protein.

• RNA transcript with a complement of the DNA code goes to a ribosome for translation into a protein.

Initiation

• The small ribosomal subunit binds to mRNA.
  → It “reads” (scans) the mRNA until it comes to a special codon (3-base codon) called the start codon (AUG).

• Each tRNA has a unique anticodon and floats in the cytoplasm.
  → tRNA with a complementary anticodon (UAC) binds to the start codon.

• The large ribosomal subunit binds, forming a complete ribosome.

Elongation

• tRNA carries a different amino acid based on its corresponding codon and binds to the A-site of the ribosome.
  → Ex: The tRNA with the anticodon UGG binds to the mRNA with the codon ACC, producing Threonine.

• On a complete ribosome, the tRNA #2 can bind at the A-site, codon to anticodon, after the tRNA #1 moves to the P-site.

• The 1^st amino acid binds to the 2^nd amino acid on the tRNA #2 via dehydration synthesis to form a new peptide bond.

• One codon is moved over, and the tRNA #1 exits the ribosome from the E-site, and tRNA #3 can now bind to the A-site.

Termination

• When a ribosome comes to one of three stop codons (UAA, UGA, UAG), the two ribosomal subunits will dissociate and fall off the mRNA.
  → A newly synthesized polypeptide is formed.

• Polysome: 1 mRNA, several ribosomes at once.
  → More efficient translation.