Lecture 12 - Translation

• Monocistronic mRNA

  • Initiation in eukaryotes is at the 5' cap, and the first AUG is the start codon
• To translate the RNA transcript we need:

  • A sequence for the ribosome to initiate translation (it doesnt just begin at the 5' end)
  • A genetic code that will specify which amino acid will be incorporated
  • A termination sequence so the ribosome knows when to stop
  • Translation begins at AUG and terminates at UAA or UAG or UGA
  • The AUG sets the code, so the next 3 nucleotides after AUG will code for another amino acid and so on
• Properties of the Genetic Code

  • There are no spaces between codons (codons are adjacent)
  • The genetic code in non-overlapping (each nucleotide is part of one codon)
  • The genetic code is degenerate (most amino acids specified by more than one codon)
  • The genetic code is (nearly) universal (with minor exceptions, each triplet/codon has the same meaning in all organisms)
• What are the major components for translation: mRNA, tRNA, rRNA and ribosomal proteins
• Ribosomes consists of a large and small subunit, including three binding sites (exit site, peptidyl site and aminoacyl site)
• tRNA contains a amino acid attachment site as well as an anticodon
• What are the three steps in translation:

  Initiation, Elongation and Termination
• What happens in inititation:

  The initiator AUG codon is recognized and Met. is established as the first amino acid in the new polypeptide chain.
• What happens in elongation:

  Successive amino acids are added one by one to the growing chain.
• What happens in termination:

  A stop codon is recognized by a releasing factor that mimics a tRNA resulting in the completed polypeptide chain being released from the ribosome.
• Note that the first base in the codon (5' end) pairs with the last base (3' end) in the anticodon
• Initiation in depth

  • A reaction transfers the Met. to the amino acid on the tRNA in the A site, forming a peptide bond
  • The ribosome moves down one codon, which puts the amino acid carrying the polypeptide into the P site and the now-uncharged tRNA into the E site, where it is ejected. A new tRNA complementary to the next codon binds to the A site.
• Elongation in depth:

  • A reaction transfers the Met to the amino acid on the tRNA in the A site, forming a peptide bond
  • The ribosome moves down one codon, which puts the amino acid carrying the polypeptide into the P site and the now uncharged tRNA into the E site, where it is ejected. A new tRNA complementary to the next codon binds to the A site
  • The polypeptide transfers to the amino acid on the tRNA in the A site. The polypeptide is elongated by repeating these steps