Biology · MU Pleven

14.Protein synthesis (translation) – mechanism and control in pro- and eukaryotes.

Walks through translation from ribosome structure and mRNA types through initiation, elongation and termination in bacteria and eukaryotes. It contrasts the prokaryotic Shine-Dalgarno start with the eukaryotic cap-dependent scanning and Kozak context, and names the elongation factors and release factors for each. Pictures of the ribosome and initiation steps accompany the text.

Inside: 12 sections · 4 labelled pictures · 20 flashcards · 5 min

Topic 14 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 12 sections, about 774 words (5 min of reading), with 20 flashcards.

In this note

  1. Protein synthesis (translation) – mechanism and control in pro- and eukaryotes.
  2. Translation
  3. Ribosome
  4. mRNA Types
  5. Initiation of Translation
  6. Prokaryotic Translation
  7. Eukaryotic Translation
  8. Initiation
  9. Elongation
  10. Translation Elongation (Same in Prokaryotes & Eukaryotes)
  11. Elongation Factors
  12. Translation Termination

Translation

Translation occurs in ribosomes, where an mRNA strand is decoded into a sequence of amino acids linked by peptide bonds to form a polypeptide or protein.

Protein synthesis has three phases:

  • Initiation
  • Elongation
  • Termination

Ribosome

Composed of ribosomal proteins + rRNA

Has large + small subunits

Contains three tRNA sites:

  • A site – aminoacyl-tRNA
  • P site – peptidyl-tRNA
  • E site – exit site

Eukaryotes: 80S (60S + 40S)

Prokaryotes: 70S (50S + 30S)

mRNA Types

Prokaryotic mRNA: Polycistronic → encodes multiple polypeptides

Eukaryotic mRNA: Monocistronic → encodes one polypeptide

Initiation of Translation

General

Ribosomal subunits + mRNA assemble.

Translation usually starts at the first AUG encountered by the small subunit.

Prokaryotic Translation

Initiation:

  • In E. coli, initiation of protein synthesis begins with assembly of the 30S small ribosomal subunit, the mRNA template, the special initiator tRNA (fMet-tRNA), and initiation factors IF-1, IF-2, and IF-3.
  • The initiator tRNA carries formylated methionine (fMet), which has a formyl group attached to the amino nitrogen.
  • fMet-tRNA binds to the start codon (AUG) and is usually removed after translation is completed.
  • When an in-frame AUG appears during elongation, a normal Met-tRNA (non-formylated methionine) is used instead.
  • In E. coli mRNA, a Shine-Dalgarno sequence (AGGAGG) lies upstream of the first AUG.
  • This sequence is complementary to part of the 3′ end of the 16S rRNA.
  • Since bacterial mRNAs lack a 5′ cap, the Shine-Dalgarno sequence positions the ribosome correctly on the start codon.
  • The Shine-Dalgarno sequence binds to the 30S subunit with the help of initiation factors.

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