Question

MCAT Tutorial: Prokaryotic Translation

Answer 1

\({\bf{Introduction:}}\)
- prokaryotic mRNA is polycistronic (multiple proteins per sequence possible), multiple start sites needed
- no promoter
- uses ribosome binding site Shine-Dalgarno sequence at about -10 bp upstream
- proceeds in 5' to 3' direction
- occurs in cytoplasm concurrently w/ transcription

Answer 2

\({\bf{Initiation:}}\)
- 30S subunits binds initiation factors IF1 and IF3, form a complex
- new complex binds to mRNA transcript
- initiator aminoacyl-tRNA (fMet) and IF2 + 1 GTP bind to complex
- 50S subunit binds to the complex (hydrolyzes 1 GTP) --> forms complete 70S complex
- prior to elongation, the initiator tRNA is at the P-site of 70S, hydrogen bonded to start codon

Answer 3

\({\bf{Elongation:}}\)
- next aminoacyl-tRNA H-bonds to the first one (hydrolysis of 1 GTP required by Tu)
- elongation factor (EF-T) removes GDP from Tu
- large subunit catalyzes peptide bond between first and second amino acid
- fMet-tRNA leaves ribosome
- first tRNA moves to E-site, second tRNA moves to P site, next codon in sequence moves to A-site (translocation, requires 1 GTP via elongation factor EF-G)
- EF-Tu removes first tRNA from E-site

Answer 4

\({\bf{Termination:}}\)
- stop codon reaches A-site
- release factor enters A-site
- RF1 and RF2 recognize the presence of a stop codon
- RF3 triggers the dissociation of RF1 and 2
- peptidyl transferase hydrolyzes the last tRNA bond
- ribosome dissociates into its component subunits, releases the peptide and transcript

Answer 5

Source material is Chapter 4.8 of MCAT: Biology Review, 2nd edition by Princeton Review