lecture 6

Question 1

translation machinery

Answer 1

• translation of mRNA into protein achieved by tRNA
• tRNA = nonidentical; each carry specific amino acid one end; anticodon other end
• anticodon pair with complementary codon on mRNA

Question 2

structure and function

Answer 2

• tRNA molecules have single RNA strand
• flattened into one plane to reveal base pairing
• hydrogen bonds = tRNA twists/folds forming 3 dimensional molecule
• tRNA roughly L-shaped

Question 3

charging of tRNAs

Answer 3

• accurate translation = tRNA anticodon matched correct amino acid; tRNA anticodons pair correctly to mRNA
• achieved by enzyme aminoacyl-tRNA synthetase
• 20 difference synthetases for each amino acid

Question 4

tRNA-mRNA pairing

Answer 4

• 61 codons; 45 tRNA’s
• flexible pairing at 3rd base of codon (wobble) allows some tRNA to bind more than one codon (accounts for some genetic redundancy)
• ribosomes facilitate specific coupling of tRNA anticodons to mRNA codons

Question 5

ribosome structure

Answer 5

• two subunits (made of proteins and ribosomal RNA)
• 3 sites
• A site: holds tRNA carrying next amino acid
• P site: hold tRNA carrying growing polypeptide chain
• E site: exit site; discharged tRNA leave ribosome

Question 6

initiation of translation

Answer 6

• binds together mRNA, tRNA with first amino acid and two ribosomal subunits
• small subunit binds mRAN and a special initiator tRNA (moves along mRNA until reached start codon AUG)
• initiation factors = proteins bringing in large subunit to complete translation initiation complex
• hydrolysis of guanosine triphosphate (GTP_ energises process

Question 7

elongation

Answer 7

• during, amino acids added one by one to the preceeding amino acid
• addition involved proteins (elongation factors)
• 3 steps: codon recognition; peptide bond formation; translocation

Question 8

termination of translation

Answer 8

• occurs when stop codon in mRNA reaches A site of ribosome
• A site accepts protein (release factor) causing addition of water molecule instead of amino acid
• reaction releases polypeptide, translation assemply come apart

Question 9

completing functional protein

Answer 9

• polypeptide chains modified after translation
• during and after synthesis, chain spontaneously coils/folds into 3D shape
• additional post-translation modifications may be needed (addition of sugars,lipids,phosphate groups; may be cleaved before protein is active; may be assembled from multiple polypeptides)

Question 10

targeting polypeptide to specific locations

Answer 10

• ribosomes may be free/bound
• bound = endomembrane system and secreted proteins
• polypeptide synthesis starts in cytosol; presence of signalling peptide targets protein to ER
• signalling peptide recognised by signal-recognition particle (escorts ribosome to receptor protein in ER membrane)
• polypeptide synthesis continues there (after release polypep. either released in ER lumen or ramains)
• other signalling peptides may target polypeptides to mitochondria

Question 11

making multiple polypeptides

Answer 11

• multiple ribosomes translate single mRNA simultaneously
• one ribosome past start codon, second ribosome can attach to mRNA (polyribosome/polysome)
• accelerates polypeptide production