Chapter 28 - Translation Flashcards
Wobble hypothesis
The third position in the codon is capable of “wobble” in its position during translation, allowing it to make alternative hydrogen-bonding arrangements with several different codon bases.
Codon bias
Use of degenerate codons by certain organisms is highly selective. Certain codons, although they code for the same amino acid, are more abundant than others.
Start signal in translation
AUG –> Methionine (eukaryotes) or N-formylmethionine (prokaryotes)
How does the ribosome know how to interpret the start codon properly to distinguish between internal Met sites and start sites?
5’ end of mRNA contains sequences to ensure that it is correctly attached to the ribosome. The FIRST AUG encountered is then interpreted as a start signal, and translation begins.
What is the difference between prokaryotic and eukaryotic mRNA?
Prokaryotic mRNA is polycistronic (code for many proteins), wherease eukaryotic mRNA is monocistronic
Open reading frame
Sequence within mRNA bounded by start and stop codons.
Shine-Dalgarno sequences
The regions 5’ to each start signal, containing sequences rich in A and G, which help align the mRNA on the ribosome so that translation can begin at the proper points and in the correct reading frame.
Structure of tRNA
(1) Anticodon loop
(2) D loop and TψC Loop
(3) Variable loop
(4) Acceptor stem (3’, where amino acid will be attached)
How is tRNA coupled to amino acids?
(1) Amino acid (bound to aminoacyl-tRNA synthetase) is activated by ATP –> aminoacyl adenylate
(2) aminoacyl adenylate reacts with one of the correct tRNAs to form the covalent bond and release AMP
- The correct tRNA is identified via identity elements on the tRNA, clustered in the anticodon loop and the acceptor stem.
Prokaryotic ribosome
70S –> 50S, 30S
Eukaryotic ribosome
80S –> 60S, 40S
New mRNA transcript is transcribed in what direction?
5’ –> 3’, based on a 3’–>5’ template strand
mRNA is read in what direction for translation?
5’ –> 3’
Protein is made in what direction?
N –> C
Prokaryotic translation
(1) IF1 and IF3 bind to preexisting 70S ribosomes, producing free 30S subunits needed for initiation.
(2) IF2 + GTP delivers the initiator tRNA to 30S, and mRNA binds. (IF2-GTP binds to Shine-Dalgarno sequence, the initiator tRNA is transferred to start codon)
(3) 50S binds, and IF3 is released. AUG initiator codon aligns with the P site.
(4) The charged tRNA for the next mRNA codon is escorted to the A site in complex with EF-Tu, which also carries a molecule of GTP
(5) Peptide bond formation. Chain transfer from the peptidyl tRNA to the aminoacyl tRNA. (Peptidyltransferase, part of the 50S subunit).
(6) Translocation of the peptidyl tRNA from A –> P. Ribosome moves one codon to the right (hydrolysis of EF-G-GTP)
(7) tRNA in the E site exits, and ribosome is ready for another cycle.
(8) Protein release factors (RF1,RF2,RF3) terminates translation.
- RF1 and RF2 recognizes stop codons
- RF3 is a GTPase that stimulates the relase process via GTP binding and hydrolysis
(9) After a release factor is bound to the ribosome, peptidyltransferase transfers the C-termianl residue of the polypeptide chain from the P-site tRNA to a water molecule, releasing the peptide chain from the ribosome.
(9) RF factors and GDP are released, followed by the tRNA. The 70S ribosome is now unstable.
- instability of 70S ribosome accentuated by the presence of the ribosome recycling factor and IF3 and IF1.
- 30S subunit may or may not dissociate from its mRNA.
