07_Protein Synthesis (cont'd) Flashcards
During which phase of the cell cycle does the nucleolus disappear? Why?
mitosis
DNA is packed, so no RNA can be made
In (eukaryotes/prokaryotes), transcription and translation occur simultaneously.
Prokaryotes
Match
70S and 80S ribosomes
eukaryotes, prokaryotes
70S: P
80S: E
What are the small and large subunits of prokaryotic ribosomes, and what are each comprised of?
small subunit: 30S (16S rRNA + 21 proteins)
large subunit: 50S (5S rRNA + 23S rRNA + 34 proteins)
What are the small and large subunits of eukaryotic ribosomes, and what are each comprised of?
small subunit: 40S (18S rRNA + 33 proteins)
large subunit: 60S (28S rRNA + 5.8S rRNA + 5S rRNA + 49 proteins)
Operons are found in (prokaryotes/eukaryotes).
Prokaryotes
When is the m7 guanine cap added to the 5’ end of the mRNA transcript?
at initiation of transcription
What type of modification is the addition of the poly-A-tail? What enzyme catalyzes this modification?
post-transcriptional; poly (A) polymerase
What prevents translation in the 5’ UTR?
secondary structure elemnents, like hairpins and loops
What does PABP bind to?
mRNA poly A tail
What protein on mRNA allows it to be exported from the nucleus through nuclear pores?
nuclear export receptor
The eIF4F complex is composed of 3 proteins. What are they, and what does each bind to?
eIF4E: cap-binding proteins, binds to 5’ cap.
eIF4A: is an RNA helicas; unwinds loops and hairpin structures in the 5’UTR
eIF4G: bridge between eIF4E and PABP
What is the ternary complex composed of?
initiatory methionyl-tRNA, eIF2 and GTP
What does the ternary complex bind to? What forms as a result?
Binds to the 40S ribosomal subunit to form a 43S pre-initiation complex
As the initiator tRNA scans the mRNA for AUG, what is used to provide energy for helicase activity by eIF4A?
Hydrolysis of ATP
What happens when the initiator tRNA anneals to a codon? (eukaryotes)
GTP is hydrolized, and eIF2 and the other initiation factors dissociate. Then, the large ribosomal subunit (60S) binds.
When the large subunit first binds, which site is methionyl tRNA loaded in?
the P site
What kind of bond is formed between the two amino acids in the P and A sites?
phosphoester bond
Which end of the peptide emerges from the ribosome first? (i.e. which direction are peptides synthesized in?)
The amino end
amino –> carboxyl
What catalyses binding of an aa-tRNA to the A-site?
eEF1α in eukaryotes
EF-TU in prokaryotes
What does eEF2 catalyze?
Movement of the amino-acid peptide from the A-site to the P-site and movement of the ribosome to the next codon.
What are the equivalents of eEF1 and eEF2 in prokaryotes?
eEF1 –> EF-TU
eEF2 –> EF-G
Which eukaryotic elongation factor is a G-protein with proofreading activity?
eEF1
What are the three stop codons?
UAA, UAG, UGA
What binds to the A-site when a stop codon is encountered?
Release factors
What are polysomes?
Numerous ribosomes translating the same mRNA strand
What does presence of the release factor in the P site trigger?
Disassembly of of the 80S ribosome-mRNA complex
How does tetracycline act as an antibiotic?
blocks binding of aminoacyl-tRNA to A-site of ribosomes
How does chloramphenicol act as an antibiotic?
blocks peptidyl transferase reaction on ribosomes
peptidyl transferase = ribozyme; forms bonds b/w adjacent amino acids
What is an example of a translation inhibitor that only acts on eukaryotes? What does it target?
Cycloheximide; blocks translocation reaction on ribosomes
