ribosomal structure Flashcards
how are bacterial ribosomes structured?
70S
50S (large subunit)
30S (small subunit)
how are eukaryotic ribosomes structured?
80S
60S (large subunit)
40S (small subunit)
what proportion of the ribosome is RNA?
2/3rds by mass
why is rRNA highly conserved?
it is catalytic RNA, with essential function
how are the large subunits held together?
60S subunits - held together with many protein-protein interactions on the outside of the ribosome
50S subunits - bacterial subunits have not so many protein-protein interactions
where does RNA enter and exit the ribosome?
enters - right hand side
exits - left hand side
what is the decoding box?
the location of codon-anticodon interactions
where is the decoding box located?
on the smaller subunit of the ribosome, on the track of the mRNA as it is threaded through
where is the peptidyl-transferase centre located?
on the large subunit, with no proteins anywhere near it
how are the decoding box and peptidyl-transferase centre linked?
tRNAs bridge the gap between the subunits
why are processing enzymes often found bound to the outside of ribosomes?
no enzymes can cleave the peptide until it pokes out of the peptide exit tunnel, which spans the body of the larger ribosomal subunit
how does the process of translation differ between prokaryotes and eukaryotes?
the main difference is in initiation, the elongation process is ubiquitous
which sites on the ribosome are mutually exclusive?
E site and A site
what are the three steps of translation?
1 - binding of the aminoacyl-tRNA into the A site of the ribosome
2- peptide bond formation
3- translocation reaction
what is required for binding of the aminoacyl-tRNA into the A site?
carrier protein (GTP) and elongation factor Tu [aa-RS-GTP-Tu]. this is the rate limiting step
why does peptide bond formation require no input of energy?
breaks the ester bond formed between the Amino acid and tRNA
what is the function of the translocation reaction?
frees up the A site in order to prepare it to receive incoming aa-tRNA
also moves the ribosome along the mRNA
why must the movement of the mRNA relative to the ribosome be exact?
must be exactly 3 nucleotides to avoid frameshift
in translocation, how does the tRNA move through the ribosome?
P/P -> E/E -> A/A
why do tRNAs move via hybrid sites?
allows them to remain attached to eachother, minimising the loss of peptidyl-tRNA
what is required for each step of translocation?
step one its spontaneous as it is exerted on tRNAs already in the hybrid state
step 2 requires EF-G and GTP
EF-G enters the A/A site, displacing tRNA from A/P -> P/P
how does EF-G displace tRNA from A/P?
- EF-G is too big to fit in the A site
- it has five folded domains, part of this is inserted into the decoding box
- it can push tRNA that is already bound in the A site out of the site.
what is the role of EF-Tu in translocation?
carries aa-tRNA into the ribosomal A site
how do EF-Tu and EF-G exhibit molecular mimicry?
both factors occupy the same site on the ribosome at different stages of the cycle
