5. protein synthesis

Question 1

vital functions of ribosome

Answer 1

1. bind codons that permits high fidelity reading of codons
2. includes binding sites for tRNA molecules
3. interact with protein factors required for polypeptide chain initiation, elongation and termination
4. catalyse peptide bond formation
5. undergo movement permitting translational sequential codons

Question 2

sites of ribosomes

Answer 2

A site (aminoacyl): accepting aminoacyl tRNA 
P site (peptidyl): accomodates the tRNA attached to growing peptide chain 
E site (exit): leaving of tRNA without aa 

A and P site tRNAs interact closely together for peptidyl transferase reaction

Question 3

how does a tRNA bind to ribosome

Answer 3

tRNA binds to 30s (small subunit)- anticodon arm and the rest of of tRNA bound to 50s

Question 4

RNA to RNA contact (targets universally conserved regions)

Answer 4

allow for peptide bond to form between amino acids

Question 5

what happens as the polypeptide extends

Answer 5

fits into tunnel on 50s subunit that extends from P site to the outer ribosomal surface

Question 6

how are eukaryotic ribosomes different

Answer 6

3tRNA binding sites- different- larger 80s (40s, 60s)

Question 7

Why eukaryotic ribosomes different

Answer 7

have to interact with nuclear membrane and secretory system (golgi and ER)

Question 8

how do ribosomes operate

Answer 8

1. synthesis proceeds from N to C terminus (5’ to 3’)

2. new peptidyl-tRNA is transferred from A to P- uncharged tRNA moves to E site

Question 9

why can’t eurkaryotic translation begin immediately

Answer 9

mRNA must be translocated out of nuclear membrane

Question 10

polysome

Answer 10

more than one ribosome is translating on mRNA at the same time due to length of mRNA

Question 11

Translation initiation:

Answer 11

assembly of translation system components before peptide formation

Question 12

Translation initiation: requires

Answer 12

initiator tRNA and initiation factors. in Ecoli initiator is N-formulated Met residue (AUG)

Question 13

how does e.coli ribosome know where to start from

Answer 13

Shine-Dalgarno sequence upstream to start codon of mRNA
16s of 30s rRNA contains special sequence after start codon
mRNA contains complementary region (SD sequence)

Question 14

Translation initiation: initiation factors

Answer 14

IF 1,2,3

Question 15

how do eukaryotes know where to start translation from

Answer 15

cap that ribosomes bind and leave on 40s

Question 16

what does initiation result in

Answer 16

fMet- tRNAfmet x mRNA x ribosome complex
with start codon under P site
IF2 GTP brings charged tRNA to P site
first tRNA binds with fMet IF2 hydrolysis of GTP
allows second half of ribosome to be clipped on (50S attaches forming 70S subunit)

tRNAfmet is the only tRNA that enters P site
all other enter A site during chain elongation

Question 17

what does eukaryotic imitation require

Answer 17

ATP cap at 5’ mRNA
recognise mRNA’s initiating AUG codon embedded in consensus sequence
7-methylguanosine cap and are monocistronic

Question 18

translation elongation stages

Answer 18

decoding
transpeptidation
translocation

Question 19

decoding

Answer 19

ribosomes select and binds an aminoacyl-tRNA of complementary anticodon to codon on mRNA on A site

Question 20

transpeptidation

Answer 20

peptide bond formation

peptidyl group in P site tRNA transferred to aminoacyl group in A site

Question 21

translocation

Answer 21

a and p site tRNAs transferred to P and E sites
uncharged tRNA from P to E
peptidyl tRNA from A to P
delivery of next aminoacyl tRNA by elongation factors
requires GTP hydrolysis bring into A site

Question 22

peptide bond formation enzyme

Answer 22

peptidyl transferase in large subunit

Question 23

termination codon

Answer 23

no tRNA bound
UAA UGA UAG
recognised by release factors 1 and 2 GTP hydrolysed
proteins and sub units break off- recycled

Question 24

decoding detail

Answer 24

ribosomes monitor correct codon-anticodon pairing
EF-Tu x GTP delivers aa-tRNA to ribosome
tRNA’s aminoacyl end moves into A site after GTP os hydrolysed and EF-Tu dissociates

Question 25

transpeptidation detail

Answer 25

ribosome is a riboenzyme
peptidyl transfer centre on large subunit- consists of rRNA
nearest protein side chain is 18 aa away from newly formed peptide bond
nearest Mg ion is 8.5 aa away

Question 26

Puromycin

Answer 26

binds to A site without need for elongation factors

Question 27

streptomycin

Answer 27

member of aminoglycosides

ribosome misreads mRNA binds to 30S

Question 28

Chloramphenicol

Answer 28

inhibit peptidyl transferase activity

Question 29

tetracycline

Answer 29

binds to 30s

prevents entry to aminoacyl tRNA into A site but allows Ef-Tu to hydrolyse GTP- energetic drain on cell

Question 30

Ricin

Answer 30

removes highly consvered adenine base of 28S stops ribosomes binding

Question 31

how do you fold a protein

Answer 31

ribosome associated chaperones

Question 32

why do you need chaperones

Answer 32

ribosomes exit channel is too narrow to allow secondary structure formation
unfolded hit with other proteins causing misfolding

Question 33

Trigger factor

Answer 33

recruited by ribosomal proteins
recognise short hydrophobic protein segments 
does not require ATP 
prevents mis folding and aggregation 
near exit of ribosomes tunnel

Question 34

Proteolysis

Answer 34

activate proproteins- cleavage

contain signal peptide to be excised

Question 35

zymogen

Answer 35

inactive enzymes

Question 36

ubiquination

Answer 36

sends protein for degradation

Question 37

glycosylation

Answer 37

to signal transfer of protein to lumen of ER from cytosol