L10 - translation

Question 1

what is translation

Answer 1

the process of translating mRNA to amino acids

Question 2

how was the genetic code deciphered

Answer 2

1. synthesised poly - U mRNA and added to tube with everything required for transcription (ribosomes, AAs)
2. only one AA radiolabelled per tube
3. incubated the filtered to wash off unbound AAs
4. radioactive signal significantly higher for phenylalanine for the poly - U strand
5. repeat with other poly base combinations
6. different combinations eg ACC , the order could not be known for sure so to combat this the synthetic DNA was made in proportions and then calculated. eg 75% A and 25% C

Question 3

what is start codon and AA

in P and E

Answer 3

AUG
MET in eukaryotes
fMET in prokaryotes

Question 4

how was the genetic code for codons such as ACC deciphered?

Answer 4

different combinations eg ACC , the order could not be known for sure so to combat this the synthetic DNA was made in proportions and then calculated. eg 75% A and 25% C

CCC would be : 0.25 x 0.25 x 0.25 = 1.5%

Question 5

what is required for translation

Answer 5

ribosome

tRNA

Question 6

what transcribes tRNA in Eukaryotes

Answer 6

DNA pol 3 in

Question 7

where is rRNA transcribed

Answer 7

nucleolus

Question 8

describe tRNA structure

Answer 8

internal base pairing 
clover leaf structure
unusual bases (D)
ends with CCA at 3' end that attaches to AA
each contains 1 anticodon

Question 9

how many tRNA molecules are there and what does this mean for its bonding to the codon

Answer 9

• 22 tRNAs and 61 AA codons

- means some tRNAs recognise more than 1 codon (wobble pairing)

Question 10

explain wobble pairing mechanism

Answer 10

the first 2 bases in codon match, but the 3rd is less critical so can follow non watson crick base pairing without altering AA sequence

Question 11

what is the bond between tRNA and AA

Answer 11

ester

Question 12

what happens to tRNA when AA attaches

Answer 12

tRNA becomes charged (activated)

Question 13

can tRNA attach any AA?

Answer 13

no , it can only attach the AA that is appropriate for its anticodon

Question 14

what are aminoacyl-tRNA-synthetases

Answer 14

enzymes that ‘load’ tRNA with correct AA

Question 15

how is the correct AA attached to tRNA?

Answer 15

by aminoacyl - tRNA - synthetases

Question 16

how is the tRNA for phenylalanine written

Answer 16

tRNA phe

Question 17

how is the activated tRNA for phenylalanine written

Answer 17

phe - tRNA phe

Question 18

describe prokaryote ribosome structure

Answer 18

70s complex

small subunit - 30s
-contains 16s rRNA
large subunit - 50s

Question 19

what happens at A site of ribosome

Answer 19

new AAs delivered by tRNA

Question 20

what happens at P site

Answer 20

where new polypeptide chain forms

Question 21

what happens at E site

Answer 21

where tRNA leaves ribosome

Question 22

explain process of translation initiation (prokaryotes)

Answer 22

1. initiation factors (IF1 and IF3) bind to small subunit
2. this complex then binds to mRNA
3. first tRNA (start codon) (fMet - tRNA fmet) enters P site in complex with IGF2-GTP
4. 16s rRNA binds to shine delgarno sequence in 5 ‘ UTR region of the mRNA to line up the first AA with the AUG start codon
5. large subunit binds
6. GTP hydrolysis
7. IFs and GDP + Pi leave released

Question 23

define UTR

Answer 23

untranslated region

Question 24

what is shine delgarno sequence and its function

prokaryotes

Answer 24

sequence in 5’UTR region of mRNA

lines up start codon of mRNA (AUG) to first activated tRNA (fmet - tRNA fmet)

Question 25

define process of translation elongation

Answer 25

1. next tRNA binds to EF-Tu - GTP (elongation factor) and enters A site
2. if anticodon is complimentary to codon, conformational change causes GTP hydrolysis
3. EF-Tu , GDP + Pi released
4. peptidyl transferase process (new AA is placed underneath the pp)
5. EF-G - GTP binds to ribosome, GTP hydrolysis causes a shape change moving the ribosome along
6. original tRNA leaves through E site
   ribosome now only has P site occupied

Question 26

explain peptidyl transferase process

Answer 26

new AA is placed ‘underneath’ PP chain

NH2 group from new AA attacks carbonyl group of existing AA forming peptide bond

Question 27

what happens if the tRNA anticodon isnt complimentary to the mRNA codon

Answer 27

the tRNA will just diffuse away

Question 28

explain process of translation termination

Answer 28

1. stop codon presented by mRNA
2. release factor (RF1 , RF2) mimics shape of tRNA and enters A site with a h2o molecule
3. pp is hydrolysed from the final tRNA using h2o molecule
4. ribosome disassembles using IF3 and ribosomal recycling factor

Question 29

how does ribosome disassemble during translation termination

Answer 29

using IF3 and ribosomal recycling factor

Question 30

how is old tRNA released from ribosome once a new AA has been added

Answer 30

1. EF-G - GTP binds to ribosome, GTP hydrolysis causes a shape change moving the ribosome along
2. original tRNA leaves through E site
   ribosome now only has P site occupied

Question 31

define polycistronic

Answer 31

an mRNA strand that may encode several proteins

Question 32

are prokaryotes or eukaryotes polycistronic

Answer 32

prokaryotes

eukaryote mRNA only encodes 1 protein