Translation and post-translational modification

Question 1

Features of Prokaryotic ribosomes

Answer 1

all free in the cytoplasm

ribosomal proteins on side and back of subunit (not near tRNA and mRNA binding sites)

3 tRNA binding sites

30s/50s subunit = 70s size (as compared to 80s in eukaryotes)

Question 2

Name the 3 tRNA binding sites on prokaryotic ribosomes

Answer 2

A site (aminacyl site) : accommodates the incoming aminoacyl-tRNA

P site (peptidyl site) : accommodates the tRNA attached to the growing peptide chain

E site (exit site) : accommodates the tRNA without an amino acid that is in the process of leaving

Question 3

Describe how tRNA binds to the ribosome

Answer 3

Anticodons bind to the 30s subunit

the rest of the tRNA molecule is bound to the 50s subunit

Question 4

How do ribosomes act as a catalyst

Answer 4

TRNAs on the A-site and P-site interact closely with the mRNA via base-pairing

acceptor ends are close together , permitting the peptidyl transferase reaction

Question 5

Features of eukaryotic ribosomes

Answer 5

40s subunit

60s subunit

made from both polypeptides and rRNA

3 tRNA binding sites

Question 6

Energy requirements for translation

Answer 6

ATP and GTP required as energy sources

1 GTP needed for binding aminoacyl-tRNA to A-site

1 GTP needed for translocation step

ATP needed for peptide bond formation

Question 7

Describe chain elongation

Answer 7

The growing polypeptide is transferred from peptidyl-tRNA in the P site to the incoming as-tRNA in the A site

the new peptidyl-tRNA is transferred from A to P site

uncharged tRNA moves to the E site

Question 8

Prokaryotic mRNA is polycistronic. Explain what this means

Answer 8

More than 1 coding regions in the mRNA ; more than 1 type polypeptide can be produced

each coding region has its own initiation and termination codons

Question 9

Eukaryotic mRNA is monocistronic ; explain what this means

Answer 9

1 coding region

each mRNA codes for 1 polypeptide

Question 10

What is a polysome ?

Answer 10

More than one ribosomes can translate a piece of mRNA simultaneously

Question 11

How does initiation occur in prokaryotes ?

Answer 11

Initiation factors (IF1/2/3 needed)

sequence at the end of 16s rRNA of the ribosome base pairs with the shine-dalgarno sequence of the prokaryotic mRNA , causing the mRNA to align

the start codon on the mRNA is on the P-site

GTP required

Question 12

How does initiation occur in eukaryotes

Answer 12

The small subunit binds to the cap on the mRNA and reads along until the start codon is reached (AUG)

AUG recognised by specific tRNA which then goes straight to P site

facilitated by elF-2-GTP

large subunit then joins the complex

Question 13

How does termination occur

Answer 13

Release factors recognise terminations codons

no amino acid comp to termination codon

Question 14

How do most antibiotics work

Answer 14

By blocking translation

Question 15

Co/Post translational modification

Answer 15

Ribosome associated chaperones will aid i protein folding

can occur during or after translation

many secreted proteins are precursor proteins that are later activated by specialised endoproteases in the ER or Golgi apparatus

phosphorylation on hydroxyl groups of amino acid residues

Question 16

What are zymogens

Answer 16

Inactive enzymes

Question 17

Phosphorylation is catalysed by _____ and reversed by ______

Answer 17

Kinases and phosphatases

Question 18

When an amino acid is added to a specific tRNA (prior to protein synthesis), the accuracy is dependent on:

Answer 18

the aminoacyl tRNA synthetase specific for that amino acid

Question 19

Which processes does the Hox family of transcription factors typically control?

Answer 19

brain and limb development

Question 20

Which ONE of the following statements about mitochondrial oxidation of a substrate without ATP synthesis is true?

It can be used to create NADPHIt does not take place in humans

It is not physiologically important

it only occurs in damaged mitochondria

It produces heat

Answer 20

it produces heat

Mitochondrial oxidation of a substrate (without ATP synthesis) can occur in the presence of uncoupling proteins which are forms of proton channel. These proteins uncouple respiration from ATP synthesis and lead to the production of heat. This is an important process within hibernating animals.