Lecture 6

Question 1

What ‘adapter’ links mRNA and amino acids?

Answer 1

tRNA.

Question 2

How long is a tRNA molecule, and what shape is it?

Answer 2

80 nucleotides roughly.

Clover leaf.

Question 3

Do base pairs form within tRNA structure?

Answer 3

Yes.

Question 4

How does tRNA base pair with a codon?

Answer 4

Anticodon.

Question 5

Where are amino acids attached on a tRNA?

Answer 5

3’ hydroxyl group on end of RNA chain.

Question 6

How specific is tRNA?

Answer 6

Specific for single amino acid determined by anticodon.

Question 7

What carries out specific attachment on tRNA?

Answer 7

amino-acyl tRNA synthetases.

Question 8

How many amino-acyl tRNA synthetases are there?

Answer 8

20. One for each amino acid.

Question 9

What are the 3 binding sites on amino-acyl tRNA synthetase for?

Answer 9

Specific tRNA.
Corresponding amino acid.
ATP.

Question 10

2 steps in attachment process of amino-acyl tRNA synthetase?

Answer 10

ATP hydrolysed and amino acid joined to AMP.

Correct tRNA binds, and amino acid transferred from AMP to tRNA.

Question 11

How many amino acid codons in genetic code, and how many tRNAs?

Answer 11

61 amino acid codons.

40-40 tRNAs.

Question 12

When G is in wobble position, what can the 3rd base in the codon be?

Answer 12

U or C.

Question 13

When A is in wobble position, what can the 3rd base in the codon be?

Answer 13

U.

Question 14

When C is in wobble position, what can the 3rd base in the codon be?

Answer 14

G.

Question 15

When U is in wobble position, what can the 3rd base in the codon be?

Answer 15

A or G.

Question 16

When I is in wobble position, what can the 3rd base in the codon be?

Answer 16

U, C or A.

Question 17

What is the ribosome composed of?

Answer 17

rRNA and proteins.

2 subunits.

Question 18

What are the 2 subunits in ribosomes called?

Answer 18

Large and small.

Question 19

What does the ribosome do?

Answer 19

Binds mRNA and amino-acyl tRNAs.

Catalyses stepwise formation of peptide bonds.

Question 20

In which direction do ribosomes move along the mRNA?

Answer 20

5’ to 3’.

Question 21

What does a ribosome ensure?

Answer 21

Correct reading frame used.

Question 22

What is the A site, P site and E site?

Which subunit are these found on?

Answer 22

A = Aminoacyl tRNA binding site.
P = peptidyl-tRNA binding site.
E = Exit site.

Large subunit.

Question 23

What’s found on the small subunit of the ribosome?

Answer 23

mRNA binding site.

Question 24

3 stages of protein synthesis, and what happens in each?

Answer 24

Initiation - small subunit binds mRNA and initiator amino acyl tRNA then large subunit binds.
Elongation - peptide bonds formed as the ribosome moves along mRNA.
Termination - one of the three stop codons enters A site, releasing completed protein.

Question 25

What is the start codon?

Answer 25

AUG.

Question 26

Where does initiation of protein synthesis get its energy from?

Answer 26

GTP hydrolysis and proteins called invitation factors.

Question 27

3 steps in elongation of protein?

Answer 27

Codon recognition - conversion of GTP to GDP.
Peptide bond formation.
Translocation - converts another GTP to GDP.

Question 28

What catalyses peptide bond formation?

Answer 28

Peptidyl transferase - RNA enzyme (ribozyme).

Question 29

What is added to end of polypeptide chain on termination?

Answer 29

Water.

Question 30

When the ribosome dissociates after polypeptide released from tRNA in P-site, what is hydrolysed?

Answer 30

2XGTP.

Question 31

What are polyribosomes?

Answer 31

Several ribosomes that translate an mRNA at once.

Question 32

Where does transcription and translation happen in prokaryotes and eukaryotes?

Answer 32

No nuclear membrane in prokaryotes, so both coupled.

mRNA must be transported to cytoplasm before translation can occur in Eukaryotes.

Question 33

How do proteins travel in prokaryotes and eukaryotes? Why?

Answer 33

Protein diffuse through cytoplasm in prokaryotes.. no organelles.
Proteins must be trafficked to correct site as several different organelles in eukaryotes.

Question 34

How are proteins targeted to ER?

Answer 34

SRP binds to signal peptide momentarily halting protein synthesis. Then SRP binds to receptor protein in ER membrane.
SRP then leaves, and polypeptide resumes growing, translocating across membrane.
Signal cleaving enzyme cuts off signal peptide.
Rest of completed polypeptide leaves ribosome, folding into final conformation.