Molecular Biology (9)

Question 1

How many codons are there?

Answer 1

4^3 = 64

Question 2

How many essential amino acids are there?

Answer 2

20

Question 3

What are the two types of point mutations?

Answer 3

Missense - substitution of one nucleotide - different AA in polypeptide
Nonsense - substitution of one nucleotide - premature stop codon introduced - early polypeptide termination - shorter

Question 4

What is a frameshift mutation?

Answer 4

Ribosome reads 4 nucleotides as 3 - or backs up one nucleotide and reads from different reading frame

Question 5

What are the stop codons?

Answer 5

UAA
UAG
UGA

Question 6

What is the start codon and which AA does it code for?

Answer 6

AUG

Methionine

Question 7

What is the role of rRNA?

Answer 7

Associates with proteins to form ribosomes - have enzymatic activity - act as ribosymes

Question 8

Which end of the growing polypeptide chain and new AAs attached to?

Answer 8

C-terminus

Question 9

Where is the AA attached to a tRNA molecule?

Answer 9

2’ or 3’ OH of ribose

Question 10

What sequence is found at the 3’ end of mature tRNA molecules and when is this added?

Answer 10

CCA

Added after tRNA processing

Question 11

Which enzyme attaches AA to appropriate tRNA and what does this form?

Answer 11

Aminoacyl tRNA synthetase

Forms aminoacyl-tRNA

Question 12

How many types of aminoacyl tRNA synthetase are there?

Answer 12

20 - one per AA

Question 13

Which bond attaches AA to tRNA and what is required for its formation?

Answer 13

High energy ester bond

Requires ATP hydrolysis

Question 14

What is the wobble position of a codon?

Answer 14

Non-standard base pairing at wobble position - 3bp of codon - can be bound by different 1st position of tRNA anticodon

Question 15

What are the possible bases in the first position of an anticodon if the third position of a codon is a C?

Answer 15

G, I

Question 16

What are the possible bases in the first position of an anticodon if the third position of a codon is an A?

Answer 16

U, I

Question 17

What are the possible bases in the first position of an anticodon if the third position of a codon is a G?

Answer 17

C, U

Question 18

What are the possible bases in the first position of an anticodon if the third position of a codon is a U?

Answer 18

A, G, I

Question 19

What is the Shine-Dalgarno sequence, what is its role, and where is it located?

Answer 19

5’-UAAGGAGGU-3’
Consensus sequence - for ribosome binding - in prokaryotes - sets reading frame
7-9 nucleotides upstream of fMet

Question 20

What are the sites of the large ribosomal subunit and what are their roles?

Answer 20

A site - aminoacyl site - aminoacyl tRNA binds
P site - peptidyl site - peptide bond forms
E site - exit site

Question 21

Describe the overall process of prokaryotic translation initiation

Answer 21

mRNA binds to small ribosomal subunit
Initiator tRNA with bound methionine recognises start codon - binds
Large subunit associates - initiator tRNA held in P site

Question 22

What are the roles of the factors involved in keeping the ribosomal subunits separate?

Answer 22

IF3 binds to E site - large subunit cannot bind
IF1 and IF2 associate together - IF2 is GTPase with bound GTP - both bind to A site
Only aminoacyl fitting P site is fMet-tRNA

Question 23

What is the effect of base pairing between codon and fMet-tRNA anticodon?

Answer 23

IF3 dissociation - allows large subunit to bind

IF2 hydrolyses GTP to GDP - irreversible - IF2 has lower affinity for ribosome and IF1 - IF1 and IF2 dissociate

Question 24

What is the role of GTP dephosphorylation by IF2?

Answer 24

Irreversible - due to conformational change and decrease in energy level
GTPase activity at checkpoints - GDP denotes correct AA sequence