MCQs on Enzymes for Ligation
1 - Question
Enzyme commonly used for carrying out ligation reaction is ____________
b) Reverse transciptase
Explanation: Ligase is the enzyme which is commonly used for carrying out the ligation reaction. They can either be obtained from E. coli or from cells that have been infected by the virus.
2 - Question
Which of the following statements is correct with respect to T4 DNA ligase?
a) It can carry out only blunt ended ligations
b) It doesn’t requires ATP
c) It requires a phosphate group at 3’ end and a hydroxyl group at 5’ end for the molecule to be joined
d) It is obtained from T4 bacteriophage upon infection by E. coli
Explanation: T4 DNA ligase is obtained from T4 bacteriophage upon infection by E. coli. It carries out ligation both in blunt ended and sticky ended molecules and requires ATP. It requires a phosphate group at 5’ end and a hydroxyl group at 3’ end.
3 - Question
If blunt ended ligations are to be carried out. Which of the following enzymes can be used?
a) E. coli DNA ligase
b) T4 DNA ligase
c) Both of these enzymes act equally in carrying out blunt ended ligations
d) None of them is able to carry out blunt ended ligations
Explanation: If blunt ended ligations are to be carried out, T4 DNA ligase should be used. E. coli DNA ligase is unable to carry out blunt ended ligation.
4 - Question
E. coli DNA ligase doesn’t require a cofactor.
Explanation: The E. coli DNA ligase requires a cofactor nicotamine adenine dinucleotide (NAD), along with 5’ phosphate group and 3’ hydroxyl group.
5 - Question
Mechanism of ligation for both T4 DNA ligase and E. coli DNA ligase makes use of Adenosine Monophosphate (AMP). Which of the steps is involved in the ligation mechanism?
a) AMP is added to the 5’ phosphate of one of the DNA molecule
b) It leads to the liberation of pyrophosphate from NAD and nicotinamide mononucleotide from ATP
c) The AMP is further displaced by an electrophilic attack
d) The AMP is further displaced by nucleophilic attack by 3’ hydroxyl of the same DNA molecule
Explanation: AMP is added to the 5’ phosphate of one of the DNA molecule. It leads to the liberation of pyrophosphate from ATP and nicotanamide mononucleotide from NAD. It is further displaced by nucleophilic attack by 3’ hydroxyl of the other DNA molecule.
6 - Question
Topoisomerase is also an enzyme which is used for carrying out ligation. The correct statement for topoisomerase is?
a) They act only on double stranded molecules
b) They alter the degree of supercoiling of DNA molecules
c) They are less effective than conventional DNA ligase
d) There are three types of topoisomerases
Explanation: Topoisomerases alter the degree of supercoiling of both single and double stranded DNA molecules. There are two types of topoisomerases, I and II. I is responsible for altering single stranded molecules and type II for altering double stranded molecules. They are more effective than conventional DNA ligase.
7 - Question
Mobile genetic elements can be transferred from one DNA portion to another. The enzyme carrying out this is ____________
Explanation: Transposase is used for transferring mobile genetic elements from one portion to another. They are useful in inserting origin of replication or anti-biotic resistance genes.
8 - Question
Phage based recombination systems are used for ____________
a) cleaving the molecules at specific sites
b) adding the molecules at specific sites
c) breakage and rejoining the molecules at specific sites
d) breakage at random sites
Explanation: Phage based recombination systems are used for breaking and rejoining the molecules at specific sites and not at random sites.
9 - Question
Bacteriophage lambda is having a phage recombination system. Following are the characteristics of this system ____________
a) It is used for inserting phage genome into the bacterium
b) It is used for inserting bacterial genome into the phage
c) The specific site in bacteria is attB and that in phage is attP
d) The specific sites in both of them are called as attP
Explanation: The phage genome is inserted into the bacterium. The site for insertion at the phage genome is attP and that in the case of a bacterium is attB
10 - Question
Ligation enzymes are used for ligating newly synthesized okazaki fragments. What holds true for okazaki fragments?
a) Okazaki fragments are short fragments of DNA formed on the leading strand
b) Okazaki fragments are large fragments of DNA formed on the lagging strand
c) Okazaki fragments are short fragments of DNA formed on the lagging strand
d) Okazaki fragments are large fragments of DNA formed on the leading strand
Explanation: Okazaki fragments are short DNA fragments on the lagging template strand during replication. They are complementary to the lagging strand.