This set of Molecular Biology Interview Questions and Answers for Experienced people focuses on “DNA Topology – 3”.
1. Topoisomerases I and II differ only on the number of steps required to change the linking number.
Explanation: Type II topoisomerase changes the linking number in two steps whereas type I in one step. Also for this process type II topoisomerase requires ATP but type I do not require ATP hydrolysis.
2. Which special enzyme introduces negative supercoils in DNA?
a) Type I topoisomerase
b) Type II topoisomerase
Explanation: A special type of type II topoisomerase found in prokaryotes, known as the DNA gyrase introduces negative supercoils rather than removing it. DNA gyrase is responsible for the negative supercoiling of DNA in prokaryotes.
3. Which of the following is not a property of topoisomerase?
a) Removal of supercoil
b) Helps in DNA replication
c) Cannot catenate and decatenate
d) Utilizes ATP to change linking number
Explanation: Topoisomerases promotes several other reactions important to maintaining the proper DNA structure within the cell. The enzymes use the same transient break and strand passage reaction for both catenation and decatenation of circular DNA molecules.
4. Topoisomerases introduces catenation to promote cell division.
Explanation: Topoisomerases play an essential role for unlinking DNA molecules and thus allowing them to separate into two daughter cells promoting cell division. The process of separation of these cccDNA is known as decatenation. This reaction is readily promoted by type II topoisomerase.
5. Type I topoisomerase can never decatenate catenated circular DNA?
Explanation: Type II topoisomerase can decatenate circular DNA in any situation. But type I topoisomerase can only decatenate catenated circular DNA when any one of the strand caries a nick or gap.
6. The promotion of DNA breaking and rejoining by topoisomerase requires______________
a) High – energy cofactor
c) DNase I
d) No external enzyme required
Explanation: Topoisomerases are able to promote both DNA cleavage and rejoining without the assistance of other proteins or high – energy cofactors. This is because they use a covalent – intermediate mechanism which stores the energy of phosphodiester bond cleavage to reseal the backbone later.
7. Attack of the topoisomerase gives rise to ____________
a) Hydroxy – valine linkage
b) Phospho – tyrosine linkage
c) Hydroxy – cystine linkage
d) Phospho – proline linkage
Explanation: DNA cleavage occurs when a tyrosine residue in the active site of the topoisomerase attacks a phosphodiester bond in the backbone of the target DNA. This attack generates a break in the DNA whereby the topoisomerase is covalently joined to one of the broken ends via a phospho – tyrosine linkage.
8. To reseal the DNA backbone where does topoisomerase gets the energy from?
a) ATP hydrolysis
b) High energy cofactor
c) Phosphodiester cleavage
d) Phospho – tyrosine cleavage
Explanation: The Phospho – tyrosine linkage conserves the energy in the phosphodiester bond that was cleaved. Therefore, the DNA can be resealed simply by reversing the original reaction: the OH group from one broken DNA end attack the Phospho – tyrosine bon reforming the DNA phosphodiester bond.
9. Which of the following does not occur during cell division in prokaryotes?
a) Multiple decatenations
b) DNA cleavage
c) DNA rejoining
d) Strand passage
Explanation: Between the steps of DNA cleavage and DNA rejoining, the topoisomerase promotes passage of a second segment of DNA through the break. Topoisomerase function thus involves DNA cleavage, strand passage and DNA rejoining in highly coordinated manner.
10. The active site of the topoisomerase contains a tyrosine residue.
Explanation: Yes the active site of the topoisomerase contains a tyrosine residue. This is a major requirement because it is used to produce the covalent DNA – tyrosine intermediate. This intermediate thus formed, stores the energy evolved during the phosphodiester bond breakage to reseal the backbone after the process of decatenation is complete.
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