This set of Cell Biology Multiple Choice Questions & Answers (MCQs) focuses on “DNA Replication”.
1. How many types of replication models were considered before the semiconservative nature of DNA replication was confirmed?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: Three models of DNA replication were considered before the semiconservative nature of DNA replication was confirmed. These included the semiconservative, conservative and dispersive replication.
2. Which isotopes were used by the researchers who first confirmed the semiconservative nature of DNA replication?
a) Nitrogen
b) Carbon
c) Oxygen
d) Sulfur
View Answer
Explanation: In 1957, researchers used the Nitrogen isotopes to confirm the semiconservative nature of DNA replication. At the California Institute of Technology, the researches grew bacteria in two subsequent mediums containing 15N and 14N isotopes.
3. Temperature-sensitive mutants can not replicate their chromosomes at non-permissive temperatures.
a) True
b) False
View Answer
Explanation: Certain mutant strains of bacteria can not replicate their genetic material (chromosomes) at elevated temperatures, termed nonpermissive (or restrictive) temperatures.
4. The bacterial oriC is the __________________
a) specific sequence
b) transcription factor
c) enzyme
d) polypeptide
View Answer
Explanation: The replication of chromosome begins at a specific site in bacteria, called the origin. The specific sequence called oriC is the origin of replication, where a vast array of proteins bind to initiate the replication process.
5. Movement of the replication fork generates ________________ in the unreplicated portion of DNA.
a) positive supercoils
b) negative supercoils
c) untranslated regions
d) amino acid substitutions
View Answer
Explanation: The replication fork moves bidirectionally starting from the origin of replication and moving in the opposite direction, unwinding the double helix along the way. Positive supercoils are generated in the unreplicated, wound portion of the DNA.
6. Which enzyme removes the positive supercoils generated when the replication fork?
a) helicase
b) endonuclease
c) exonuclease
d) gyrase
View Answer
Explanation: DNA gyrase is an enzyme belonging to the topoisomerase-II type of enzymes. The basic function of DNA gyrase is to relieve the mechanical stress and the positive supercoils that build up during replication.
7. When was DNA polymerase III discovered as the major enzyme behind DNA replication in bacteria?
a) 1949
b) 1959
c) 1969
d) 1989
View Answer
Explanation: In 1960s, there were hints that Kornberg enzyme – DNA polymerase I was not the only polymerase in a bacterial cell. In 1969, it came to be known that the major enzyme behind DNA replication is DNA polymerase III.
8. How many copies (approximately) of DNA polymerase III are present in a typical bacterial cell?
a) 10
b) 50
c) 100
d) 200
View Answer
Explanation: DNA polymerase III is the major enzyme responsible for DNA replication in a bacterial (and eukaryotic) cell. There are approximately 10 copies per cell of a DNA polymerase III.
9. The lagging strand is synthesized discontinuously.
a) True
b) False
View Answer
Explanation: The two newly synthesized strands are synthesized by two different mechanisms. The leading strand is synthesized continuously in 5’ to 3’ direction while the lagging strand is synthesized discontinuously in 3’ to 5’ direction.
10. Who made the discovery that one DNA strand is synthesized discontinuously in fragments?
a) Francis Crick
b) Reiji Okazaki
c) James Watson
d) Louis Pasteur
View Answer
Explanation: Reiji Okazaki of Nagoya University, Japan; made the discovery that one of the strands during DNA replication is synthesized discontinuously (termed the lagging strand) in short fragments – Okazaki fragments.
11. Which enzyme joins the Okazaki fragments into a continuous strand?
a) gyrase
b) topoisomerase
c) ligase
d) helicase
View Answer
Explanation: The Okazaki fragments are synthesized discontinuously in 3’ to 5’ direction and then later joined into a continuous strand by the enzyme DNA ligase. This strand is termed as the lagging strand.
12. Which of the following enzyme is responsible for initiating DNA replication?
a) polymerase
b) ligase
c) primase
d) gyrase
View Answer
Explanation: The enzyme primase is a type of RNA polymerase that is responsible for initiating DNA replication. This enzyme constructs a short primer composed of RNA, that serve as primers for the synthesis of DNA by DNA polymerase.
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