CLEP - Biology DNA Structure and Replication Questions and Answers — Questions and Answers
Question 1: During DNA replication, the enzyme responsible for unzipping the double helix by breaking the hydrogen bonds between complementary bases is known as:
- DNA Polymerase
- Helicase (Correct answer)
- Ligase
- Primase
Correct answer: Helicase
Helicase is the enzyme that unwinds the DNA double helix at the replication fork, separating the two parental strands so they can be used as templates for the synthesis of new DNA strands. DNA Polymerase synthesizes the new DNA strands, Ligase joins DNA fragments together, and Primase synthesizes RNA primers.
Question 2: A segment of DNA is found to have 30% adenine. According to Chargaff's rules, what percentage of the segment is composed of guanine?
- 30%
- 70%
- 20% (Correct answer)
- 40%
Correct answer: 20%
According to Chargaff's base-pairing rules, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Therefore, the percentage of adenine equals the percentage of thymine. If A = 30%, then T = 30%, for a total of 60%. The remaining 40% must be composed of guanine and cytosine. Since G pairs with C, they must be in equal amounts. Thus, guanine makes up half of the remaining 40%, which is 20%.
Question 3: The Meselson-Stahl experiment provided evidence for which model of DNA replication, demonstrating that each new DNA molecule consists of one parental strand and one newly synthesized strand?
- Conservative replication
- Dispersive replication
- Semi-conservative replication (Correct answer)
- Liberal replication
Correct answer: Semi-conservative replication
The Meselson-Stahl experiment confirmed the semi-conservative model of DNA replication. This model accurately describes how each new DNA double helix is a hybrid, containing one strand from the original molecule and one newly synthesized strand. The conservative model proposed the original molecule remained intact, while the dispersive model suggested the new molecules were a mix of old and new DNA fragments.
Question 4: In a laboratory setting, a scientist observes that the joining of Okazaki fragments is impaired during DNA replication. A mutation in the gene coding for which of the following enzymes is the most likely cause of this issue?
- Helicase
- DNA Polymerase III
- Topoisomerase
- DNA Ligase (Correct answer)
Correct answer: DNA Ligase
DNA ligase is the enzyme responsible for joining the Okazaki fragments on the lagging strand into a continuous DNA strand by forming phosphodiester bonds. A mutation in the gene for DNA ligase would prevent this sealing process. Helicase unwinds DNA, DNA Polymerase III synthesizes the new DNA, and Topoisomerase relieves supercoiling.
Question 5: Which of the following statements accurately describes the synthesis of the lagging strand during DNA replication?
- It is synthesized continuously in the 5' to 3' direction toward the replication fork.
- It is synthesized discontinuously as a series of Okazaki fragments. (Correct answer)
- It uses the leading strand as a template for its synthesis.
- It does not require an RNA primer to initiate synthesis.
Correct answer: It is synthesized discontinuously as a series of Okazaki fragments.
The lagging strand is synthesized discontinuously in the direction away from the replication fork. Because DNA polymerase can only add nucleotides in the 5' to 3' direction, synthesis on the lagging strand must be done in short segments called Okazaki fragments, each initiated by its own RNA primer.
Question 6: A nucleotide, the basic building block of DNA, is composed of which three components?
- A ribose sugar, a phosphate group, and an amino acid
- A deoxyribose sugar, a phosphate group, and a nitrogenous base (Correct answer)
- A ribose sugar, a nitrogenous base, and an amino acid
- A deoxyribose sugar, a carboxyl group, and a nitrogenous base
Correct answer: A deoxyribose sugar, a phosphate group, and a nitrogenous base
Each nucleotide in a DNA molecule consists of three parts: a five-carbon sugar called deoxyribose, a phosphate group, and one of four nitrogenous bases (adenine, guanine, cytosine, or thymine). Ribose is the sugar found in RNA, not DNA.
CLEP - Biology DNA Structure and Replication Questions and Answers
During DNA replication, the enzyme responsible for unzipping the double helix by breaking the hydrogen bonds between complementary bases is known as: