CLEP Biology DNA Structure and Replication 2 — Questions and Answers
Question 1: Which enzyme is responsible for unwinding the DNA double helix during replication?
- DNA polymerase
- Helicase (Correct answer)
- Ligase
- Primase
Correct answer: Helicase
Helicase breaks hydrogen bonds between base pairs to unwind and separate the two DNA strands at the replication fork.
Question 2: What is the term for the short RNA sequences that initiate DNA synthesis?
- Okazaki fragments
- Telomeres
- Primers (Correct answer)
- Exons
Correct answer: Primers
Primers are short RNA sequences synthesized by primase that provide a free 3'-OH end for DNA polymerase to begin adding nucleotides.
Question 3: The leading strand is synthesized continuously because it grows in the same direction as:
- The lagging strand
- The replication fork movement (Correct answer)
- The primer removal
- The Okazaki fragment ligation
Correct answer: The replication fork movement
The leading strand is synthesized continuously in the 5' to 3' direction toward the replication fork, matching its direction of movement.
Question 4: Which of the following correctly describes the antiparallel nature of DNA?
- Both strands run 5' to 3' in the same direction
- One strand runs 5' to 3' while the complementary strand runs 3' to 5' (Correct answer)
- DNA strands run perpendicular to each other
- The two strands have identical base sequences
Correct answer: One strand runs 5' to 3' while the complementary strand runs 3' to 5'
Antiparallel means the two strands of DNA run in opposite directions: one 5'→3' and its complement 3'→5'.
Question 5: Chargaff's rules state that in any DNA molecule:
- A + G = C + T (Correct answer)
- The amount of A equals C and the amount of G equals T
- Purines always pair with purines
- The ratio of A to T varies by species
Correct answer: A + G = C + T
Chargaff's rules state that %A = %T and %G = %C, which means the total purines (A+G) equal total pyrimidines (C+T).
Question 6: What does the enzyme DNA ligase do during replication?
- Synthesizes new DNA strands
- Removes RNA primers
- Joins Okazaki fragments by forming phosphodiester bonds (Correct answer)
- Unwinds the double helix
Correct answer: Joins Okazaki fragments by forming phosphodiester bonds
DNA ligase seals the nicks between Okazaki fragments on the lagging strand by forming phosphodiester bonds after the RNA primers are replaced with DNA.
Question 7: Which of the following is NOT a component of a DNA nucleotide?
- Deoxyribose sugar
- Phosphate group
- Nitrogenous base
- Ribose sugar (Correct answer)
Correct answer: Ribose sugar
DNA nucleotides contain deoxyribose (lacking an -OH on the 2' carbon), not ribose, which is found in RNA nucleotides.
CLEP Biology DNA Structure and Replication 2
Which enzyme is responsible for unwinding the DNA double helix during replication?