CLEP Biology DNA Structure and Replication 3 — Questions and Answers
Question 1: During replication, the lagging strand is synthesized as Okazaki fragments because:
- DNA polymerase can only add nucleotides in the 3' to 5' direction
- DNA polymerase can only add nucleotides in the 5' to 3' direction, which runs away from the fork on that strand (Correct answer)
- Helicase moves too slowly on the lagging strand
- The lagging strand has no template
Correct answer: DNA polymerase can only add nucleotides in the 5' to 3' direction, which runs away from the fork on that strand
Since DNA polymerase synthesizes only 5'→3', the lagging strand template (running 5'→3' toward the fork) must be copied in discontinuous fragments.
Question 2: What is the role of single-strand binding proteins (SSBPs) during DNA replication?
- They catalyze the formation of phosphodiester bonds
- They stabilize the unwound single strands and prevent reannealing (Correct answer)
- They remove RNA primers from the template
- They recruit RNA polymerase to the origin
Correct answer: They stabilize the unwound single strands and prevent reannealing
SSBPs coat the separated single-stranded DNA at the replication fork, keeping the strands apart so they can serve as templates.
Question 3: The Watson-Crick model of DNA describes its structure as a:
- Single-stranded helix
- Right-handed antiparallel double helix (Correct answer)
- Left-handed parallel double helix
- Triple helix held by covalent bonds
Correct answer: Right-handed antiparallel double helix
Watson and Crick described DNA as a right-handed antiparallel double helix stabilized by hydrogen bonds between complementary bases.
Question 4: In the Meselson-Stahl experiment, which replication model was supported?
- Conservative replication
- Dispersive replication
- Semi-conservative replication (Correct answer)
- Rolling circle replication
Correct answer: Semi-conservative replication
Meselson and Stahl showed that after one generation, all DNA contained one heavy and one light strand, confirming the semi-conservative model.
Question 5: Which purine base pairs with cytosine in DNA?
- Adenine
- Guanine (Correct answer)
- Thymine
- Uracil
Correct answer: Guanine
Guanine (a purine) pairs with cytosine (a pyrimidine) via three hydrogen bonds in the DNA double helix.
Question 6: What is a replication origin (ori)?
- The end of a chromosome where replication terminates
- A specific DNA sequence where replication is initiated (Correct answer)
- The enzyme that begins DNA synthesis
- The point where the two new strands are joined
Correct answer: A specific DNA sequence where replication is initiated
A replication origin is a specific DNA sequence where replication machinery assembles and strand separation begins.
Question 7: Topoisomerases relieve replication stress by:
- Synthesizing new nucleotides ahead of the fork
- Cutting and rejoining DNA to relieve supercoiling tension ahead of the replication fork (Correct answer)
- Replacing RNA primers with DNA
- Joining the two daughter chromosomes after replication
Correct answer: Cutting and rejoining DNA to relieve supercoiling tension ahead of the replication fork
As helicase unwinds DNA, positive supercoils build up ahead of the fork; topoisomerases cut the DNA to relieve this torsional stress and then reseal it.
CLEP Biology DNA Structure and Replication 3
During replication, the lagging strand is synthesized as Okazaki fragments because: