AP Biology: DNA Replication Practice Questions
Test yourself on AP Biology: DNA Replication with 12 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
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1. After one round of DNA replication, what does the semiconservative model predict about each daughter DNA molecule?
Explanation. In semiconservative replication, the two parental strands separate and each serves as a template. Complementary synthesis produces two daughter molecules, each composed of one original strand paired with one newly made strand.
2. A short template DNA strand has the sequence 3'-TACGGA-5'. Which newly synthesized strand is complementary and antiparallel to it?
Explanation. DNA polymerase reads the template in the 3'-to-5' direction and builds the complementary strand 5'-to-3'. Applying A-T and C-G base pairing to 3'-TACGGA-5' gives 5'-ATGCCT-3'.
3. At a replication fork, which enzyme directly separates the parental DNA strands by disrupting hydrogen bonds between complementary bases?
Explanation. Helicase unwinds the double helix at the replication fork by separating paired bases. Primase makes RNA primers, DNA polymerase extends DNA, and ligase seals breaks between adjacent DNA fragments.
4. Why is one new strand synthesized as Okazaki fragments at a typical replication fork?
Explanation. The parental templates are antiparallel, but DNA polymerase can extend only from a free 3' end. On the template oriented opposite fork movement, repeated priming allows short fragments to be synthesized 5'-to-3' and later joined.
5. A mutation disables primase but leaves helicase and DNA polymerase functional. Which immediate effect is most likely during DNA replication?
Explanation. Primase supplies short RNA primers with free 3' hydroxyl groups that DNA polymerases can extend. Without those primers, initiation on the leading strand and repeated initiation on the lagging strand are strongly impaired.
6. A cell produces normal-length Okazaki fragments, but many unsealed breaks remain between adjacent fragments after replication. Which enzyme is most directly defective?
Explanation. DNA ligase forms phosphodiester bonds that seal remaining nicks between adjacent DNA fragments after primer replacement. Normal fragment production indicates that unwinding, priming, and polymerase extension have already occurred.
7. Researchers briefly expose replicating cells to a labeled DNA precursor. Label first appears at many separated sites along each eukaryotic chromosome. Which conclusion is best supported?
Explanation. Simultaneous labeled regions at separated chromosomal sites indicate that DNA synthesis begins at multiple origins. Multiple replication bubbles shorten the time required to copy a long eukaryotic chromosome.
8. Two cultures begin with DNA containing only heavy nitrogen. Both are moved to light-nitrogen medium. After one complete replication round, all DNA molecules form one band of intermediate density. Which model is most consistent with this observation?
Explanation. After one round under semiconservative replication, every daughter duplex contains one heavy parental strand and one newly synthesized light strand, giving a single intermediate-density band. Conservative replication would instead produce separate heavy and light bands.
9. A double-stranded DNA region contains 300 total nucleotides, and 90 of them are adenine. How many cytosine nucleotides are present in the region?
Explanation. In double-stranded DNA, adenine pairs with thymine, so 90 adenines imply 90 thymines. The remaining 120 nucleotides are equally divided between cytosine and guanine, leaving 60 cytosines.
10. A DNA polymerase variant has normal synthesis speed but lacks proofreading activity. Compared with normal cells, what outcome is most likely?
Explanation. Proofreading removes many incorrectly paired nucleotides during DNA synthesis. If polymerization continues but proofreading is absent, replication can proceed while leaving a higher frequency of mismatches for other repair systems to address.
11. Cells are treated with a drug that blocks topoisomerase activity during S phase. Which problem is most likely to develop ahead of advancing replication forks?
Explanation. As helicase unwinds DNA, rotation and torsional strain build ahead of the fork. Topoisomerases normally relieve that strain by transiently breaking and rejoining DNA, so inhibiting them causes excess supercoiling to accumulate.
12. A researcher compares equal populations of cells before and after S phase. No cell division occurs during the interval. Which change is expected after successful DNA replication?
Explanation. S phase copies each chromosome to form two sister chromatids joined at one centromere. The amount of DNA therefore doubles, but chromosome number counted by centromeres and the number of homologous sets remain unchanged until chromatids separate.
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FAQ
Why can DNA polymerase synthesize DNA only in the 5'-to-3' direction?
DNA polymerase adds each incoming nucleotide to the free 3' hydroxyl group of the growing strand. This chemistry makes the new strand extend in the 5'-to-3' direction while the template is read antiparallel to it.
How do the leading and lagging strands differ?
Both strands are synthesized 5'-to-3'. The leading strand is extended continuously toward the replication fork, while the lagging strand is built away from the fork in repeatedly primed Okazaki fragments that DNA ligase joins.
Does DNA replication change chromosome number or ploidy?
Replication doubles DNA content by producing sister chromatids, but it does not by itself change chromosome number when chromosomes are counted by centromeres, and it does not change the number of homologous chromosome sets.