How is DNA copied accurately before cell division?
Topic 6.2 Replication: explain how DNA is replicated semiconservatively, including the roles of the key enzymes and the leading and lagging strands.
A focused answer to AP Biology Topic 6.2, covering semiconservative replication, helicase, DNA polymerase, the leading and lagging strands, Okazaki fragments and ligase, with a worked replication problem.
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What this topic is asking
The College Board (Topic 6.2) wants you to explain how DNA is replicated semiconservatively, including the roles of helicase, DNA polymerase and ligase, and why the two new strands (the leading and lagging strands) are made differently.
Semiconservative replication
This conserves the information accurately, because each new strand is built directly from the sequence of an old one.
The enzymes and the fork
Leading and lagging strands
This difference is a direct consequence of the strands being antiparallel combined with DNA polymerase only working to . As the fork opens, the leading-strand template is exposed in the right direction for continuous copying toward the fork, but the lagging-strand template is exposed in the wrong direction, so polymerase has to keep restarting behind the fork and making short pieces.
Replication is also remarkably accurate. DNA polymerase proofreads as it works, removing a mismatched nucleotide before adding the next, and further repair systems fix most remaining errors. This high fidelity is essential: because each daughter cell must inherit a faithful copy of the genome, even a small error rate would accumulate damaging mutations over many divisions. The rare errors that do slip through are one source of the new variation that evolution acts on.
Try this
Q1. Define semiconservative replication. [1 point]
- Cue. Each new DNA molecule has one original strand and one newly synthesized strand.
Q2. Explain why the lagging strand is made in fragments. [2 points]
- Cue. DNA polymerase works only to ; because the strands are antiparallel, the lagging strand template runs the wrong way, so synthesis proceeds away from the fork in short Okazaki fragments.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (long FRQ excerpt). (a) Explain what is meant by semiconservative replication. (b) Describe the role of DNA polymerase, and explain why one new strand (the lagging strand) is made in fragments while the other (the leading strand) is made continuously.Show worked answer →
A 4-point explain-and-describe FRQ on replication.
(a) Explain (1 point): semiconservative means each new DNA molecule has one original (parental) strand and one newly made strand.
(b) Describe and explain (3 points): (1 point) DNA polymerase adds complementary nucleotides to a template strand, building the new strand only in the to direction; (1 point) because the strands are antiparallel, the leading strand template allows continuous synthesis toward the replication fork; (1 point) the lagging strand template runs the other way, so it is made in short Okazaki fragments away from the fork, which are then joined by ligase.
Markers reward defining semiconservative replication and linking the antiparallel strands to continuous versus fragmented synthesis.
AP 2017 (style)1 marksSection I (multiple choice). Which enzyme unwinds and separates the two strands of the DNA double helix during replication? (A) DNA polymerase. (B) Helicase. (C) Ligase. (D) RNA polymerase.Show worked answer →
The correct answer is (B).
Helicase unwinds and separates the two DNA strands at the replication fork. DNA polymerase (A) then adds nucleotides; ligase (C) joins Okazaki fragments; RNA polymerase (D) is used in transcription, not replication.
Related dot points
- Topic 6.1 DNA and RNA Structure: describe the structure of DNA and RNA and explain how it suits their role in storing and transmitting genetic information.
A focused answer to AP Biology Topic 6.1, covering the double helix, antiparallel strands, complementary base pairing, the sugar-phosphate backbone, and the differences between DNA and RNA, with a worked base-pairing calculation.
- Topic 6.3 Transcription and RNA Processing: explain how RNA polymerase transcribes a gene into mRNA and how the primary transcript is processed in eukaryotes.
A focused answer to AP Biology Topic 6.3, covering RNA polymerase, the template strand, the differences between transcription and replication, and eukaryotic RNA processing (cap, tail, splicing), with a worked transcription example.
- Topic 6.7 Mutations: explain the types of mutations and how they affect gene products, phenotype and the variation available to a population.
A focused answer to AP Biology Topic 6.7, covering point mutations (silent, missense, nonsense), frameshift mutations, chromosomal mutations, their effects on proteins and phenotype, and their role as the source of new variation, with a worked example.
- Topic 4.5 Cell Cycle: describe the phases of the cell cycle, including interphase and mitosis, and explain how the events of each phase produce two genetically identical cells.
A focused answer to AP Biology Topic 4.5, covering G1, S, G2, the phases of mitosis, cytokinesis and G0, and how the cycle produces two genetically identical daughter cells, with a worked timing calculation.
- Topic 1.6 Nucleic Acids: describe the structural similarities and differences between DNA and RNA and explain how the directionality and base pairing of nucleic acids support their function.
A focused answer to AP Biology Topic 1.6, covering nucleotide structure, the antiparallel double helix, base pairing, the 5' to 3' directionality, and the structural differences between DNA and RNA.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)