What are the phases of the cell cycle, and what happens in each?
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.
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What this topic is asking
The College Board (Topic 4.5) wants you to describe the phases of the cell cycle, interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis), and explain how the events of each phase produce two genetically identical daughter cells. You should be able to say what happens in each phase and interpret data on phase durations.
Interphase
DNA replication in S phase is what guarantees that each daughter cell can receive a complete, identical copy of the genome.
The mitotic phase
The mitotic phase separates the duplicated chromosomes and then divides the cell.
In animal cells, cytokinesis pinches the membrane inward (a cleavage furrow); in plant cells, a new cell plate forms between the two cells.
G0 and why most cells are in interphase
Because interphase is much longer than the mitotic phase, at any moment most cells in a tissue sample are in interphase. The proportion of cells seen in each phase reflects how long that phase lasts, which is the basis of a common data question: count the cells in each phase, and the fraction in a phase equals the fraction of the cycle time spent in it. This works because a large, randomly sampled population of cells is spread across the cycle in proportion to each phase's duration, so a long phase simply contains more cells at any instant.
Try this
Q1. Identify the phase in which DNA is replicated and state what each chromosome becomes. [2 points]
- Cue. S phase; each chromosome becomes two identical sister chromatids joined at a centromere.
Q2. Explain why mitosis produces two genetically identical daughter cells. [2 points]
- Cue. DNA is copied exactly in S phase, then mitosis separates one identical copy of each chromosome into each new cell, so both receive the same genome.
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 2020 (style)4 marksSection II (long FRQ excerpt, data). In a sample of 1000 dividing cells, 800 are in interphase, 80 in prophase, 30 in metaphase, 40 in anaphase and 50 in telophase. The whole cycle takes 20 hours. (a) Calculate the time spent in interphase. (b) Explain why most cells in the sample are found in interphase.Show worked answer →
A 4-point quantitative-and-explain FRQ on the cell cycle.
(a) Calculate (2 points): fraction in interphase ; (1 point) time hours. (1 point)
(b) Explain (2 points): (1 point) the proportion of cells in a phase reflects how long that phase lasts; (1 point) interphase (G1, S, G2) is the longest part of the cycle because the cell grows, replicates its DNA and prepares for division, so at any moment most cells are in it.
Markers reward the correct calculation and linking the proportion of cells in a phase to the duration of that phase.
AP 2018 (style)1 marksSection I (multiple choice). During which phase of the cell cycle is DNA replicated? (A) G1. (B) S phase. (C) G2. (D) Mitosis.Show worked answer →
The correct answer is (B).
DNA is replicated during S (synthesis) phase of interphase, producing two identical copies of each chromosome (sister chromatids). G1 and G2 are growth and preparation phases, and mitosis separates the already-copied chromosomes.
Related dot points
- Topic 4.6 Regulation of the Cell Cycle: explain how checkpoints and regulatory molecules control progression through the cell cycle, and how loss of control leads to cancer.
A focused answer to AP Biology Topic 4.6, covering cell-cycle checkpoints, cyclins and cyclin-dependent kinases, growth factors, the link to signal transduction, and how loss of regulation causes cancer.
- Topic 4.3 Signal Transduction Pathways: explain how signalling pathways relay and amplify a signal to produce a response, and how mutations or chemicals that change the pathway affect the cell.
A focused answer to AP Biology Topic 4.3, covering relay molecules, phosphorylation cascades, signal amplification, the variety of cellular responses, and how mutations and chemicals alter pathways.
- Topic 4.4 Feedback: explain how negative feedback maintains homeostasis and how positive feedback amplifies a response, using examples from cellular and organismal systems.
A focused answer to AP Biology Topic 4.4, covering negative feedback and homeostasis, positive feedback and amplification, set points, and how feedback data are analyzed, with a worked chi-square example.
- 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.
- Topic 2.1 Cell Structure: Subcellular Components: describe the structures and functions of the subcellular components and organelles of prokaryotic and eukaryotic cells.
A focused answer to AP Biology Topic 2.1, covering the organelles of eukaryotic cells (nucleus, ribosomes, ER, Golgi, mitochondria, chloroplasts, lysosomes, vacuoles) and the endomembrane system, with structure-to-function reasoning.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)