Stages of the cell cycle
BiologyΒ· 45 min read
1. Overview of the Cell Cycleβ β ββββ± 10 min
Cell Cycle
The regulated sequence of events that occurs between one cell division and the next, producing two genetically identical daughter cells
The cell cycle follows a highly regulated order, alternating between a long growth phase and a shorter division phase. It can be split into three core sequential stages.
Interphase (Gβ, S, Gβ sub-phases)
Mitosis (prophase, metaphase, anaphase, telophase)
Cytokinesis (cytoplasm division)
A scientist observes 1000 dividing onion root tip cells, with 910 in interphase. If the total cell cycle is 24 hours, calculate the duration of mitosis.
- 1
Calculate the proportion of cells in mitosis:
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- 3
Multiply proportion by total cycle length to get mitosis duration:
- 4
2. Interphase and Cell Cycle Checkpointsβ β β βββ± 15 min
Interphase
The longest, most active phase of the cell cycle where the cell prepares for division
Interphase is split into three distinct sub-phases, each with specific functions:
Gβ (Gap 1): Cell grows, synthesizes proteins and organelles, checks for sufficient size and nutrients
S (Synthesis): DNA replicates, producing two identical sister chromatids connected at the centromere
Gβ (Gap 2): Cell continues growing, checks for complete DNA replication and repairs damage
Checkpoints are regulatory control mechanisms that prevent errors. Key checkpoints occur at the end of Gβ, end of Gβ, and during metaphase of mitosis.
What happens if a cell has irreparable damaged DNA detected during Gβ?
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The Gβ checkpoint halts the cell cycle to prevent the cell entering mitosis with damaged DNA.
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If damage cannot be repaired, the cell initiates apoptosis (programmed cell death) to prevent abnormal daughter cells from forming.
3. Stages of Mitosisβ β β βββ± 20 min
Mitosis
Nuclear division that produces two genetically identical daughter nuclei with the same chromosome number as the parent cell
Name and describe the stage of mitosis where sister chromatids separate.
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This stage is anaphase. It follows metaphase, where chromosomes were aligned at the cell equator.
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Key events: 1. The centromere joining each pair of sister chromatids divides. 2. Spindle fibres shorten, pulling genetically identical sister chromatids to opposite poles of the cell. 3. By the end of anaphase, there is one complete set of chromosomes at each pole.
Stage | Key Events |
|---|---|
Prophase | Chromosomes condense, nucleolus disappears, nuclear envelope breaks down, spindle forms |
Metaphase | Chromosomes align at cell equator, spindle fibres attach to centromeres |
Telophase | Chromosomes decondense, nuclear envelopes reform, nucleoli reappear, spindle breaks down |
Exam tip:
CIE exams frequently ask you to identify mitosis stages from diagrams or micrographs. Always check chromosome position and whether centromeres have divided.
4. Cytokinesis: Animal vs Plant Cellsβ β ββββ± 10 min
Cytokinesis
Division of the cytoplasm after nuclear division to produce two separate daughter cells
Cytokinesis differs between animal and plant cells due to the rigid cell wall present in plant cells.
Compare how cytokinesis occurs in animal and plant cells.
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In animal cells: A cleavage furrow forms around the cell equator, pulled inwards by actin microfilaments. The furrow deepens until the cytoplasm is completely split into two daughter cells.
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In plant cells: Golgi vesicles carry cell wall material and assemble at the equator, fusing to form a new cell plate. The cell plate develops into a new cell wall and cell membrane, splitting the cell into two daughter cells.
5. Common Pitfalls
Wrong move:
Claiming interphase is a resting phase between cell divisions.
Why:
Interphase is highly active, with cell growth, DNA replication and protein synthesis occurring continuously.
Correct move:
Interphase is the active preparation phase for cell division.
Wrong move:
Stating DNA replication occurs in prophase of mitosis.
Why:
DNA replication is completed during S phase of interphase, before mitosis begins.
Correct move:
Chromosomes are already duplicated at the start of mitosis.
Wrong move:
Claiming sister chromatids separate in metaphase.
Why:
Chromosomes only align at the cell equator during metaphase; separation happens after the spindle checkpoint.
Correct move:
Sister chromatid separation occurs during anaphase.
Wrong move:
Claiming cytokinesis is a stage of mitosis.
Why:
Mitosis refers only to nuclear division; cytokinesis is cytoplasmic division that occurs after mitosis.
Correct move:
The M phase of the cell cycle includes both mitosis and cytokinesis.
Wrong move:
Stating all cells proceed to mitosis after G1.
Why:
If conditions are unfavourable, cells exit the cycle into G0, a non-dividing resting state.
Correct move:
Cells only proceed to S phase after passing the G1 checkpoint.
6. Quick Reference Cheatsheet
Phase | Key Events |
|---|---|
G1 | Cell growth, checkpoint for size/nutrients |
S | DNA replication, forms sister chromatids |
G2 | Growth, checkpoint for DNA damage |
Prophase | Chromosomes condense, spindle forms |
Metaphase | Chromosomes align at cell equator |
Anaphase | Sister chromatids separate to opposite poles |
Telophase | New nuclei form, chromosomes decondense |
Cytokinesis (animal) | Cleavage furrow splits cytoplasm |
Cytokinesis (plant) | Cell plate forms new cell wall |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2023 Β· 1
MCQ on interphase events
- 2022 Β· 2
Identify mitosis stages from diagram
- 2021 Β· 1
MCQ on G2 checkpoint function
What's Next
Understanding the stages of the cell cycle is the foundation for all further study of cell division, growth and cancer in eukaryotes. The genetic stability provided by the cell cycle and mitosis is critical for organismal growth, tissue repair, and asexual reproduction. Errors at any stage, particularly checkpoints, can lead to mutations and uncontrolled cell division, which you will explore when learning about the development of cancer. Next, you will build on this knowledge to explore the detailed behaviour of chromosomes during mitosis, and how mitosis differs from meiosis for sexual reproduction.