# Stages of the cell cycle

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u5-stages-of-the-cell-cycle/

This module covers the ordered sequence of events eukaryotic cells undergo to produce genetically identical daughter cells. You will learn key events of interphase, mitosis, cytokinesis and the role of regulatory checkpoints.

**Prerequisites:** [Structure of eukaryotic chromosomes](https://www.owlsprep.com/study/cie-9700-u5-chromosome-structure/); Basic understanding of cell division

## Learning objectives

- Identify the main stages of the eukaryotic cell cycle
- Describe key events occurring in each sub-stage of interphase and mitosis
- Explain the role of cell cycle checkpoints
- Compare cytokinesis in animal and plant cells

## Overview of the Cell Cycle

**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.

1. Interphase (G₁, S, G₂ sub-phases)
2. Mitosis (prophase, metaphase, anaphase, telophase)
3. Cytokinesis (cytoplasm division)

**Worked example:** 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:
2. $$\frac{1000 - 910}{1000} = 0.09$$
3. Multiply proportion by total cycle length to get mitosis duration:
4. $$0.09 \times 24 = 2.16 \text{ hours}$$

## Interphase and Cell Cycle Checkpoints

**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:

- **G₁ (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
- **G₂ (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 G₁, end of G₂, and during metaphase of mitosis.

**Worked example:** What happens if a cell has irreparable damaged DNA detected during G₂?

1. The G₂ checkpoint halts the cell cycle to prevent the cell entering mitosis with damaged DNA.
2. If damage cannot be repaired, the cell initiates apoptosis (programmed cell death) to prevent abnormal daughter cells from forming.

> **info**
>
> Cells that bypass checkpoints and divide uncontrollably can develop into tumours and cancer.

## Stages of Mitosis

**Mitosis** — Nuclear division that produces two genetically identical daughter nuclei with the same chromosome number as the parent cell

**Worked example:** Name and describe the stage of mitosis where sister chromatids separate.

1. This stage is anaphase. It follows metaphase, where chromosomes were aligned at the cell equator.
2. 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.

## Cytokinesis: Animal vs Plant Cells

**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.

**Worked example:** Compare how cytokinesis occurs in animal and plant cells.

1. 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.
2. 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.

> **tip**
>
> Remember: Most plant cells do not have centrioles, so centrioles are not required for spindle formation in flowering plants.

## Common pitfalls

- **Wrong:** Claiming interphase is a resting phase between cell divisions.
  - Why it fails: Interphase is highly active, with cell growth, DNA replication and protein synthesis occurring continuously.
  - Correct: Interphase is the active preparation phase for cell division.
- **Wrong:** Stating DNA replication occurs in prophase of mitosis.
  - Why it fails: DNA replication is completed during S phase of interphase, before mitosis begins.
  - Correct: Chromosomes are already duplicated at the start of mitosis.
- **Wrong:** Claiming sister chromatids separate in metaphase.
  - Why it fails: Chromosomes only align at the cell equator during metaphase; separation happens after the spindle checkpoint.
  - Correct: Sister chromatid separation occurs during anaphase.
- **Wrong:** Claiming cytokinesis is a stage of mitosis.
  - Why it fails: Mitosis refers only to nuclear division; cytokinesis is cytoplasmic division that occurs after mitosis.
  - Correct: The M phase of the cell cycle includes both mitosis and cytokinesis.
- **Wrong:** Stating all cells proceed to mitosis after G1.
  - Why it fails: If conditions are unfavourable, cells exit the cycle into G0, a non-dividing resting state.
  - Correct: Cells only proceed to S phase after passing the G1 checkpoint.

## 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 |

## 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.

- [Mitosis and cytokinesis](https://www.owlsprep.com/study/cie-9700-u5-mitosis-and-cytokinesis/)
- [Cell cycle control and cancer](https://www.owlsprep.com/study/cie-9700-u5-cell-cycle-control-and-cancer/)
- [Stem cells](https://www.owlsprep.com/study/cie-9700-u5-stem-cells/)

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