The Cell Cycle and Mitosis
The cell cycle consists of interphase, where the cell grows and copies its DNA, and mitosis, where the nucleus divides through prophase, metaphase, anaphase and telophase to produce two genetically identical diploid daughter cells.
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The cell cycle
Content standard 6.2 covers the full cell cycle, not just mitosis itself. Interphase is the longest part of the cycle and has three stages: G1, where the cell grows and produces organelles and proteins; S, where DNA is replicated so each chromosome now consists of two identical sister chromatids; and G2, where the cell continues to grow and prepares for division.
Mitosis (M phase) then follows, dividing the nucleus, and cytokinesis divides the cytoplasm to complete the formation of two new cells.
The four stages of mitosis
- Prophase, chromatin condenses into visible chromosomes, each made of two sister chromatids; the nuclear membrane breaks down and spindle fibres begin to form.
- Metaphase, chromosomes line up individually along the equator (middle) of the cell, attached to spindle fibres from opposite poles.
- Anaphase, the centromere of each chromosome splits, and spindle fibres pull the sister chromatids apart toward opposite poles of the cell.
- Telophase, chromatids arrive at opposite poles and uncoil back into chromatin; a nuclear membrane re-forms around each set, creating two new nuclei.
Cytokinesis and significance of mitosis
After telophase, cytokinesis splits the cytoplasm to form two separate daughter cells. In animal cells, the cell membrane pinches inward at the middle (cleavage furrow) until it splits into two.
In plant cells, which have a rigid cell wall, a new cell plate forms across the middle and develops into a new cell wall separating the two cells. Mitosis is significant because it produces two daughter cells that are genetically identical to the parent cell and to each other, which is essential for growth, tissue repair and asexual reproduction.
Uncontrolled mitosis and how it is examined
When the mechanisms that normally control the rate of mitosis fail, cells may divide uncontrollably, forming a mass of abnormal cells known as a tumour, which is associated with cancer. Exam questions commonly show a diagram or micrograph of a cell and ask you to identify the stage of mitosis and describe the chromosome behaviour occurring at that stage, or ask you to sequence the stages of mitosis correctly and describe events at each one.
Summary of the four stages
The four stages of mitosis always follow the same order, remembered as PMAT. This table sets the main event of each stage beside how the chromosomes are arranged, which is what diagram questions test.
| Stage | Main event | Chromosome arrangement |
|---|---|---|
| Prophase | Chromosomes condense and become visible; nuclear membrane breaks down; spindle fibres form | Scattered, each made of two sister chromatids |
| Metaphase | Chromosomes attach to spindle fibres from opposite poles | Lined up singly along the equator |
| Anaphase | Centromeres split; sister chromatids are pulled apart | Chromatids moving to opposite poles |
| Telophase | Chromatids reach the poles and uncoil; nuclear membranes re-form | Two groups, one at each pole |
Worked exam-style question
Question. A diagram shows a cell during mitosis in which the chromosomes are lined up singly along the equator of the cell. (a) Name this stage.
(b) Name the stage that immediately follows, and describe what happens to the chromosomes in it. (c) A parent cell has 8 chromosomes.
State the number of chromosomes in each daughter cell produced by mitosis. (d) Explain why the two daughter cells are genetically identical to the parent cell.
Model answer. (a) Metaphase, because the chromosomes are arranged singly along the equator. (b) Anaphase: the centromere of each chromosome splits and the sister chromatids are pulled to opposite poles by the spindle fibres.
(c) 8 chromosomes in each daughter cell, the same as the parent cell. (d) During interphase the DNA is replicated to form identical sister chromatids; in anaphase these chromatids are separated equally, so each daughter cell receives an identical copy of every chromosome.
Practice question
Try this. The stages of mitosis are given here out of order: anaphase, telophase, prophase, metaphase. (a) Write the stages in the correct order.
(b) If the parent cell was diploid, state whether the daughter cells are diploid or haploid. (c) State one process in the body that depends on mitosis.
Exam tip
Key terms
- Mitosis, nuclear division producing two genetically identical diploid cells.
- Chromosome, a condensed structure of DNA carrying genes.
- Diploid, a cell with two complete sets of chromosomes.
- Haploid, a cell with one complete set of chromosomes.
- Meiosis, division producing four genetically varied haploid cells.
Source:SRC-DSKP-EN
Frequently asked questions
What happens to the chromosomes during each stage of mitosis?
How does cytokinesis differ between animal cells and plant cells?
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