How is the cell cycle organised and controlled, and how does mitosis produce two genetically identical cells?
Describe the cell cycle and the stages of mitosis, and explain the significance of mitosis and the consequences of uncontrolled division
A focused answer to the H2 Biology Cell Biology outcome on the cell cycle and mitosis. Interphase and its sub-phases, the stages of mitosis, the role of checkpoints, the genetic significance of mitosis, and how loss of control leads to cancer.
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What this dot point is asking
SEAB wants you to describe the cell cycle (interphase and mitosis), to set out the four stages of mitosis in order, to explain how the process produces two genetically identical cells, and to explain how the cycle is controlled and what happens when control is lost. It connects cell biology to DNA replication and to the genetics of cancer.
The answer
The cell cycle
The cell cycle is the ordered sequence of events from one cell division to the next. It has two main parts.
- Interphase, the longest phase, has three sub-phases: G1 (growth and protein synthesis), S (DNA replication, so each chromosome becomes two identical sister chromatids), and G2 (further growth and preparation for division).
- Mitosis (M phase), the division of the nucleus, followed by cytokinesis, the division of the cytoplasm.
The four stages of mitosis
- Prophase. Chromosomes condense and become visible as two sister chromatids joined at a centromere; the nuclear envelope breaks down and the spindle begins to form.
- Metaphase. Chromosomes line up at the equator of the spindle (the metaphase plate); spindle fibres attach to the centromeres.
- Anaphase. Centromeres divide and the spindle fibres shorten, pulling sister chromatids to opposite poles.
- Telophase. Chromosomes decondense at each pole and two new nuclear envelopes form; cytokinesis then divides the cell into two.
Why the daughter cells are identical
DNA was replicated accurately in S phase, so each chromosome consists of two identical sister chromatids. Mitosis separates these equally, so each daughter cell receives one exact copy of every chromosome. The result is two cells genetically identical to each other and to the parent.
Control and its loss
Checkpoints verify that conditions are correct before the cycle proceeds, halting division if DNA is damaged or incompletely replicated. The cycle is regulated by genes including proto-oncogenes and tumour suppressor genes. Mutations that disable this control allow uncontrolled division, forming a tumour, which may become cancer.
Examples in context
Example 1. Wound healing. When tissue is damaged, cells at the wound edge undergo repeated mitosis to replace the lost cells with genetically identical ones. This relies on the accurate replication and equal separation that mitosis guarantees.
Example 2. Proto-oncogenes and tumour suppressors. Proto-oncogenes normally promote division and tumour suppressor genes normally restrain it. A gain-of-function mutation in a proto-oncogene or a loss-of-function mutation in a tumour suppressor can release the brakes on the cycle, illustrating how cancer is fundamentally a disease of cell-cycle control.
Try this
Q1. Name the sub-phase of interphase in which DNA is replicated. [1 mark]
- Cue. The S phase (synthesis phase).
Q2. Explain why mitosis is important in the growth of a multicellular organism. [2 marks]
- Cue. It produces new cells that are genetically identical to existing cells, increasing cell number while maintaining the same genetic information throughout the body.
Q3. State what is meant by a checkpoint in the cell cycle. [1 mark]
- Cue. A control point at which the cell verifies that conditions (such as correct DNA replication or spindle attachment) are met before the cycle is allowed to continue.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksDescribe the events of the four stages of mitosis, and explain how mitosis ensures that the two daughter cells are genetically identical to the parent cell.Show worked answer →
Examiners want the four stages in order, then the genetic point.
Prophase: chromosomes condense and become visible as two sister chromatids joined at a centromere; the nuclear envelope breaks down and the spindle forms.
Metaphase: chromosomes line up at the equator (the metaphase plate), with spindle fibres attached to the centromeres.
Anaphase: the centromeres divide and the spindle fibres shorten, pulling the sister chromatids to opposite poles.
Telophase: chromosomes decondense at each pole and two nuclear envelopes reform; cytokinesis then divides the cytoplasm into two cells.
Mitosis produces genetically identical cells because DNA was accurately replicated during interphase to give two identical sister chromatids per chromosome, and these are separated equally so each daughter receives one copy of every chromosome.
Markers reward the correct ordered events of each stage and a clear explanation linking prior replication and equal separation to genetic identity.
Original4 marksExplain the role of checkpoints in the cell cycle and describe what can happen if this control is lost.Show worked answer →
The answer should explain control and then the consequence of its failure.
Checkpoints are control points in the cell cycle at which the cell verifies that conditions are correct before proceeding. For example, a checkpoint before mitosis checks that the DNA has been correctly and fully replicated and is undamaged, and another checks that chromosomes are correctly attached to the spindle before separation. If a check fails, the cycle is halted so the problem can be repaired or the cell can be destroyed by apoptosis.
If this control is lost, for example through mutations in the genes that regulate the cycle (proto-oncogenes and tumour suppressor genes), cells may divide repeatedly and uncontrollably. This produces a mass of cells called a tumour, and if the cells invade other tissues the result is cancer.
Markers reward the purpose of checkpoints, at least one specific check, the role of regulatory genes, and the link between loss of control and tumour formation or cancer.
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