How does a cell copy and divide its contents accurately during the cell cycle?
The cell cycle, including interphase (DNA replication) and mitosis as a controlled process producing two genetically identical daughter cells; the stages of mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis; the calculation of a mitotic index; the role of mitosis in growth and repair, and how uncontrolled cell division can lead to the formation of tumours and cancer.
A focused answer to the AQA 3.2 dot point on the cell cycle and mitosis. Covers interphase and the four mitotic stages, cytokinesis, mitotic index calculation, and how uncontrolled division produces tumours and cancer.
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What this dot point is asking
AQA wants you to describe the cell cycle, recognise and order the stages of mitosis, calculate a mitotic index, and explain how loss of control over mitosis leads to tumours and cancer.
The answer
The cell cycle
The cell cycle is the regulated sequence a cell follows to grow and divide.
Interphase is divided into three parts:
- G1 (first growth): the cell grows and makes proteins and organelles.
- S (synthesis): DNA replicates, so each chromosome becomes two identical sister chromatids joined at a centromere.
- G2 (second growth): the cell continues to grow and checks the replicated DNA for errors.
Interphase is the longest stage; most cells spend most of their life here.
The stages of mitosis
Mitosis produces two genetically identical diploid daughter cells. Remember the order with the four stages:
- Prophase. Chromosomes condense and become visible as two sister chromatids. The nuclear envelope breaks down and spindle fibres form from the centrioles.
- Metaphase. Chromosomes line up along the equator (the metaphase plate) and attach to spindle fibres by their centromeres.
- Anaphase. The centromeres divide and the spindle fibres pull the sister chromatids to opposite poles. This requires ATP. The V-shape of separating chromatids is a recognition clue.
- Telophase. Chromatids reach the poles, decondense, and a new nuclear envelope forms around each set.
Cytokinesis then divides the cytoplasm into two separate cells. In animal cells the membrane pinches inwards; in plant cells a new cell wall (cell plate) forms.
Mitotic index
The mitotic index measures the proportion of cells actively dividing:
A high mitotic index indicates rapid division, seen in growing tissue (a root tip) or in a tumour.
Mitosis in growth, repair and cancer
Mitosis is essential for growth, repair of damaged tissue, and asexual reproduction. Because daughter cells are genetically identical, it preserves the genome.
Cell division is normally tightly controlled by genes. Cancer arises when control is lost:
- Proto-oncogenes normally stimulate division; a mutation can turn them into oncogenes that drive continuous division.
- Tumour suppressor genes normally slow division or trigger apoptosis; a mutation can switch them off, removing the brake.
Either change leads to uncontrolled mitosis, forming a mass of cells called a tumour. A benign tumour stays in place; a malignant tumour (cancer) invades surrounding tissue and can spread (metastasise) through the blood or lymph.
Examples in context
Example 1. Root tip squash to count dividing cells. Garlic or onion root tips are stained and squashed to view dividing cells. Because the meristem at the tip grows rapidly, many cells are caught in mitosis, giving a high mitotic index. Students count cells in each stage to estimate the relative time spent in each phase.
Example 2. Chemotherapy and rapidly dividing cells. Many chemotherapy drugs target cells with a high mitotic index, for example by disrupting spindle formation in metaphase or blocking DNA replication in S phase. Because cancer cells divide rapidly they are hit hardest, but normal fast-dividing tissues such as hair follicles and gut lining are also affected, explaining common side effects.
Try this
Q1. Put the stages of mitosis in order and give one event of each. [4 marks]
- Cue. Prophase (chromosomes condense, nuclear envelope breaks down); metaphase (chromosomes line up at equator); anaphase (chromatids pulled to poles); telophase (nuclear envelopes reform).
Q2. In a tissue sample of 80 cells, 12 are in mitosis. Calculate the mitotic index as a percentage. [1 mark]
- Cue. percent.
Q3. Explain the difference between a proto-oncogene mutation and a tumour suppressor gene mutation in causing cancer. [3 marks]
- Cue. A proto-oncogene mutated to an oncogene actively stimulates excess division; a tumour suppressor mutation removes the brake that normally slows division or triggers apoptosis; both result in uncontrolled mitosis.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA2 marksIn a sample of 120 cells viewed under a microscope, 18 were undergoing mitosis. Calculate the mitotic index.Show worked answer →
Mitotic index is the proportion of cells in mitosis, given as a decimal or percentage.
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As a percentage this is 15 percent. Markers reward the correct ratio and value (accept 0.15 or 15 percent).
2021 AQA3 marksExplain how a mutation in a tumour suppressor gene can lead to the formation of a tumour.Show worked answer →
A 3-mark answer links the gene's normal job, the mutation, and uncontrolled division.
- A tumour suppressor gene normally slows the cell cycle or triggers programmed cell death (apoptosis) in damaged cells.
- A mutation can inactivate the gene so it no longer stops division.
- Cells then divide uncontrollably by mitosis, forming a mass of cells (a tumour).
Markers reward the normal regulatory role, the loss of function, and uncontrolled mitosis.
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