How does a cell copy itself exactly, and what happens when that control fails?
2.1.6 Cell division: the cell cycle and its regulation by checkpoints; the main stages of mitosis (prophase, metaphase, anaphase and telophase) and cytokinesis; the significance of mitosis in growth, repair and asexual reproduction; the calculation and use of the mitotic index.
A focused answer to the OCR H420 2.1.6 dot point on the cell cycle and mitosis. Covers interphase and checkpoints, the four stages of mitosis and cytokinesis, the significance of mitosis, the link to cancer, and the mitotic-index calculation.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
OCR wants you to describe the cell cycle and its checkpoints, sequence and describe the four stages of mitosis and cytokinesis, explain why mitosis matters, link loss of control to cancer, and calculate and interpret the mitotic index.
The answer
The cell cycle
The cell cycle is a long interphase followed by mitosis and cytokinesis. Interphase has three sub-phases:
- G1 (first growth): the cell grows and makes organelles and proteins.
- S (synthesis): DNA replicates, producing two identical sister chromatids per chromosome.
- G2 (second growth): the cell grows further and checks the replicated DNA for errors.
Crucially, DNA replicates during interphase, not during mitosis. The cycle is regulated at checkpoints (notably the G1 and G2 checkpoints) that verify cell size, DNA integrity and successful replication before the cell is allowed to proceed; this prevents damaged DNA being passed on.
The stages of mitosis
Mitosis produces two genetically identical diploid daughter nuclei. Remember the order with "PMAT":
- Prophase: chromosomes condense and become visible (each as two chromatids), the nuclear envelope breaks down, and the spindle forms from centrioles (in animal cells).
- Metaphase: chromosomes line up on the equator; spindle fibres attach to each centromere.
- Anaphase: centromeres divide, spindle fibres shorten, and sister chromatids are pulled to opposite poles. This requires ATP.
- Telophase: chromosomes decondense at each pole and new nuclear envelopes form, giving two nuclei.
Cytokinesis then divides the cytoplasm: animal cells pinch in by a cleavage furrow, while plant cells form a new cell wall along a cell plate.
Significance and the link to cancer
Mitosis drives growth, tissue repair and replacement, and asexual reproduction, always producing genetically identical cells. It is tightly controlled by genes. Cancer arises when control is lost: a mutation can turn a proto-oncogene into an oncogene that drives continuous division, or switch off a tumour suppressor gene that normally slows division or triggers apoptosis. Either causes uncontrolled mitosis and a tumour. A benign tumour stays localised; a malignant tumour (cancer) invades surrounding tissue and can spread (metastasise).
The mitotic index
The mitotic index is the proportion of cells in mitosis:
A high value indicates rapidly dividing tissue (a root tip, a meristem or a tumour). It is a common practical and data context, often from a stained root-tip squash (PAG-style microscopy).
Examples in context
Example 1. Wound healing. When skin is cut, basal cells divide by mitosis to produce genetically identical cells that replace the lost tissue, illustrating the repair role of mitosis.
Example 2. Chemotherapy. Many cancer drugs target rapidly dividing cells, for example by disrupting spindle formation in mitosis; this is why they also affect fast-dividing healthy tissue such as hair follicles, a frequent applied context.
Try this
Q1. In which phase of the cell cycle does DNA replication occur? [1 mark]
- Cue. The S (synthesis) phase of interphase.
Q2. Explain why anaphase requires ATP. [2 marks]
- Cue. Spindle fibres (microtubules) shorten to pull the chromatids to opposite poles; this movement is an active process that uses energy from ATP.
Q3. A tissue sample of 200 cells contains 30 in mitosis. Calculate the mitotic index. [1 mark]
- Cue. 30 divided by 200 = 0.15 (15 percent).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20184 marksDescribe what happens to the chromosomes during metaphase and anaphase of mitosis.Show worked answer →
Two stages, two clear descriptions (about 2 marks each).
Metaphase: the chromosomes (each made of two sister chromatids) line up along the equator (metaphase plate) of the cell. Spindle fibres from the poles attach to the centromere of each chromosome.
Anaphase: the centromeres divide and the spindle fibres shorten, pulling the sister chromatids to opposite poles of the cell. This requires ATP. Each pole now receives an identical set of chromosomes.
Markers reward "line up on the equator" and "spindle attaches to centromere" for metaphase, and "centromeres divide", "chromatids pulled to opposite poles" and the ATP requirement for anaphase.
OCR H420/02 20213 marksA microscope field of view contains 80 cells, of which 12 are in mitosis. Calculate the mitotic index, and explain what a high mitotic index suggests about a tissue.Show worked answer →
A calculation plus an interpretation.
Mitotic index (or 15 percent) (2 marks: working and answer).
For the third mark: a high mitotic index means a large proportion of cells are actively dividing, which suggests rapidly growing tissue such as a meristem or root tip, or, in animal tissue, possibly a tumour.
Markers reward the correct ratio and a sensible biological interpretation.
Related dot points
- 2.1.6 Cell division, diversity and organisation: how meiosis produces haploid gametes and generates genetic variation through crossing over and independent assortment; the meaning and potential of stem cells (totipotent, pluripotent and multipotent); cell specialisation and the organisation of cells into tissues, organs and organ systems.
A focused answer to the OCR H420 2.1.6 dot point on meiosis, stem cells and cell organisation. Covers meiosis and how it generates variation, the potency of stem cells, their uses and ethics, and the organisation of cells into tissues, organs and systems.
- 2.1.1 Cell structure: the ultrastructure of eukaryotic and prokaryotic cells, the function of organelles including the role of the rough endoplasmic reticulum and Golgi apparatus in producing and secreting proteins; the use, calibration and resolution of light and electron microscopes.
A focused answer to the OCR H420 2.1.1 dot point on cell structure and microscopy. Covers every required eukaryotic and prokaryotic organelle, the protein secretory pathway, the three microscopes, eyepiece-graticule calibration and the magnification equation.
- 2.1.3 Nucleotides and nucleic acids: the semi-conservative replication of DNA and the roles of DNA helicase, DNA polymerase and the complementary base pairing rule; the nature of the genetic code as a triplet code that is degenerate and non-overlapping; the roles of mRNA and tRNA in protein synthesis.
A focused answer to the OCR H420 2.1.3 dot point on DNA replication and the genetic code. Covers semi-conservative replication, the roles of DNA helicase and DNA polymerase, the Meselson-Stahl evidence, and the triplet, degenerate, non-overlapping code with transcription and translation.
- 6.1.1 Cellular control: the nature of gene mutations and their effects on proteins; the control of gene expression at the transcriptional level, including operons (the lac operon) and transcription factors; the role of homeobox (Hox) genes in body plan development; and the role of apoptosis (programmed cell death).
A focused answer to the OCR H420 6.1.1 dot point on cellular control. Covers gene mutations and their effects, the control of transcription by the lac operon and transcription factors, the role of homeobox (Hox) genes in body plan development, and apoptosis.
- 6.1.2 Patterns of inheritance: monohybrid and dihybrid crosses; the inheritance of codominant and multiple alleles, sex linkage and epistasis; the use of genetic diagrams to predict phenotypic ratios; and the chi-squared test to compare observed and expected results.
A focused answer to the OCR H420 6.1.2 dot point on patterns of inheritance. Covers monohybrid and dihybrid crosses, codominance and multiple alleles, sex linkage and epistasis, genetic diagrams and phenotypic ratios, and the chi-squared test.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)