How do cells divide to grow, repair and make gametes, and how does meiosis create variation?
Cell division: the cell cycle and its control; mitosis and its role in growth and repair; meiosis and the production of genetic variation; and the mitotic index.
A focused answer to the Eduqas Biology Core Concepts statement on cell division. Covers the cell cycle and its checkpoints, the stages of mitosis, meiosis and the sources of variation it creates, and the mitotic index calculation.
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What this dot point is asking
Eduqas wants you to describe the cell cycle and its control, explain mitosis and its role in growth and repair, explain meiosis and how it creates variation, and calculate the mitotic index. This is a Core Concepts statement, examined on every paper and central to the genetics content in Component 2.
The cell cycle and its control
The cell cycle has two main parts:
- Interphase, the longest part: G1 (the cell grows and makes organelles), S phase (DNA replicates, so each chromosome becomes two sister chromatids), and G2 (more growth and the cell checks the copied DNA).
- Mitosis and cytokinesis, when the nucleus then the cytoplasm divide.
Mitosis
Mitosis produces two genetically identical diploid nuclei, in four stages:
- Prophase: chromosomes condense and become visible (each as two sister chromatids joined at the centromere); the nuclear envelope breaks down and the spindle forms.
- Metaphase: chromosomes line up on the equator (metaphase plate), attached to spindle fibres at their centromeres.
- Anaphase: the centromeres divide and the sister chromatids are pulled to opposite poles by the shortening spindle fibres (using ATP).
- Telophase: the chromatids reach the poles, decondense, and new nuclear envelopes form; cytokinesis then splits the cytoplasm into two cells.
Mitosis allows growth, repair of damaged tissue, and asexual reproduction, always producing cells genetically identical to the parent.
Meiosis
Meiosis is a reduction division of two stages (meiosis I and meiosis II) that produces four genetically different haploid cells from one diploid cell, halving the chromosome number so that fertilisation restores the diploid number.
These two processes, plus random fertilisation of one of many possible gametes by another, are the sources of genetic variation in sexually reproducing organisms.
The mitotic index
The mitotic index is the proportion of cells in a sample that are in mitosis:
A high value means many cells are dividing (rapid growth, or uncontrolled division in a tumour).
Examples in context
Example 1. Why cancer drugs target dividing cells. Many chemotherapy drugs block mitosis (for example by preventing spindle formation), so they hit rapidly dividing tumour cells hardest, but also affect healthy fast-dividing tissues such as hair follicles, which is why hair loss is a side effect.
Example 2. Meiosis and the scale of variation. With 23 homologous pairs in humans, independent assortment alone gives possible combinations of chromosomes per gamete, before crossing over and random fertilisation, which is why siblings differ so much.
Try this
Q1. State the products of mitosis and the products of meiosis. [2 marks]
- Cue. Mitosis: two genetically identical diploid cells. Meiosis: four genetically different haploid cells.
Q2. Name the stage of meiosis in which crossing over occurs. [1 mark]
- Cue. Prophase I.
Q3. A tissue sample of 40 cells contains 6 cells in mitosis. Calculate the mitotic index. [1 mark]
- Cue. (or 15 percent).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksDescribe two ways in which meiosis produces genetic variation among the gametes formed.Show worked answer →
Crossing over: during prophase I, homologous chromosomes pair up and sections of chromatid are exchanged at chiasmata, producing new combinations of alleles on each chromosome.
Independent assortment: during metaphase I, the homologous pairs line up randomly on the equator, so maternal and paternal chromosomes are distributed to the gametes in different combinations.
Markers reward crossing over (exchange of chromatid sections between homologous chromosomes in prophase I) and independent assortment (random arrangement of homologous pairs in metaphase I).
Eduqas 20213 marksA section of root tip contained 50 cells, of which 8 were in mitosis. Calculate the mitotic index, and explain what a high mitotic index in a tissue sample indicates.Show worked answer →
The mitotic index is the proportion of cells in mitosis: (or 16 percent).
Markers reward the correct calculation and value.
A high mitotic index means a large proportion of cells are actively dividing, indicating a rapidly growing tissue; in a clinical sample it can indicate uncontrolled division such as in a tumour.
Markers reward a high proportion of dividing cells indicating rapid growth or (in a tumour) uncontrolled division.
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Sources & how we know this
- Eduqas A Level Biology Specification (A400) — Eduqas (2015)