What this chapter is about
Every living organism begins as a single cell, and the growth, development and repair of tissues all depend on the ability of cells to divide. This chapter introduces the orderly sequence of events by which a cell duplicates its contents and splits into two daughter cells. You will learn about the cell cycle — the defined phases a cell passes through between one division and the next — and the two fundamentally different modes of division: mitosis, which produces genetically identical cells, and meiosis, which halves the chromosome number and generates genetic variation.
Understanding cell division is essential for grasping how a multicellular body develops from a fertilised egg, how wounds heal, and how errors in division can lead to diseases such as cancer. The chapter also connects with genetics, because meiosis is the process that shuffles parental chromosomes and ensures each gamete carries a unique combination of alleles. By the end, you should be able to describe the stages of mitosis and meiosis, explain their significance, and distinguish one from the other.
Key ideas
- The cell cycle has two broad parts: interphase (the longer phase when the cell grows and copies its DNA) and the M phase (mitosis or meiosis plus cytokinesis).
- Interphase is subdivided into G₁ (growth and normal metabolism), S (DNA synthesis, when chromosomes replicate), and G₂ (preparation for division).
- Mitosis consists of prophase, metaphase, anaphase and telophase; it produces two daughter cells with the same chromosome number as the parent (2n → 2n in a diploid organism).
- Meiosis involves two successive divisions, meiosis I and meiosis II, and produces four haploid cells (2n → n) from one diploid cell.
- Crossing over (exchange of segments between homologous chromosomes during prophase I) and independent assortment of chromosomes generate genetic diversity.
- Cytokinesis differs in plant and animal cells: animal cells divide by cleavage furrow formation, while plant cells build a cell plate from the centre outward.
- Checkpoints in G₁, G₂ and metaphase ensure that DNA is intact and chromosomes are properly attached before the cell proceeds; failure of checkpoints can lead to uncontrolled division.
Formulas and facts to remember
1. Cell cycle duration: In a rapidly dividing human cell, the entire cycle takes about 24 hours; interphase occupies roughly 95 % of this time. 2. DNA content change: During S phase the DNA content doubles from 2C to 4C (where C is the amount in a haploid set); after mitosis each daughter returns to 2C. 3. Chromosome terminology: A replicated chromosome has two sister chromatids joined at the centromere; these separate during anaphase (anaphase II in meiosis). 4. Meiosis I is reductional: Homologous chromosomes pair (synapsis) and then separate, halving the chromosome number. 5. Meiosis II is equational: Sister chromatids separate, similar to mitosis but starting from haploid cells. 6. Chiasmata: The X-shaped points where crossing over occurs; visible during diplotene of prophase I. 7. Significance of mitosis: Growth, repair, asexual reproduction; maintains genetic consistency. 8. Significance of meiosis: Formation of gametes and spores; maintains constant chromosome number across generations and introduces variation.
Worked examples
### Example 1: Calculating DNA content after S phase
A somatic cell of a plant has 2n = 20 chromosomes. Before S phase the DNA content is 10 pg (picograms). What is the DNA content immediately after S phase ends?
Solution During S phase each chromosome replicates, so the total DNA doubles. DNA after S phase = 10 pg × 2 = 20 pg. Note that the chromosome number is still 20 (each now consists of two sister chromatids joined at the centromere).
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### Example 2: Identifying the stage from a description
A student observes onion root tip cells and sees one cell where the chromosomes are lined up at the centre of the cell in a single plane. Each chromosome appears to be attached to spindle fibres from opposite poles. Name the stage and the type of division.
Solution Chromosomes aligning at the cell's equator (metaphase plate) with spindle attachments to both poles is characteristic of metaphase of mitosis. In meiosis I the homologous pairs align (bivalents), whereas here individual chromosomes align, indicating mitotic metaphase.
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### Example 3: Predicting gamete chromosome number
A diploid animal has 2n = 46 chromosomes. After meiosis, how many chromosomes will each gamete contain, and how many gametes result from one cell undergoing meiosis?
Solution Meiosis halves the chromosome number: 46 / 2 = 23 chromosomes per gamete. One diploid cell produces four haploid gametes (or four products; in oogenesis only one becomes a functional egg while three become polar bodies).
Common mistakes
- Thinking S phase increases the number of chromosomes → S phase doubles the DNA but the chromosome count stays the same; sister chromatids are counted as one chromosome until they separate.
- Confusing homologous chromosomes with sister chromatids → Homologous chromosomes are two separate chromosomes (one from each parent) carrying genes for the same traits; sister chromatids are identical copies of one chromosome held together at the centromere.
- Believing crossing over occurs in mitosis → Crossing over happens only in prophase I of meiosis, when homologous chromosomes are synapsed.
- Stating that meiosis produces two cells → Meiosis produces four haploid cells; two divisions occur.
- Mixing up the terms haploid and diploid when describing gametes and somatic cells → Somatic (body) cells are diploid (2n); gametes are haploid (n).
Quick revision
- Interphase = G₁ + S + G₂; cell grows and replicates DNA here.
- Mitosis: one division, two identical diploid daughters; for growth and repair.
- Meiosis: two divisions, four genetically varied haploid cells; for sexual reproduction.
- Crossing over and independent assortment create genetic variation.
- Checkpoints prevent division if DNA is damaged or chromosomes misaligned.
- Cytokinesis: cleavage furrow in animals, cell plate in plants.