Cell Division: Mitosis and Meiosis
Compare the processes of mitosis and meiosis, understand their stages, and explore the significance of each type of cell division for growth, repair, and reproduction.
Mitosis
Mitosis is the process of cell division that produces two genetically identical daughter cells, each with the same number of chromosomes as the parent cell (diploid, 2n). It is essential for growth, repair, and asexual reproduction.
Stages of Mitosis
Prophase
Chromosomes condense and become visible. Spindle fibres form. Nuclear membrane begins to break down.
Metaphase
Chromosomes line up along the cell's equator (metaphase plate). Spindle fibres attach to centromeres.
Anaphase
Sister chromatids are pulled apart to opposite poles of the cell by spindle fibres.
Telophase & Cytokinesis
Nuclear membranes reform. Chromosomes decondense. Cytoplasm divides, producing two identical cells.
Remember PMAT: Prophase, Metaphase, Anaphase, Telophase. Before mitosis, DNA replicates during S phase of interphase so each chromosome has two identical sister chromatids.
Meiosis
Meiosis is cell division that produces four genetically unique haploid cells (gametes), each with half the chromosome number (n). It involves two rounds of division (meiosis I and meiosis II) and introduces genetic variation through crossing over and independent assortment.
Meiosis I
Homologous chromosomes pair up and are separated.
- Prophase I: Crossing over occurs between homologous pairs
- Metaphase I: Homologous pairs line up at the equator
- Anaphase I: Homologous chromosomes separate (independent assortment)
- Telophase I: Two haploid cells formed
Meiosis II
Sister chromatids are separated (similar to mitosis).
- Prophase II: Chromosomes condense again
- Metaphase II: Chromosomes line up at equator
- Anaphase II: Sister chromatids pulled apart
- Telophase II: Four unique haploid cells formed
Sources of genetic variation: (1) Crossing over in prophase I swaps segments between homologous chromosomes. (2) Independent assortment in anaphase I randomly distributes homologous pairs. (3) Random fertilisation combines different gametes.
Comparing Mitosis and Meiosis
While both are forms of cell division, mitosis and meiosis differ in their purpose, process, and outcomes.
Biological Significance
Mitosis: Ensures all body (somatic) cells have the complete set of chromosomes. Essential for tissue repair and organism growth.
Meiosis: Halves the chromosome number so that when two gametes fuse at fertilisation, the diploid number is restored. Generates genetic diversity essential for evolution.
In humans: Somatic cells have 46 chromosomes (2n = 46). Gametes (eggs and sperm) have 23 chromosomes (n = 23).
Key Vocabulary
Diploid (2n)
A cell containing two complete sets of chromosomes -- one from each parent. In humans, 2n = 46.
Haploid (n)
A cell containing one set of chromosomes. Gametes (sperm and egg) are haploid. In humans, n = 23.
Crossing Over
The exchange of genetic material between homologous chromosomes during prophase I of meiosis, increasing genetic variation.
Homologous Chromosomes
A pair of chromosomes (one from each parent) that have the same genes at the same positions but may have different alleles.
Worked Examples
A human cell with 46 chromosomes undergoes mitosis. How many chromosomes does each daughter cell have?
Step 1: Mitosis produces daughter cells with the same chromosome number as the parent.
Answer: Each daughter cell has 46 chromosomes (2n = 46), identical to the parent cell.
How many genetically different gametes can be produced by an organism with 2n = 4 through independent assortment alone?
Step 1: Number of homologous pairs = n = 2.
Step 2: Independent assortment produces 2n combinations = 2² = 4 different gametes.
Answer: 4 genetically different gametes. For humans (n = 23), this is 223 = 8,388,608 combinations.
Explain why meiosis is necessary for sexual reproduction.
Step 1: If gametes were diploid (2n), fertilisation would double the chromosome number each generation.
Step 2: Meiosis halves the chromosome number, producing haploid gametes (n).
Answer: When two haploid gametes fuse (n + n = 2n), the diploid number is restored. Without meiosis, the chromosome number would increase uncontrollably each generation.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
How many daughter cells are produced by meiosis?
Question 2
During which stage of mitosis do chromosomes line up at the cell's equator?
Question 3
Crossing over occurs during which phase?
Question 4
Which type of cell division is used for growth and tissue repair?
Question 5
A cell with 2n = 12 undergoes meiosis. How many chromosomes will each gamete have?
Key Concepts Summary
- ●Mitosis produces 2 identical diploid cells for growth and repair (PMAT).
- ●Meiosis produces 4 unique haploid gametes through two divisions.
- ●Crossing over and independent assortment create genetic variation in meiosis.
- ●Meiosis halves the chromosome number so fertilisation restores the diploid number.
- ●In humans: somatic cells = 46 chromosomes (2n); gametes = 23 chromosomes (n).