Cell Division: Mitosis and Meiosis

Grade 9 · Science · Study Guide

Understand how cells divide to produce identical copies for growth and repair, and how sex cells are made for reproduction.

Cell division is fundamental to life — it allows organisms to grow, repair damaged tissues, and reproduce. There are two major types of cell division: mitosis and meiosis, each with a distinct purpose and outcome.

Mitosis produces two genetically identical daughter cells, each with the same chromosome number as the parent cell. In humans, that means 46 chromosomes (diploid, 2n). Mitosis proceeds through four stages: prophase (chromosomes condense, spindle forms), metaphase (chromosomes align at the cell's equator), anaphase (sister chromatids separate and are pulled to opposite poles), and telophase (nuclear envelopes reform, chromosomes decondense). This is followed by cytokinesis — the splitting of the cytoplasm. Mitosis is used for growth, tissue repair, and asexual reproduction.

Meiosis produces four genetically unique daughter cells, each with half the chromosome number of the parent cell (haploid, 1n — 23 chromosomes in humans). These are the sex cells (gametes): sperm and eggs. Meiosis involves two rounds of division (Meiosis I and Meiosis II). A critical event in Meiosis I is crossing over, where homologous chromosomes exchange segments of DNA — this shuffles genetic information and creates genetic diversity. When an egg and sperm fuse during fertilization, the normal diploid number (46) is restored.

Mitosis vs. Meiosis Comparison
Feature	Mitosis	Meiosis
Purpose	Growth and repair	Sexual reproduction
Divisions	1	2
Daughter cells	2	4
Chromosome number	Diploid (2n)	Haploid (1n)
Genetic identity	Identical to parent	Unique (crossing over)
Where it occurs	Body (somatic) cells	Reproductive organs (gonads)

Key terms

Mitosis: Cell division producing two genetically identical diploid daughter cells; used for growth and repair.
Meiosis: Cell division producing four genetically unique haploid daughter cells (gametes); used for sexual reproduction.
Diploid (2n): Cell containing two sets of chromosomes (one from each parent); 46 chromosomes in humans.
Haploid (1n): Cell containing one set of chromosomes; 23 chromosomes in human gametes.
Chromosome: A coiled structure of DNA and protein that carries genetic information; humans have 46 in somatic cells.
Crossing Over: Exchange of DNA segments between homologous chromosomes during Meiosis I; increases genetic diversity.
Gamete: A reproductive cell (sperm or egg) with half the normal chromosome number; produced by meiosis.
Cytokinesis: Division of the cytoplasm following nuclear division; completes cell division.

Flashcards

Mitosis: Produces 2 identical diploid cells; for growth and repair.
Meiosis: Produces 4 unique haploid gametes; for sexual reproduction.
Diploid (2n): Two full sets of chromosomes; 46 in humans.
Haploid (1n): One set of chromosomes; 23 in human gametes.
Crossing Over: Exchange of DNA between homologs during meiosis I; creates genetic diversity.
Phases of Mitosis: Prophase → Metaphase → Anaphase → Telophase (PMAT).

Practice questions

What is the main purpose of mitosis?
1. Produce sex cells
2. Growth and repair of tissues
3. Increase genetic diversity
4. Reduce chromosome number
Show answer
Answer: Growth and repair of tissues
Mitosis replaces damaged or worn-out cells with exact copies — perfect for growth, healing, and maintenance. Meiosis is reserved for producing gametes.
Meiosis produces two identical diploid daughter cells.
Show answer
Answer: False
Meiosis produces four haploid (1n) daughter cells that are genetically unique due to crossing over and independent assortment. Mitosis produces two identical diploid cells.
Human gametes contain ___ chromosomes (haploid number).
Show answer
Answer: 23
Meiosis halves the diploid number (46) to produce haploid gametes with 23 chromosomes each. When sperm (23) and egg (23) fuse, the zygote restores the diploid count of 46.
During which process do homologous chromosomes exchange DNA segments, increasing genetic diversity?
1. Cytokinesis
2. Mitosis
3. Crossing over
4. Telophase
Show answer
Answer: Crossing over
Crossing over (recombination) occurs in Prophase I of meiosis — homologous chromosomes pair up and exchange DNA segments, creating new gene combinations that enhance genetic diversity.
Which stage of mitosis do chromosomes align along the cell's equator?
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Show answer
Answer: Metaphase
During metaphase, chromosomes are fully condensed and aligned at the metaphase plate (cell's equatorial plane). The spindle fibers attach to each chromosome's centromere, ready to pull them apart.

Fun facts

Your body produces about 25 million new cells every second through mitosis — mostly red blood cells.
Crossing over during meiosis means every single sperm or egg cell is genetically unique, which is why siblings look similar but not identical.
Cancer is essentially mitosis gone wrong — cells divide uncontrollably because the normal checkpoints that regulate the cell cycle are disrupted.

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