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🧬 Science · Biology · Lesson 3 of 7 · 6 min

Mitosis and meiosis

🧫 Your skin heals by mitosis; you exist because of meiosis. The TEAS loves to ask which is which and how many chromosomes come out.

💡 Key idea

Mitosis: 1 division, 2 identical diploid cells. Meiosis: 2 divisions, 4 different haploid cells.

🧠 The idea

The CELL CYCLE has interphase (G1 growth, S phase when DNA is copied, G2 preparation) followed by division. MITOSIS makes two genetically identical DIPLOID daughter cells for growth and repair. Its phases are prophase (chromosomes condense, spindle forms), metaphase (chromosomes line up at the middle), anaphase (sister chromatids pull apart) and telophase (nuclei re-form), then cytokinesis splits the cytoplasm. MEIOSIS makes gametes. It has two divisions and produces four genetically different HAPLOID cells. In prophase I, homologous chromosomes pair and CROSSING OVER swaps segments; meiosis I separates homologous pairs and meiosis II separates sister chromatids, much like mitosis. Humans have 46 chromosomes (2n = 46) in body cells and 23 (n = 23) in eggs and sperm. Fertilization restores 46.

✏️ Worked example

A dog body cell has 78 chromosomes. How many are in a dog sperm cell and in a skin cell made by mitosis? Step 1: sperm come from meiosis, which halves the number: 78 ÷ 2 = 39. Step 2: mitosis keeps the number the same, so the new skin cell has 78. Step 3: when sperm (39) meets egg (39), the zygote has 78 again.

📌 Takeaways

  • DNA replicates in the S phase of interphase, before division starts.
  • PMAT: prophase, metaphase (line up), anaphase (pull apart), telophase.
  • Crossing over in prophase I and independent assortment make gametes genetically unique.

📖 Terms in this lesson

Mitosis: Cell division that produces two genetically identical body cells; for example, skin cells dividing to heal a cut.

Meiosis: Cell division that produces four sex cells, each with half the usual number of chromosomes; for example, the formation of egg and sperm cells.

✅ Check yourself

Question 1 of 3

During which phase of mitosis do chromosomes line up along the middle of the cell?

  1. Metaphase
  2. Telophase
  3. Prophase
  4. Anaphase
Show the answer and rationale

Answer: A

In metaphase the chromosomes line up at the cell's equator. In prophase they condense, in anaphase the sister chromatids separate, and in telophase new nuclear envelopes form.

Question 2 of 3

A human egg cell contains how many chromosomes?

  1. 46
  2. 23
  3. 12
  4. 92
Show the answer and rationale

Answer: B

Eggs and sperm are haploid, made by meiosis, so they carry 23 chromosomes. Body cells are diploid with 46, and 92 would be a doubled diploid set, not a gamete.

Question 3 of 3

Which statement correctly describes the products of meiosis?

  1. Two identical diploid cells
  2. Four identical diploid cells
  3. Two different haploid cells
  4. Four different haploid cells
Show the answer and rationale

Answer: D

Meiosis has two divisions, giving four haploid cells that differ because of crossing over and independent assortment. Two identical diploid cells describe mitosis.

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