Lesson · B.3.2
Meiosis and reproduction
Why meiosis halves the chromosome number to make gametes, how it differs from mitosis, and the crossing over step the HESI A2 tests most often.
ObjectiveB.3.2Read4 minExam weightCell Division and Genetics ≈ 20% of Biology
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- Explain why meiosis, not mitosis, is used to produce sperm and egg cells.
- List the phases of meiosis I and meiosis II and say what changes in each.
- Describe crossing over and state when in meiosis it happens.
What the lesson concludes
- Meiosis produces four haploid daughter cells from one diploid parent cell, each with half the chromosome number.
- Human somatic cells carry 46 chromosomes, diploid, and human gametes carry 23, haploid.
- Meiosis I separates homologous chromosome pairs, meiosis II separates sister chromatids, the same way mitosis does.
- Crossing over happens in prophase I, between nonsister chromatids of homologous chromosomes, and it is the main source of genetic variation.
- Fertilisation restores the diploid number by combining one haploid sperm and one haploid egg.