Biology / Unit 4: Cell Growth and Division
4.4 Meiosis
Notes
Meiosis makes gametes (sex cells: sperm and egg) with half the chromosome number.
- Diploid ($2n$) cells have two sets of chromosomes; in humans $2n = 46$. Haploid ($n$) cells have one set; $n = 23$.
- Homologous chromosomes are matching pairs, one from each parent, with genes for the same traits.
- Meiosis I separates homologous pairs. Meiosis II separates sister chromatids.
- Result: 4 genetically different haploid cells.
- Crossing over in prophase I swaps pieces between homologous chromosomes, adding variation. Independent assortment of pairs adds more.
- Fertilization joins two haploid gametes to form a diploid zygote.
Practice
A dog's body cells have 78 chromosomes. How many chromosomes are in a dog sperm cell?
Gametes are haploid, with half the diploid number.
During which event do homologous chromosomes exchange pieces of DNA?
Crossing over creates new combinations of alleles.
What does meiosis I separate?
Meiosis I separates homologs; meiosis II separates sister chromatids.
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