1. Meiosis
Chapter 11.4

2. Function of Meiosis Produces gametes (sex cells)
Males – sperm
Females – egg or ovum
Only occurs in gonads (testes or ovaries)
Sperm + egg → zygote (fertilized egg)

3. Chromosome Count Chromosome number = ploidy number (n)
Haploid: Half the normal number of chromosomes (1 set = n)
Diploid: Normal number of chromosomes (2 sets = 2n)
Sperm (n) + Egg (n) → Zygote (2n)

4. Human Chromosomes Humans have 2 sets of 23 chromosomes
n = 23 2n = 46
22 pairs of autosomes
1 pair of sex chromosomes
XX = female XY = male
Meiosis makes haploid gametes from diploid cells

5. Sex chromosomes

6. Homologous Chromosomes
Each pair of chromosomes are said to be homologous
One from mom and one from dad
Carry the same genes, but different forms of each gene
Ex – both chromosomes may have eye color gene, but one has blue and one has brown

7. Meiosis Two cell divisions Meiosis I (PMAT I) Meiosis II (PMAT II)
Homologous chromosomes separate
Results in 2 haploid cells
Meiosis II (PMAT II)
Sister chromatids separate
Results in 4 haploid cells

8. Meiosis I Prophase I Metaphase I nuclear env. disappears
homologous chromosomes pair up
Form a tetrad (tetra = four)
Metaphase I
tetrads line up along cell’s equator

9. Meiosis I Anaphase I Telophase I
Tetrad splits, homologous chromosomes separate
Telophase I
Cell splits into two haploid cells

10. Meiosis II Similar to mitosis Prophase II uneventful Metaphase II
Replicated chromosomes line up at equator
Anaphase II
Sister chromatids separate
Telophase II
Cells divide = 4 haploid gametes

11. Gametogenesis Spermatogenesis Oogenesis 4 tiny haploid sperm
1 large haploid egg, 3 small polar bodies

12. Creating Variation Meiosis creates genetic variation in two ways:
Random shuffling of chromosomes in Metaphase I
More than 8 million possible gametes
Crossing over in Prophase I
Homologous chromosomes swap pieces of DNA when tightly paired as tetrad

13. Crossing Over