Meiosis involves two cell divisions that reduce the chromosome number by half to produce haploid gametes, such as eggs and sperm. It occurs in the ovaries and testes. During prophase I of meiosis I, homologous chromosomes pair up and genetic recombination occurs via crossing over. This increases genetic variation. Meiosis I separates the homologous chromosomes, followed by meiosis II which separates sister chromatids without intervening DNA replication, resulting in four haploid cells in males and one in females.

1. Meiosis
vs.
Mitosis

2. Chromosome Number

3. Chromosome Number
• Species have characteristic numbers of
chromosomes
• Eukaryotic cells have pairs of homologous
chromosomes
• Cells with a double set of chromosomes are
diploid (2n)
• Cells with one set of chromosomes are
• haploid (n)

4. • Humans body cells are
diploid (2N)
– 2N = 46
• Gametes - sex cells
– Males – sperm
– Females – eggs
• Gametes are HAPLOID
– N = 23
• Fertilization restores the
diploid state
• (N + N = 2N)
• Zygote - a fertilized egg
(2N)

5. Meiosis…bottom line
• Process to make haploid
gametes
• Chromosome # is cut in half
– 2N  N
• Occurs in ovaries & testes
• TWO cell divisions
• Increases genetic variation

6. Prior to
•Meiosis….INTERPHASE
What happens during Interphase???
– DNA REPLICATION!!!
2N
2N

7. Meiosis I
2N
• Prophase I – homologous
chromosomes form tetrads &
crossing over occurs
• Metaphase I – tetrads line up
along the middle
• Anaphase I – homologous
chromosomes separate
• Telophase I – two nuclei form
• Cytokinesis I – cells separate
N
N

8. Crossing Over
• Occurs in Prophase I
• Chromosomes exchange portions of their
chromatids
• Increases genetic variation

9. Meiosis II
N
• DNA replication DOES NOT occur
between Meiosis I & II
• Prophase II – spindle
forms, prepares for division
N
• Metaphase II – chromosomes
align along the center
• Anaphase II – sister
chromatids separate
• Telophase II – nuclei
reform; cytokinesis
N
N
N
N
Looks A LOT like
MITOSIS!

10. Meiosis in Men vs. Women
• Men: 4
sperm are
made
• Women: only
1 egg is
made

11. Meiosis vs. Mitosis

12. Meiosis Summary
• Chromosome number is cut in half!
• Produces haploid gametes – egg and sperm
• Crossing-over occurs during Prophase I
– leads to increased genetic variation
• Two cell divisions, but DNA does not
replicate between divisions
• Results: 4 sperm in men and 1 egg in women