Meiosis is a type of cell division that produces haploid cells through two divisions of a single diploid cell. It is used in sexual reproduction to create eggs and sperm. During meiosis I, homologous chromosome pairs separate, reducing the chromosome number by half. Meiosis II then separates the sister chromatids, resulting in four haploid cells total. This ensures genetic variation from the combination of DNA from two parents in the zygote.

1. Meiosis
Textbook Section 8.3

2. Sexual Reproduction
► Organisms receive half of their
DNA from each parent
► Sperm and eggs cells (haploid)
combine to create a zygote
(diploid)
► Used by most animals, fungi,
plants, and algae

3. Remember…
► Organisms have two
copies of each
chromosome, called
“homologous
chromosomes”
► Can be autosomes or
sex chromosomes

4. Meiosis
► A cell replicates its DNA,
then divides twice, forming
four haploid cells
► Stages are the same, but
they occur TWICE!

5. Meiosis I vs. Meiosis II

6. Steps of Meiosis
► PROPHASE I

7. Steps of Meiosis
► METAPHASE I
 Homologous
chromosomes line up
along center in pairs

8. Steps of Meiosis
► ANAPHASE I
 Sister chromatids DO
NOT separate.
 Homologous
chromosomes DO!
► TELOPHASE I
 Nuclear membrane does
not re-form

9. Steps of Meiosis
► PROPHASE II
 Cell has been split in
two, so all “II” stages
take place in both cells

10. Steps of Meiosis
► METAPHASE II

11. Steps of Meiosis
► ANAPHASE II
 Sister chromatids finally
separate
► TELOPHASE II

12. AN IMPORTANT DIFFERENCE
►In the first cell division, CHROMOSOME
PAIRS (HOMOLOGOUS CHROMOSOMES) are
separated.
►In the second cell division, SISTER
CHROMATIDS are pulled apart.

13. Comparing Mitosis and Meiosis
► Mitosis makes
cells for growth,
development,
repair, and
asexual
reproduction
► Meiosis makes
cells for sexual
reproduction

14. Comparing Mitosis and Meiosis
► Mitosis produces two
cells identical to the
parent cell (diploid -
called daughter cells)
► Meiosis produces four
cells with half the
number of
chromosomes as the
parent (haploid)

15. Comparing Mitosis and Meiosis
► In meiosis, “crossing
over” occurs during
prophase I.
► Chromosomes form tetrads
► Homologous chromosomes
“swap” parts of their DNA
 leads to genetic
variation

16. Sperm vs. Eggs
►Spermatogenesis creates four equally-sized
sperm cells
►Oogenesis creates three tiny cells that will
die (polar bodies) and one large ovum (egg)

17. Spermatogenesis vs. Oogenesis