1. Problem:
Our cells have 46 chromosomes—so why don’t fertilized eggs have 92 chromosomes?

2. Answer:
Meiosis

3. Meiosis
The form of cell division by which gametes, with half the number of chromosomes, are produced.
Diploid (2n)  haploid (n)
Meiosis is sexual reproduction.
Two divisions (meiosis I and meiosis II).

4. Fertilization n=23 egg 2n=46 zygote
The fusion of a sperm and egg to form a zygote.
A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote

5. Meiosis Sex cells divide to produce gametes (sperm or egg).
Gametes have half the # of chromosomes.
Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
Meiosis is similar to mitosis with some chromosomal differences.

6. Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) meiosis II human
sex cell
diploid (2n)
n=23
meiosis I

7. Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

8. Meiosis I (four phases)
Cell division that reduces the chromosome number by one-half.
four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I

9. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

10. Crossing Over
Crossing over (variation) may occur between nonsister chromatids at the chiasmata.
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Chiasmata (chiasma) are the sites of crossing over.

11. Crossing Over - variation
Tetrad
nonsister chromatids
chiasmata: site of crossing over
variation

12. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

13. Metaphase I
metaphase plate OR
metaphase plate

14. Anaphase I

15. Telophase I

16. Meiosis II No interphase II (or very short - no more DNA replication)
Remember: Meiosis II is similar to mitosis

17. Prophase II
same as prophase in mitosis

18. Metaphase II
same as metaphase in mitosis
metaphase plate

19. Anaphase II
same as anaphase in mitosis
sister chromatids separate

20. Telophase II