1. MEIOSIS

2. 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).

3. 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

4. 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.

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

6. Interphase I Similar to mitosis interphase.
Chromosomes replicate (S phase).
Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
Centriole pairs also replicate.

7. Interphase I Nucleus and nucleolus visible. chromatin nuclear membrane
cell membrane
nucleolus

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 Longest and most complex phase (90%). Chromosomes condense.
Synapsis occurs: homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

10. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

11. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry genes controlling the same inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes

12. Karyotype
A method of organizing the chromosomes of a cell in relation to number, size, and type.

13. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

14. 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.

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

16. 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.

17. Prophase I
centrioles
spindle fiber
aster
fibers

18. Metaphase I Shortest phase Tetrads align on the metaphase plate.
INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation

19. Metaphase I
metaphase plate OR
metaphase plate

20. Anaphase I Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.

21. Anaphase I

22. Telophase I Each pole now has haploid set of chromosomes.
Cytokinesis occurs and two haploid daughter cells are formed.

23. Telophase I

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

25. Prophase II
same as prophase in mitosis

26. Metaphase II
same as metaphase in mitosis
metaphase plate

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

28. Telophase II Same as telophase in mitosis. Nuclei form.
Cytokinesis occurs.
Remember: four haploid daughter cells produced.
gametes = sperm or egg

29. Telophase II

30. Meiosis n=2 n=2 2n=4 sperm haploid (n) meiosis II meiosis I sex cell
diploid (2n)

31. Variation
Important to population as the raw material for natural selection.
Question:
What are the three sources of
genetic variation?

32. Answer: Remember: variation is good! 1. crossing over (prophase I)
2. independent assortment (metaphase I)
random fertilization
mutations (next chapter)
Remember: variation is good!

33. Question:
A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

34. Answer:
10 chromosomes (haploid)

35. 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

36. Sources www.ursulinehs.org/powerpoint/meiosis.ppt