1. Word(s) of the Day
Diploid : Is a Cell that contains two identical sets of DNA
Haploid: Cell that contains Half the total number of chromosomes

2. Goals for the day !
Identify the order in which different cellular organelles produce haploid cells Use new vocabulary such as haploid and diploid to describe meiosis Compare and Contrast Mitosis and Meiosis

3. MEIOSIS Not

4. 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).

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
“Sperm” “Formation”
2n=46
human
sex cell
diploid (2n)
n=23
meiosis I

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

8. 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

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

10. Interphase I

11. 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

12. 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).

13. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

14. 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.

15. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

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

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

18. And once again because sex cells are weird ….

19. Sex Chromosomes DO NOT crossover !
XX chromosome - female
XY chromosome - male

20. Prophase I
centrioles
spindle fiber or “Fishing Line”

21. Metaphase I Shortest phase Tetrads align on the metaphase plate.
INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations

22. Metaphase I
metaphase plate OR
metaphase plate

23. Answer 223 = ~8 million combinations Formula: 2n
Human chromosomes: 2n = 46
n = 23
223 = ~8 million combinations

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

25. Anaphase I
metaphase plate

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

28. Telophase I

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

30. Prophase II
same as prophase in mitosis

31. Metaphase II
same as metaphase in mitosis
metaphase plate

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

33. Telophase II Same as telophase in mitosis. Nucleus reforms .
Cytokinesis occurs.
Remember: four haploid daughter cells produced.
gametes = sperm or egg

34. Telophase II

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