1. Mitosis & Meiosis

2. Mitosis Review Cell makes a copy of itself
1) Copy its DNA (part of a chromosome).
2) Copies are separated & sorted into two sides of the cell.
3) The cell then splits in two.
4) Part of each parent is carried to the two new cells.
5) Two EXACT daughter cells (same number of chromosomes)
Examples: skin, bones, internal organs

3. Meiosis – Sexual Reproduction
Cell division that forms gametes (egg and sperm cells)
Gametes have half the # of chromosomes.
Two divisions (meiosis I and meiosis II).
Part of each parent is carried to the four new cells.
Meiosis is similar to mitosis with some chromosomal differences.

4. Each resulting cell still has chromosomes from Mom & Dad
Mitosis: Meiosis:
Each resulting cell still has chromosomes from Mom & Dad

5. Tips MITosis takes one cell and Makes It Two
Meiosis has to do with sex
From the cell’s point of view:
mITosis results in Identical Twins CELLS
mEioSis results in Egg and Sperm

6. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs carry genes controlling the same inherited traits.
Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 1 pair of sex chromosomes

7. Homologous chromosomes
sister chromatids
paternal
maternal
Tetrad

8. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

9. Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues.

10. Autosomes (The Autosomes code for most of the offspring’s traits)
In Humans the “Autosomes” are sets

11. In Humans the “Sex Chromosomes” are the 23rd set
Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. Two “X” chromosomes = female. One “X” chromosome and one “Y” chromosome = male.
In Humans the “Sex Chromosomes” are the 23rd set
XX chromosome - female
XY chromosome - male

12. Cell Division in Sex Cells
Interphase I
Meiosis I and Meiosis 2
Similar to Mitosis EXCEPT changes in chromosomes

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

14. 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

15. Prophase I
centrioles
spindle fiber
TETRAD

16. Prophase I Homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

17. Homologous chromosomes
sister chromatids
paternal
maternal
Tetrad

18. Prophase 1 – Crossing Over
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Crossing over causes variation

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

20. Metaphase I Tetrads align on the in center of cell on spindle
INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation OR

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

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

23. Meiosis II
No interphase II
Remember: Meiosis II is similar to mitosis

24. Prophase II
same as prophase in mitosis

25. Metaphase II
same as metaphase in mitosis

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

27. Telophase II Four haploid daughter cells produced
gametes = sperm or egg

28. 2n diploid
n haploid
n haploid

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

30. Oogenesis

31. 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

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

33. Answer: Remember: variation is good! 1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!

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

35. Answer:
10 chromosomes (haploid)

36. Question:
A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

37. Answer:
10 chromosomes

38. Meiosis animation: