1. Meiosis

2. Cell Division
Mitosis is the division of one cell into two IDENTICAL DIPLOID cells
Identical to each other
Identical to the original cell
Same number of chromosomes

3. Cell Division
MEIOSIS is the division of one cell into four genetically DIFFERENT HAPLOID cells (known as gametes or sex cells)
Different from each other
Different from the original cell
HALF the number of chromosomes

4. Sexual Reproduction When a sperm fertilizes an egg…
Does the fertilized egg have twice as many chromosomes?
Why not?
Because the sperm and the egg have HALF the chromosomes of a normal cell

5. Chromosome Pairs Humans have 46 chromosomes
Each of our 46 chromosomes has a “twin”
23 sets of “twins”

6. Chromosome Pairs HOMOLOGOUS PAIRS
Two chromosomes with same genetic info
Mom’s gene for hair color match up with dad’s gene for hair color to determine your hair color
23 chromosomes come from mom
23 chromosomes come from dad
= 46
23 HOMOLOGOUS PAIRS

7. Sexual Reproduction Gamete – any reproductive cell
Egg or sperm
Gametes are haploid (23)
HAPLOID = 1 set of unpaired chromosomes
HA = Half
PLOID = Chromosomes

8. Sexual Reproduction Haploid + Haploid = Diploid
Egg + Sperm = Fertilized Egg
Meiosis is the cell division in which the resulting cells (gametes) have half the number of chromosomes as the original cell

9. Meiosis
Meiosis has two parts:
Meiosis I
Meiosis II

10. PMAT PMAT Meiosis I & II Meiosis I Meiosis II Prophase I Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
PMAT
PMAT

11. Meiosis Overview Very similar to mitosis Here are the differences…
Starts as diploid (2n)
Ends as haploid (1n)

12. Meiosis Overview Very similar to mitosis Here are the differences…
Starts as diploid (2n)
Ends as haploid (1n)

13. Meiosis I: Homologous Pairs Separate
Prophase I
DNA condenses
Nuclear envelope breaks down
Spindle fibers form
Difference: Homologous pairs begin to pair up

15. Crossing Over
In Prophase I, the homologous pairs, pair up with each other
During this time, crossing over can occur
The chromatids are so close to each other, they can literally cross each other and swap genetic information
Always in equal amounts
Crossing over creates genetic variation

16. Genetic Variation: Crossing Over

18. Meiosis I: Homologous Pairs Separate
Metaphase I
Homologous pairs
Line up at the middle
DIFFERENT FROM METAPHASE
Metaphase each one lines up
Metaphase 1 pairs line up
SAME – spindle fibers attach

20. Meiosis I: Homologous Pairs Separate
Anaphase I
Homologous pairs separate and move to opposite poles
SIMILAR TO ANAPHASE
BUT Sister chromatids are still together

22. Meiosis I: Homologous Pairs Separate
Telophase I
Chromosomes gather
Cytokinesis begins
SAME AS TELOPHASE

24. Meiosis II: SISTER Chromatids Separate
Prophase II
DNA stays condensed
Nuclear envelope breaks down
Spindle fibers form
SAME AS PROPHASE AND PROPHASE 1

26. Meiosis II: SISTER Chromatids Separate
Metaphase II
Chromosomes line up at equator
SAME AS METAPHASE
Metaphase I – pairs separate
Metaphase II – chromatids separate

28. Meiosis II: SISTER Chromatids Separate
Anaphase II
Chromatids separate and move to opposite poles
SAME AS ANAPHASE

30. Meiosis II: SISTER Chromatids Separate
Telophase II
Chromosomes gather and nuclear envelope forms
Cytokinesis begins
SAME AS TELOPHASE

32. Oogenesis- formation of ovum (egg cells)
Oo = egg
Genesis = beginning or make
Oogenesis is meiosis for females
Female mammals are believed to be born with finite number of potential eggs
7 million at 20 weeks of gestation
1-2 million at birth
400,000 at puberty
~500 will be released in their lifetime
Few if any complete meiosis

33. Spermatogenesis- formation of sperm cells
Spermatogenesis is meiosis for males
Begins at puberty, ends at death
One spermatocyte yields four spermatozoa
Process takes about 64 days
170 million sperm made a day

35. Meiosis Video

36. Meiosis