1. Meiosis: cell division in which gametes are produced.
Chapter 10
Sexual Reproduction and Genetics
10.1 Meiosis
Meiosis I
Meiosis: cell division in which gametes are produced.
When gametes
combine in fertilization, the number of
chromosomes is restored.

2. Every cell has 46 chromosomes (23 pairs) EXCEPT the sex cells (egg and sperm)
These cells are called gametes
Human gametes contain 23 chromosomes total.

3. Chromosomes and Chromosome Number
Chapter 10
Sexual Reproduction and Genetics
10.1 Meiosis
Chromosomes and Chromosome Number
Human body cells (somatic cells) have 46 chromosomes
Each parent contributes 23 chromosomes
23 from mom
23 from dad

5. Homologous chromosomes —one of two paired chromosomes, one from each parent

6. Same centromere position
Chapter 10
Sexual Reproduction and Genetics
10.1 Meiosis
Homologous Chromosomes and Chromosome Number
Same length
Same centromere position
Carry genes that control the same inherited traits

7. Males- Spermatogenesis Females- Oogenesis
Males produce sperm in their testicles. Special cells there called spermatogonia start the process. Males constantly make sperm. Remember 4 are made each time! Billions will be made in a lifetime.
Females-
Oogenesis
Females produce eggs in their ovaries. Special cells there called oogonia start the process. Females make eggs according to their menstrual cycle. Remember only 1 viable egg is made each time meiosis occurs. The other three die off and are called Polar Bodies. About 400 eggs will be made in a lifetime.

8. Production of Sperm cells
Meiosis in Males

9. INTERPHASE and PROPHASE 1
MEIOSIS
INTERPHASE and PROPHASE 1
special division to produce gametes
Chromosomes are in homologous pairs
Cell is “diploid” - has 2 chromosomes of each type
Cell is “2n” where n=the number of types of chromosomes
DNA Replication takes place – makes cell “4n”

10. MEIOSIS
METAPHASE 1
Chromosomes line up next to their homologous partners - .
Homologous pairs in this alignment are called tetrads
Randomly sorted (which goes on left & which on right)

11. MEIOSIS ANAPHASE 1. Connections between chromosome pairs break.
Chromosomes move away from their homologous partners -.
centromeres do not break.
Mistakes at this stage may cause both chromosomes to go to the same side

12. MEIOSIS TELOPHASE 1. Division into 2 new cells.
New cells have different chromosomes.
New cells are “diploid” or “2n”

13. MEIOSIS
METAPHASE 2
Cells line up in 1 line like in mitosis

14. MEIOSIS
ANAPHASE 2.
Centromeres break and chromosomes split into chromatids like in mitosis

15. MEIOSIS
TELOPHASE 2
Cells each divide into 2 cells

16. MEIOSIS 4 GAMETES New cells are haploid gametes.
. could be sperm, pollen, or eggs (in species that need many eggs)

18. Production of eggs
Meiosis in Females

19. MEIOSIS (forming some egg cells).
PROPHASE 1
Same as normal meiosis so far
Cell is called “Primary Oocyte”

20. MEIOSIS METAPHASE 1
Chromosomes line up near edge of cell
otherwise like normal meiosis

21. MEIOSIS ANAPHASE 1
Cell divides nuclear material normally
other cell contents are divided unevenly

22. MEIOSIS TELOPHASE 1
Small cell being formed is called “Polar Body”
Large cell is called “Secondary Oocyte”
Polar Body may die now or may live long enough to divide

23. MEIOSIS METAPHASE 2
Chromosomes line up near edge in Secondary Oocyte
Polar Body’s purpose was to remove chromosomes without losing much cell content

24. MEIOSIS ANAPHASE 2
Cell contents divide unevenly again

25. MEIOSIS TELOPHASE 2
New small cell is “2nd Polar Body”
This polar body dies also

26. MEIOSIS EGG
1 Living Cell becomes “ovum”
Ovum - haploid egg cell

27. Meiosis Mistakes and Crossing Over

30. Results of Nondisjunction
Down's Syndrome, a common form of mental retardation, is caused by possession of three copies of chromosome 21, instead of the usual two.
Turner's Syndrome, a condition that causes infertility and immature sexual development in women, is due to the possession of only one X chromosome, instead of the two always present in normal women.

31. Crossing Over
Homologous chromosome tetrads in metaphase I may exchange portions of their chromatids
Crossing-over produces new combinations of alleles.
KEY POINT: CROSSING OVER INCREASES GENETIC VARIATION IN THE GAMETES
Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.

32. MALES

55. FEMALES

77. Chromosome Pairing
Embryo
Fetus
Baby

78. Comparing Meiosis and Mitosis
Meiosis
Mitosis
Number of cells produced 4 2
Type of cells produced
Haploid
Diploid
Cells are genetically…
Different
The Same
Crossing over…
Occurs
Does not occur
Number of divisions 1
Type of cells…
Sex Cells
Everything else