1. Meiosis
Sexual Reproduction

2. Mitosis Review
Mitosis: division of somatic (body) cells during the cell cycle.
Product = 2 identical daughter cells.
Start with 46 chromosomes and ends with 46 chromosomes.
2n = 2n when n = a single chromosome
Ie: n = 23 chromosomes and 2n = 46 chromosomes or 23 pairs of chromosomes.

3. Notes Meiosis is: The division of sex cells (gametes)
Humans have 46 chromosomes
Fruit fly has 8 chromosomes
Diploid = “two sets” 2n Haploid = “one set” n
Meiosis is division of gametes, while mitosis is division of somatic (body) cells.
There are 2 cell divisions during meiosis.
Gametes (sex cells):
Males = XY
Females = XX
Fertilizations: Joining of a sperm (n) and egg (n) to create a zygote (2n)

4. How about the rest of us? + 46 46 92
What if a complex multicellular organism (like us) wants to reproduce?
joining of egg + sperm
Do we make egg & sperm by mitosis?
No!
if we did, then…. 46 + 46 92
egg
sperm
zygote
Doesn’t work!

5. Karyotype A “picture” of our chromosomes used by genetists.
If you remember: our parents gave each of us 1/2 set of chromosomes (23 of them to be exact) to pair up and complete the set (46).
Used by scientists and doctors to look for abnormalities and gender
Pictures are taken during the prophase/metaphase stage of Mitosis. When chromosomes become visible.
Each chromosome is paired up with it’s match and then laid out in order by size and shape.

8. Homologous chromosomes
Paired chromosomes
both chromosomes of a pair carry “matching” genes
control same inherited characters
homologous = same information
single stranded homologous chromosomes
diploid 2n
2n = 4
double stranded homologous chromosomes

9. How do we make sperm & eggs?
Must reduce 46 chromosomes  23
must reduce the number of chromosomes by half 23 46 23
zygote 46
egg 23
meiosis 46 23
fertilization
sperm
gametes

10. Steps of meiosis Meiosis 1 Meiosis 2 interphase prophase 1 metaphase 1
anaphase 1
telophase 1
Meiosis 2
prophase 2
metaphase 2
anaphase 2
telophase 2
1st division of meiosis separates homologous pairs
(2n  1n)
“reduction division”
2nd division of meiosis separates sister chromatids
(1n  1n)
* just like mitosis *

11. Overview of meiosis I.P.M.A.T.P.M.A.T 2n = 4 interphase 1 prophase 1
metaphase 1
anaphase 1
n = 2
n = 2
prophase 2
metaphase 2
anaphase 2
telophase 2
n = 2
telophase 1

12. Preparing for meiosis 1st step of meiosis Duplication of DNA
Why bother?
meiosis evolved after mitosis
convenient to use “machinery” of mitosis
DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis)
2n = 6
single
stranded
2n = 6
double
stranded
M1 prophase

13. Meiosis 1 1st division of meiosis separates homologous pairs synapsis
single
stranded
1st division of meiosis separates homologous pairs
2n = 4
double
stranded
prophase 1
synapsis
2n = 4
double
stranded
metaphase 1
tetrad
reduction
1n = 2
double
stranded
telophase 1
Repeat after me!
I can’t hear you!

14. 4 Meiosis 2 2nd division of meiosis separates sister chromatids
double
stranded
prophase 2
What does this division look like?
1n = 2
double
stranded
metaphase 2
1n = 2
single
stranded
telophase 2 4

15. Meiosis 1 & 2

16. Trading pieces of DNA Crossing over
during Prophase 1, sister chromatids intertwine
homologous pairs swap pieces of chromosome
DNA breaks & re-attaches
prophase 1
synapsis
tetrad

17. Mitosis vs. Meiosis

18. Mitosis vs. Meiosis Mitosis Meiosis 1 division
daughter cells genetically identical to parent cell
produces 2 cells
2n  2n
produces cells for growth & repair
no crossing over
Meiosis
2 divisions
daughter cells genetically different from parent
produces 4 cells
2n  1n
produces gametes
crossing over

19. Pedigrees
Pedigrees are family trees that show specific traits passed down through each generation.
Ms. Greb’s family tree:

20. How to read a pedigree