1. Cell Division - Meiosis

2. Who Does Meiosis and Why?
All Multicellular Eukaryotes
Animals, Plants, Fungi
Meiosis is needed for sexual reproduction
Sexual Reproduction is beneficial because….
It allows genetic recombination
New combinations of genes may ..
1. better suit environments
2. allow offspring to survive radical change in environment
3. allow offspring to survive a disease outbreak

3. Vocab
Cells produced by meiosis =
Gametes
Sex Chromosomes = determine sex
(in humans XX = female, Xy = male)
Autosomes = any chromosome not used to determine sex of offspring

4. .
Gametes = haploid cells made by meiosis
Haploid gametes have one set of chromosomes (1 of each kind)
Called 1n where n = number of
different C types
Humans have
23 different types
22 autosomes +
1 sex determining

5. .
Diploid cells have 2 of each kind of chromosome
Diploid cells are called 2n where n=
The number of different Chromosome types
Humans = 23 types
Somatic cells have
2 of each type ->

6. .
To determine 1n chromosome number multiply 1 * n
Humans 1n C number is … 23
To determine 2n chromosome number multiply 2 * n
Human’s 2n C number is…
2 * 23 = 46

7. Based on the Cat Karyotype below, calculate the 1n and 2n number for cats
18 different autosomes (C # 1-18) one pair of sex chromosomes
So 19 different kinds
1n = 1(19) = 19
2n = 2(19) = 38

8. Haploid Cells produced by Meiosis
Animal Meiosis only happens in gonads
Gonads = ovary or testis
Specialized cells called spermatocytes or oocytes are the only gonad cells that do meiosis
Spermatocytes and oocytes are constantly replaced by specialized cells in gonads called
Spermatogonia cells & oogonia cells

9. Spermatogenesis
Spermatogonia do
Mitosis to make
1Primary spermatocyte
Which…
Does meiosis to make 4 spermatids

10. Spermatids differentiate into sperm
Acrosome = vesicle w/ penetrating enzymes
Nucleus = 1n
Midpiece makes ATP powers flagellum
Tail = flagellum

11. Oogenesis
Oogonia do
Mitosis to make
1 primary Oocyte
Which does…
Meiosis I to make
1 secondary oocyte and one polar body
Meiosis II makes
1 ovum &1 polar body

12. Meiosis I:separate homologous pairs (cells become 1n with X shape C)
Prophase I :
same as mitosis +
Synapsis &
crossing over
Metaphase I :
C line up in pairs

13. .
Anaphase I :
Homologs move to
opposite poles
(cohesin along arms breaks down)
(cohesin at centromeres remains)

14. Telophase I :
Cytokinesis sinultaneously
X shaped C reach poles
Many cells do not decondense C or
reform nuclear E
Spindle only partly broken down

15. Homologous Chromosomes
Homologous Chromosomes are
same C type
same length
code for same genes
(but can be different versions)
Unless they are X and y which are the only pair of Homologues that do not look alike

16. Synapsis
Homologous Chromosomes pair up
Line up gene to gene then physically connect along their arms
Proteins form a synaptonemal complex that bind them together

17. .
Crossing Over : Non-sister chromosomes trade corresponding segments of DNA
Genes far from centromere cross over most often
Location of a gene on a chromosome =
the gene’s locus (loci)
Distance from centromere measured in
map units

18. Chiasmata
X shaped region where crossing over has happened.
Synaptonemal complex has broken down but …
Cohesions still hold sister chromatids together

20. Meiosis in Fungi
Fungal bodies formed of hyphae
Filaments made of chains of 1n cells
Hyphae interweave to form fungus body
The body mass is called a mycelium

22. Phylum Ascomycota don’t make mushrooms
Hypha can be + or –
If one of each meet then…
2 cells (one + one-) fuse cytoplasm
(plasmogamy)
Then fuse their nuclei … now 2n
(karyogamy)
Then do meiosis…making 4 haploid nuclei

24. Each 1n nucleus does mitosis
So now 8 nuclei in the cell
Each nucleus grows a tough casing around it and becomes a
Spore (ascospore)
Spores disperse on wind
Germinate to make new 1n hyphae .

25. The ascospores that form from one cell stay together in a capsule called an ascus

26. Homologous C both have gene for spore color…one tan color one black color
No crossing over
= 4:4
Any other pattern results from crossing over

27. Genetic Variation
Increased by crossing over
Increased by Independent Assortment..
Each pair of homologues lines up randomly and Independent of how other pairs line up
# different gametes formed = 2n
Humans =
223 or 8,399,608