1. Meiosis – The spice of life

2. Meiosis
A cell division that separates homologous pairs of chromosomes forming haploid sex/germ cells
Whaaaa?
Let’s back it up a few steps 

3. Parents give you your genes
A. Gene – a unit of hereditary info consisting of a specific nucleotide sequence of DNA, via a gamete Genes are packaged into chromosomes.

4. B. A Karyotype
is a display of all a cell’s chromosome pairs in order of shape and size

5. C. Chromosomes come in pairs…in somatic cells that is!
-Homologous chromosomes (one from mom and one from dad) have the same
-size
-centromere location
-shape
-gene type location (locus)
-humans have 46 chromosomes, or 23 pairs!
mom dad

6. Typical Karyotypes
Normal Female Karyotype
Normal Male Karyotype
Autosomes 1-22
Sex chromosomes – 23 23
Sex chromosomes
Because they meet at fertilization
They must be separated
These are all double stranded chromosomes
These are all double stranded chromosomes
1. How many pairs of homologous chromosomes are found in a typical human cell?
2. Which chromosome pair is not homologous in humans?
3. Why are chromosomes found in pairs? (hint – sexual reproduction)
4. What must happen to homolog chromosomes in cells used for sexual reproduction?

7. This karyotype has 2 copies of every chromosome, and we call that diploid (2n). The “n” stands for a set of chromosomes. This means it came from a somatic (or body) cell.
If two somatic cells fertilized each other, what would the offspring’s karyotype look like, or what would the chromosome number be?
***ALMOST ALL your cells are diploid – skin, muscle, bone, fat, nerve Y I K E S !

8. What does meiosis do to the # of chromosomes?
It reduce the chromosome number to one chromosome from each homologous pair! The chromosome number is now haploid (n). The type of cell formed is a gamete (aka germ, sex, reproductive , sperm, egg)
Humans – somatic cell = 46 chromosomes
Humans – germ cell = ____ chromosomes
When is a human cell’s ploidy (chromosome set) restored to diploid? 23
Fertilization

9. There are 3 ways to define MEIOSIS
***Assume you start meiosis with a somatic cell***
What happens to homologous chromosomes?
What cell type is formed?
What happens to the ploidy/chromosome number?
Meiosis separates homologous pairs of chromosomes into new cells
Meiosis makes gametes (sex, reproductive cells)
Meiosis reduces the diploid # (2n) to haploid # (n),
aka (reduction division)

10. II. Meiosis follows the cell cycle
Interphase (G1, S, G2) followed by TWO cell divisions – Meiosis I and Meiosis II.
Don’t forget that chromosomes duplicate during the S phase!

11. homologous chromosomes
Overview of Meiosis
MEIOSIS I: Separates
homologous chromosomes
MEIOSIS II: Separates
sister chromatids
Telophase I
and
Cytokinesis
Telophase II
and
Cytokinesis
Prophase I
Metaphase I
Anaphase I
Prophase II
Metaphase II
Anaphase II
Sister
chromatids
remain
attached
Sister
chromatids
Chiasmata
Spindle
Sister
chromatids
separate
Haploid
daughter
cells
forming
Figure 13.8 Exploring meiosis in an animal cell
Homologous
chromo-
somes
separate
Cleavage
furrow
Homologous
chromosomes
Microtubules
attached to kinetochore
Meiosis II = “equational division” as sister chromatids separate (start double-stranded, centromeres break and become single-stranded)
Meiosis I = “reductional division” as chromosome number goes from diploid to haploid

12. A. Importance of Meiosis I
In Prophase I of Meiosis I, crossing over occurs. This is when the double-stranded homologous pairs connect and exchange DNA (some of mom’s genes go onto dad’s chromosomes and vice versa). This is also known as recombination.

13. Crossing Over and Synapsis in Prophase I1 3
Crossover – DNA exchanged
Paternal
sister
chromatids 4 2
2. Synapsis occurs – When homologous pairs “associate” (loosely bond together)
4. Chiasmata - points where crossing over occurred

14. B. Steps of Meiosis I Prophase I Metaphase I Anaphase I Unique Events:
1. Synapsis: Homologous Chromosomes Pair up
2. Crossing over: Homologous chromosomes exchange parts
Metaphase I
Homologous Chromosomes line up opposite one another
Centromeres are not lined up on the metaphase plate
(different arrangement than mitosis)
Anaphase I
Complete DS Chromosome moves the poles.
CENTROMERES DO NOT SPLIT

15. Meiosis II - ***The two cells are haploid! But they
Telophase I
Cytokinesis
Meiosis II -
***The two cells are haploid! But they
have double-stranded chromosomes!
Prophase II
May be skipped (human males)
Will not start until fertilization (human females)
Metaphase II
Chromosomes line up at equator
***Centromeres line up on the metaphase plate (like mitosis)
Anaphase II
CENTROMERES SPLIT
***Double Stranded chromosomes become single stranded
Telophase II
Cytokenesis
Maturation of gametes into 4 HAPLOID daughter cells

16. III. How Meiosis Increases Genetic Variety in Offspring
Crossing over during Prophase 1 – recombination of genes!
Independent Assortment of chromosomes
during Metaphase 1 & II.
-this means how they align on the ‘metaphase plate)
C. Random fertilization to form zygote

17. IV. Who does Meiosis? 1. Adults always diploid
Meiosis only used to make gametes
Adults can be diploid or haploid
2. Gametes made by mitosis
3. Spores made by meiosis
Meiosis used to produce haploid adult
2. Gametes made by mitosis
3. Only a zygote is diploid

18. TAKE-HOME POINTS
Sexual reproduction GREATLY INCREASES genetic variation!
Sexual reproduction is possible via meiosis, formation of gamete cells with half the chromosomes (haploid) than somatic cells (diploid)

19. Crossing over in Prophase I
Slide 16

20. Independent Assortment of Homologous Chromosomes
Slide 16

21. Meiosis Cell Cycle
Meiosis
Meiosis Phase
Slide 10