1. Meiosis
PAP Biology

2. Our bodies have 2 types of cells:
Somatic cells = Body Cells
These are diploid (2n)
Ex. Blood cells, lung cells, muscle cells, heart cells, etc.
Gametes = Sex Cells
These are haploid (n)
Ex. Egg and Sperm

3. Our cells have 2 types of chromosomes:
Autosomes
Carry traits that make you who you are
Ex. Chromosomes 1-44
Sex chromosomes
Carry traits that
make you who you
are AND determine
your gender
Ex. Chromosomes
45-46 (X or Y)
Karyotype: a diagram that shows the number and appearance of chromosomes in a cell.

4. Homologous Chromosomes
Homologous chromosomes = chromosome pairs that have the same types of genes
One from mom and one from dad
Different from…
Sister chromatids = 2 identical copies of the same chromosome

5. During sexual reproduction, the number of cells is important
Diploid Cells = 2 full sets of chromosomes
(2n)
A set from mom and a set from dad
Ex. Human somatic cells 2n = 46
Haploid Cells = 1 full set of chromosomes
(n)
Only one set that is a combination of chromosomes from mom and dad
Ex. Gametes = Egg and Sperm n = 23

6. During sexual reproduction, the number of cells is important
MEIOSIS
Duplicated
chromosomes 2n
Meiosis I
Duplicated
chromosomes
Duplicated
chromosomes n n
Meiosis II
Unduplicated
chromosomes
Unduplicated
chromosomes n n n n

7. Sexual Reproduction
Sexual reproduction –fusing the genetic information (gametes) from two parents to produce offspring that are a genetic mixture of both parents.
Fertilization – the actual fusion of egg and sperm to form a zygote

8. Purpose of Meiosis
Meiosis is the process of creating gametes – sex cells that have HALF the normal number of chromosomes (only 1 set)
We call these cells haploid because they only have one of each chromosomes. These cells are used ONLY for sexual reproduction (sperm in males or eggs in females)
Each parent only passes on HALF of their chromosomes (23) – one from each pair – so the offspring ends up with a full set (46)

9. Before Meiosis I… Interphase = growth phase of cell.
Remember it is divided into 3 phases:
G1 (gap 1) phase - the cell grows and makes proteins.
S (synthesis) phase - chromosomes replicate
G2 (gap 2) phase - cells continue to grow and make proteins
**At the end of interphase the cell has 2 full sets of duplicated chromosomes**

10. Meiosis I Meiosis II A separation of homologous chromosomes
A reduction from diploid duplicated chromosomes to haploid duplicated chromosomes
A separation of sister chromatids
Duplicated haploid daughter cells from Meiosis I divide into individual chromosomes

11. Prophase I Nuclear membrane breaks down.
Centrioles separate and make spindle fibers
Homologous chromosomes pair up & become visible.
Tetrad - cluster of 4 chromatids.
Crossing over may occur between homologous chromosomes

12. Crossing Over
During Prophase 1 homologous chromosomes are lined up together
Sometimes chromosomes can cross over each other and get “tangled”
When this happens, they swap pieces of DNA
This process creates new combinations of genes – chromosomes that are “part mom/part dad”
Image: © AndreaLaurel

13. Metaphase I
Homologous chromosomes are lined up in the middle of the cell in pairs

14. Anaphase I
Homologous chromosomes pairs separate, one chromosome (2 sister chromatids) to each side of the cell
Sister chromatids remain attached!

15. Telophase I (and Cytokinesis)
Chromosomes gather at poles
Nuclear membrane MAY form
Cytoplasm divides (cytokinesis)
End Result: Results in 2 haploid daughter cells with duplicated chromosomes that are different from the sets in the original diploid cell

16. Prophase II
Spindle fibers form & attach to the centromeres of the sister chromatids.
Nuclear membrane breaks down.

17. Metaphase II
Sister chromatids line up in the middle of the cell single file

18. Anaphase II Sister chromatids are pulled apart
One chromatid from each pair travels to each pole

19. Telophase II (and Cytokinesis)
Nuclear membrane forms around each set of chromosomes.
Spindle fibers dissolve
Cytoplasm divides
End Result: 4 genetically unique, haploid daughter cells, each containing one set of chromosomes

20. Mitosis vs. Meiosis Creation of diploid somatic cells
What
When
Where
Why
How
Result
Type of Reproduction
Creation of diploid somatic cells
Creation of haploid sex cells
Females: before you are born; Males: throughout life
Throughout your life
Throughout body
In ovaries and testes
Resources I love:
Crash Course Mitosis:
Crash Course Meiosis:
For growth and repair
To make babies
PMAT once
PMAT twice
2 identical diploid somatic cells
4 unique haploid gametes
Asexual
Sexual
(c) It's Not Rocket Science