1. Re-Cap
OK so we learned that in mitosis, somatic cells go through cell division.
In humans, one cell starting with 46 chromosomes will result in 2 cells each with 46 chromosomes.
So what type of cells don’t go through mitotic cell division??

2. Sex Cells!
Sex cells, or gametes, do NOT go through mitosis. They do a different division called meiosis.
The result from this type of division is 4 cells that have half the number of chromosomes than the original cell.
So one human cell that has 46 chromosomes will produce 4 sex cells (gametes) that each have 23 chromosomes.

3. But… Why? Every organism has a specific chromosome number.
Humans = 46 chromosomes
Chimpanzees = 48 chromosomes
Ferns = 1010 chromosomes
This chromosome number is called the organism’s diploid (2n) number. That is because there are 2 of each type of chromosome. One comes from ma and one comes from pa.
These chromosomes are called homologous chromosomes because they code for the same traits.

4. But… Why? continued*
In order for a ma and pa to make a baby, they have to join two gametes (egg & sperm) together.
When 2 sex cells fuse, or unite, that is call fertilization and the resulting organism is called a zygote.
If those cells were both diploid (2n) then that would mean…
Not Good… The chromosome number is not the same.

5. The Solution?
To keep the chromosome number constant, one diploid cell with go through a division called meiosis to produce 4 haploid (n) gametes.
That way when sperm and egg come together we can have a whole diploid zygote.

6. Lets Get Started
Before meiosis can begin, the chromosomes have to replicate.
This occurs during interphase.
Cells will grow in size to prepare to divide
Cells will synthesize the necessary proteins

7. Phases of Meiosis
Meiosis occurs in 2 parts. First you have Meiosis I, then you have Meiosis II.
In meiosis I you start with one diploid cell and you end with 2 haploid cells.
In meiosis II you start with the 2 haploid cells from meiosis I and end with 4 haploid cells.
Total, there are 2 divisions in the process of meiosis.

8. Prophase I Chromosomes become visible Nuclear envelope disappears
Centrioles head to opposite poles and spindle forms
Homologous chromosomes (one pair of sister chromatids from the mother and one from the father) pair up to form a tetrad
The tetrad pairs up so tightly that crossing over occurs

9. Metaphase I Spindle fibers attach to the centromeres
Tetrads line up along the equator (or middle of the cell)
Note that homologous chromosomes line up together along the equator in Meiosis where in Mitosis, they lined up independently to one another.

10. Anaphase I
Homologous chromosomes separate and head to opposite ends of the cell
Centromeres DO NOT split – Sister chromatids will stay together until the next division

11. Telophase I Spindle is broken down Chromosomes uncoil
Cytoplasm divides into two cells

12. Prophase II Sister Chromatids are visible
If the nuclear envelope has reformed from meiosis I, it will break down again
Centrioles will head to the poles and spindle fibers form.

13. Metaphase II
Spindle pulls the sister chromatids to the middle of the cell where they line up along the equator in random order (just as they did during Mitosis)

14. Anaphase II
Centromere of each sister chromatid splits and each chromatid heads for an opposite pole

15. Telophase II and Cytokinesis
Nuclei reform (nuclear envelope reappears)
Spindle breaks down
Chromosomes uncoil
Cytoplasm divides into a total of four haploid cells that will become gametes
Each cell contains one chromosome from each homologous pair

16. Variability
Meiosis has a large role in maintaining variability in a species.
Through sexual reproduction, offspring are not simply replicas of one organism but a genetic combination of two organisms
Crossing over during Prophase I insures that a parent organism can pass on different gametes each time it reproduces, creating a variety of offspring.

17. Chromosomal Mutations
Chromosomal mutations can happen when chromosomes break and do not repair correctly.

18. NON-DISJUNCTION… oops!
Errors can also occur during Meiosis.
Sometimes the homologous chromosomes do not separate properly – this is called nondisjunction
This results in gametes with either an extra copy of a chromosome or no copy at all.
Normal: Non-disjunction examples:

19. Disorders From Non-Disjunction…
Remember: In normal fertilization, a zygote would get one copy of a chromosome from each parent resulting in one pair of each type of chromosome (humans: 23 pairs)
Monosomy – when the zygote gets a copy of a chromosome from only one parent so it is missing one chromosome
Most zygotes with monosomy do not survive
One exception is the case of Turner’s Syndrome
Females have only one X chromosome instead of two
These people will still have female sexual characteristics but they will generally be underdeveloped

21. Identifying a Chromosomal Disorder
To determine whether or not an organism has the proper number of each chromosome, one can look at a karyotype
To make a karyotype a photograph is taken of the paired chromosomes during metaphase
These pairs are cut out and arranged in a chart according to length and location of centromere
Once arranged, it is easy to see if there are any extra or missing chromosomes