Meiosis

Meiosis is the process of turning one diploid cell into four haploid gametes. It would probably help to define some of those words before moving on.

Diploid - having two sets of chromosomes (di- two; ploid- has to do with sets of chromosomes)
- Most cells in your body have 2 sets of chromosomes
- 1 set comes from your mother; 1 set comes from your father
Haploid - having one set of chromosomes in a cell
- Parents can only pass on 1 set each, otherwise a child would end up with too many chromosomes
Gamete - a cell used for sexual reproduction (sperm or egg)
- Gametes combine during fertilization to form a zygote
- Zygote - the one cell that is a combination of the egg and sperm
  - zygotes are diploid (2 sets) and they multiply using mitosis so that all future cells have the same DNA

Making Gametes

Meiosis sounds like "mitosis," and it is very similar - a cell separates chromosomes to form more nuclei for new cells. While mitosis is used to make identical copies of cells, meiosis creates cells that are different from the original.

The only cells in the body that perform meiosis are diploid "germ cells" (not bacteria etc) in the testes or ovaries.
Interphase is the same as mitosis - chromosomes replicate in the S phase, then the cell grows a bit.
Prophase I - Chromosomes coil up and the nuclear membrane disappears, but they start to arrange differently:
- Homologous Pairs start to form tetrads (groups of 4)
  - Homologous pair - the same types of chromosomes that you got from your mom and dad
  - Each one of the pair is also attached to its copy (sister chromatids)
- Crossing Over - pieces of homologous pairs break off and reattach to the other in the pair
  - This creates more genetic variation by mixing up genes on a chromosome.
Metaphase I - homologous pairs line up in the middle of the cell and attach to spindle fibers
Anaphase I - homologous pairs are separated, but they are still attached to sister chromatids (copies)
Telophase I - nuclear membranes form around each haploid nucleus as the cell prepares to divide
Prophase II - In the new cells, everything starts again - chromosomes coil up
Metaphase II - Sister chromatids are lined up in the middle of the cell
Anaphase II - sister chromatids are pulled apart
Telophase II - nuclei reform

End Result - four cells (gametes) with only one set of chromosomes each (haploid)

Diploid v Haploid Numbers

Be sure you can distinguish diploid cells from those that are haploid. Keep in mind, not all organisms have the same number of chromosomes, so you may need to do a little math.

Diploid Cells

Any cell that is part of the body - brain, liver, skin, etc.
Zygotes
2 sets of chromosomes (mom and dad)
Human = 46 chromosomes
Cow = 60 chromosomes
Cat = 38 chromosomes

Haploid Cells

Gametes (unfertilized egg or sperm)
1 set of chromosomes (after meiosis)
Human = 23
Cow = 30
Cat = 19