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Meiosis, condensed.

What Is Meiosis?

Meiosis is the formation of gametes; the sex cells used in sexual reprod­uction (egg and sperm cells).
Usually, during interp­hase, body cells are diploid cells; they contain a full set of chromo­somes (2n). Gametes are haploid (n). Each parent's gametes contain half the genetic material needed, so when a gamete from each parent fuse together, the offspring has a full set of genetic material (2n); half from each parent.

The Stages of Meiosis

1) Interphase
DNA within the cell is duplic­ated.
2n 4n
2) Prophase I
Chromo­somes form homologous pairs (tetrads), and crossing over occurs, where certain genes from each chromosome physically interact and swap over.
3) Metaphase I
Tetrads line up at the equator of the cell.
4) Anaphase I
Spindle fibers pull 1 chromosome from each tetrad to opposite poles of the cell.
5) Telophase I
A cleavage furrow forms, which pinches the cell in the middle.
6) Cytoki­nesis
The cell divides into 2 daughter cells.
4n 2n
7) Prophase II
The reformed nuclear envelope breaks down.
8) Metaphase II
Chromo­somes line up at the cell's equator.
9) Anaphase II
Each chromo­some's sister chromatids are pulled to opposite poles of the cell by spindle fibers.
10) Telophase II
A cleavage furrow forms.
11) Cytoki­nesis
Each daughter cell divides into 2 cells, forming a total of 4 haploid cells for the whole process.
2n n

What Causes Genetic Variation?

Indepe­ndent assort­ment, which occurs when chromo­somes randomly line up at the equator.
Crossing over, during prophase I, where genetic material is swapped by chroma­tids.
Random fertil­isa­tion. During sexual reprod­uction, only one sperm will fertilise one egg, meaning it is random chance which combin­ation of genetic inform­ation will be present in the parents' gametes.

Meiosis Diagram

How Are Mitosis and Meiosis Different?

Number of cell divisions
Number of daughter cells
Genetic compos­ition of daughter cells
Role in animals
Tissue growth, asexual reprod­uction
Formation of gametes (for sexual reprod­uction)


this is so outragous i cryed
Do not use "tetrads" for the homologous pairs, "tetrads" is an out-of-date, but still often-used, incorrect term for the paired homologues in prophase I of meiosis. Use "bivalent." That way if the homologues are not paired (asynapsis), each is called a "univalent." If there are three homologues (viz., trisomy), when synapsed form a "trivalent." Also, in plants and many lower organisms meiosis results in the formation of haploid spores, not gametes. The four hapolid spores are a "tetrad." In plants the spores develop into gametophytes, the haploid plants that produce the gametes. Mitosis occurs in cells of different ploidy levels, i.e., n, 2, 3n, etc, not just in diploid (2n) cells. Examples: In hapolid (n) cells the development of gametophytes in plants. In 3n (triploid) cells the develop of the endosperm in seeds of flowering plants. In the diagram, for prophase I say "chromosomes pair," no need to add the "up."

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