1. Cellular Reproduction: Meiosis
The Process by which Sperm and Egg are formed in the Sex Organs

2. Mitosis vs. Meiosis Meiosis Mitosis Two parents of different genders
Two sets of phases
Chromosomes divide twice after DNA replication
4 gamete cells, each with haploid number of chromosomes
Mitosis
One Parent
One set of phases
Chromosomes divide once after DNA replication
Two daughter cells with diploid number of chromosomes

3. Meiosis: Gametogenesis
The process of making gametes in animals which reproduce by sexual means is called gametogenesis
Eggs are produced in the ovary by a process called Oogenesis—diploid cell is called the oocyte—haploid cell is the ovum;
However, of the four resulting ova, only one will be functional
Sperm is produced in the testes by Spermatogenesis—diploid cell is called the spermatogonia—resulting haploid cells are call spermatids and all should be viable
What might cause sperm to be non-viable?

4. Meiosis: Chromosomal number
Normal Body Cells have a diploid (2n) number of chromosomes—these are normal skin, nervous, bone, blood, and all other types of cells—they will divide by mitosis as the tissues develop
Sexual organs (ovaries and testes) produce gametes with a haploid (1n) number of chromosomes—they must unite to form a complete organism with a full chromosomal set

5. Chromosomal Number—See p.388
Humans have 46 chromosomes (haploid 23)
Dogs have 78 chromosomes (haploid 39)
Why do dogs have more?
Much more diverse with regard to size, hair length, ear length, tails, color, etc.
Domestic cats have 38 chromosomes
Rats have 42 chromosomes
Chimps and most apes have 48 chromosomes
Corn and most vegetables have between chromosomes

6. Meiosis In Meiosis, the chromosomal number is reduced by one half
Humans 46/2 =23; Dogs 39; Cats 19
In Meiosis, the chromosomes are duplicated once before Prophase as in Mitosis, but cell division continues with each cell dividing again, resulting in gametes with half the chromosomes
The resultant gametes are said to have the haploid or one-half the number of chromosomes

7. Meiosis I: Phases Meiosis I involves:
Prophase I: replicated pairs of homologous (identical) chromosomes condense and attach to each other
Metaphase I: spindle fibers arrange the chromosome pairs in the middle of the cell
Anaphase I: the fibers separate the pairs and pull them to opposite sides of the cell
Telophase I: Cytokinesis takes place resulting in two diploid cells, just as in Mitosis.
But there is no Interphase and no further replication of DNA

8. Meiosis II: Phases
The resulting two cells go through a second set of division, but this time the chromosomes are NOT replicated before the process continues
Prophase II: Spindle fibers form again and the diploid chromosomes condense again
Metaphase II: the fibers align the chromosomes along the center of the cell attached to each sister chromatids
Anaphase II: sister chromatids are separated and pulled to opposite poles of the cell
Telophase II: cytokinesis divides these two cells into four haploid cells with half the number of chromosomes