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

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

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?

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

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 14-26 chromosomes

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

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

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