Reproduction and Meiosis

Reproduction Reproduction—the process of producing offspring Asexual—one parent, offspring are genetically identical to parent Budding, fission, fragmentation, parthenogenesis Sexual—two parents, offspring genetically different from parents Parents produce gametes Females=egg Males=sperm Two gametes combine=fertilization; produces first cell called the zygote Offspring will have traits of both parents

Homologous Chromosomes Each species has a characteristic number of chromosomes Humans=46; dogs=78; fruit flies=8 Chromosomes come in pairs—homologous chromosomes Chromosomes with the same size, shape, and carry the codes for the same genes Each chromosome in a homologous pair comes from one of the parents In humans, 23 chromosomes come from mom and 23 from dad

Meiosis Form of cell division that produces 4 daughter cells with half the number of chromosomes as the parent cell 2 distinct stages Meiosis I Prophase I, Metaphase I, Anaphase I, Telophase I Meiosis II Prophase II, Metaphase II, Anaphase II, Telophase II

Meiosis I Prophase I Chromosomes condense Homologous chromosomes pair up Crossing over Chromatids exchange genetic material Increases genetic variability

Meiosis I Metaphase I Anaphase I Telophase I Homologous chromosomes line up along equator of cell Anaphase I Homologous chromosomes move apart, pulled by spindle fibers Telophase I Cytoplasm divides, resulting in two cells with the same number of chromosomes as parent

4 different haploid cells Meiosis II Prophase II Metaphase II Telophase II 4 different haploid cells Anaphase II

Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

Mitosis vs Meiosis Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent Where Body cells Sex cells When Throughout life At sexual maturity Role Growth and Repair Sexual Reproduction

Gamete Production Gametes have half the number of chromosomes of the organism Sperm (23 chromosomes) + Egg (23 chromosomes) = Human (46 chromosomes) Diploid (2n) A cell with two of each chromosome, which are in pairs {FULL set} Haploid (n) A cell with only one of each chromosome {half set}

Gamete Production: Sperm Occurs in the testes Two divisions produce 4 spermatids Spermatids mature into sperm Men produce about 250,000,000 sperm per day

Gamete Production: Egg Occurs in the ovaries Two divisions produce 3 polar bodies that die and 1 egg Polar bodies die because of unequal division of cytoplasm Immature egg called oocyte Starting at puberty, one oocyte matures into an ovum (egg) every 28 days