Transcript Slide 1
Meiosis Formation of Gametes (Eggs & Sperm) 1 Facts About Meiosis Preceded by interphase which includes chromosome replication Two meiotic divisions --- Meiosis I and Meiosis II Called Reduction- division Original cell is diploid (2n) Four daughter cells produced that are haploid (1n) 2 Facts About Meiosis Produces gametes (eggs & sperm) Occurs in the testes in males starting at puberty (Spermatogenesis) Occurs in the ovaries in females (Oogenesis). Meiosis I finishes before the female is born and Meiosis II happens for one egg each month before fertilization. Therefore the female is born with all her eggs but are not ready for ferilization until Meiosis II is completed. 3 Why Do we Need Meiosis? It is the fundamental basis of sexual reproduction Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote 4 Fertilization – “Putting it all together” 2n = 6 1n =3 5 Replication of Chromosomes Replication is the process of duplicating a chromosome Occurs prior to division Replicated copies are called sister chromatids Held together at centromere Occurs in Interphase 6 Meiosis: Two Part Cell Division Sister chromatids separate Homologs separate Meiosis I Diploid Meiosis II Diploid Haploid 7 Meiosis I: Reduction Division Spindle fibers Nucleus Early Prophase I (Chromosome number doubled) Late Prophase I Metaphase I Nuclear envelope Anaphase I Telophase I (diploid) 8 Prophase I Early prophase Homologs pair. Crossing over occurs. Late prophase Chromosomes condense. Spindle forms. Nuclear envelope fragments. 9 Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids) Join to form a TETRAD Called Synapsis 10 Crossing-Over Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring 11 Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment at points called chiasmata. 12 Metaphase I Homologous pairs of chromosomes align along the equator of the cell 13 Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres. 14 Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. 15 Meiosis II Gene X Only one homolog of each chromosome is present in the cell. Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. 16 Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Anaphase II Telophase II 4 Genetically Different haploid cells 17 Prophase II Nuclear envelope fragments. Spindle forms. 18 Metaphase II Sister chromatids align along equator of cell. 19 Equator Anaphase II Pole Sister chromatids separate and move to opposite poles. 20 Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. 21 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 22 Meiosis Animation 23 Is there an advantage to sexual reproduction? Yes, Sexual reproduction creates differences or diversity. Because sexual reproduction creates so many different combination, when the environment changes, there is a better chance that some organisms will survive. 24 Gametogenesis Oogenesis or Spermatogenesis 25 Spermatogenesis Occurs in the testes Two divisions produce 4 spermatids Spermatids mature into sperm Men produce about 250,000,000 sperm per day 26 Spermatogenesis in the Testes Spermatid 27 Spermatogenesis 28 Oogenesis 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 29 Oogenesis in the Ovaries 30 Oogenesis First polar body may divide (haploid) a X a a X a Mitosis Oogonium (diploid) A X X Primary oocyte (diploid) Polar bodies die X Meiosis I Meiosis II (if fertilization occurs) A X A X Secondary oocyte (haploid) Ovum (egg) A Mature egg X Second polar body (haploid) 31 Twins Division after Fertilization Conjoined (Siamese) Twins Brittany and Abby Hensel of Minnesota http://www.top1.ro/Nicolae/abigail-sibrittany-hensel-unite-pentrutotdeauna.html Comparing Mitosis and Meiosis 36 Comparison of Divisions Mitosis Meiosis 2 Number of divisions 1 Number of daughter cells 2 4 Yes No Same as parent Half of parent Where Somatic cells Germ (gamete) cells When Throughout life At sexual maturity Growth and repair Sexual reproduction 37 Genetically identical? Chromosome # Role