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Transcript
1 s 2 Lesson #1.3 Meiosis 3 Mitosis/Meiosis Overview • Difference between Mitosis and Meiosis – Mitosis •Reproduction – Asexual, required 1 parent cell – Division increases population •Growth – Organism develops from a single cell – 100 trillion cells in a human body from single cell – RBC reproduce 1 million/second •Repair 4 Mitosis/Meiosis Overview –Meiosis •Reproduction –Sexual, requires TWO parents –Produces gametes: egg and sperm cells –Leads to genetic variability and diversity within species 5 Meiosis Formation of Gametes (Eggs & Sperm) 6 Facts About Meiosis Preceded by interphase which includes chromosome replication Goes through Two meiotic divisions Meiosis I and Meiosis II Original cell is diploid (2n) Four daughter cells produced, are haploid (1n) 7 Facts About Meiosis Daughter cells contain half the number of chromosomes as the original cell Produces gametes (eggs & sperm) Occurs in the testes in males Occurs in the ovaries in females 8 More Meiosis Facts Start with 46 double stranded chromosomes After 1 division - 23 double stranded chromosomes After 2nd division - 23 single stranded chromosomes Occurs in our germ cells that produce gametes 9 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 10 Fertilization – “Putting it all together” 2n = 6 1n =3 11 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 12 A Replicated Chromosome Gene X Homologous chromosomes (same genes, different alleles) Sister Chromatids (same genes, same alleles) Meiosis Forms Haploid Gametes Meiosis must reduce the chromosome number by half Fertilization then restores the 2n number from mom from dad child too much! meiosis reduces genetic content The right number! 14 Meiosis: Two Part Cell Division Sister chromatids separate Homologs separate Meiosis I Meiosis II Diploid Haploid Haploid 15 Meiosis I: Reduction Division Spindle fibers Nucleus Early Prophase I (Chromosome number doubled) Late Prophase I Nuclear envelope Metaphase Anaphase Telophase I I I 16 Prophase I Early prophase Homologs pair. Crossing over occurs. Late prophase Chromosomes condense. Spindle forms. Nuclear envelope fragments. 17 Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids) Join to form a TETRAD 18 Crossing-Over Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring 19 Homologous Chromosomes During Crossing-Over 20 Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment 21 Metaphase I Homologous pairs of chromosomes align along the equator of the cell Microtubules from one pole are attached to the centromere of one chromosome of each tetrad, while those from the other pole are attached to the other. 22 Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres. 23 Telophase I -Homologous chromosomes move until there is a haploid set at each pole. -Each chromosome consists of linked sister chromatids. -Cytokinesis by the same mechanisms as mitosis usually occurs simultaneously Nucleus reforms, but there is no further replication of chromosomes. 24 Meiosis II •NO interphase between Meiosis I and Meiosis II 25 Meiosis II: Reducing Chromosome Number Prophase II Metaphase Telophase II Anaphase 4 unique II II haploid cells 26 Prophase II During prophase II a spindle apparatus forms, attaches to centromeres of each sister chromatids, and moves them around. Spindle fibers from one pole attach to the centromere of one sister chromatid and those of the other pole to the other sister chromatid. 27 Metaphase II Chromosomes align along equator of cell. 28 Anaphase II Equator Pole Sister chromatids separate and move to opposite poles. 29 Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. 30 Results of Meiosis -Gametes (egg & sperm) form -Four haploid cells with one copy of each chromosome -One allele of each gene is inherited from each parent -Different combinations of alleles for different genes along the chromosome Lesson#1.3 Gametogenesis 32 Gametogenesis Oogenesis or Spermatogenesis 33 Spermatogenesis Occurs in the testes…begins at puberty Two divisions produce 4 spermatids Spermatids mature into sperm Men produce about 250,000,000 sperm per day 34 Spermatogenesis 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 36 Oogenesis First polar body may divide (haploid) a Mitosis Oogonium (diploid) X A X Primary oocyte (diploid) X Meiosis II a X a a Polar bodies die X Meiosis I (if fertilization occurs) A X A X Secondary oocyte (haploid) Ovum (egg) Mature egg A X Second polar body (haploid) 37 Putting it all together All your body cells (somatic cells) go through mitosis for repair and growth 46 chromosomes, 2n (diploid) At puberty, your sex cells (gametes) go through meiosis to prepare for fertilization 23 chromosomes, n (haploid) FERTILIZATION 1 haploid egg + 1 haploid sperm = 1 diploid zygote mitosis Mitosis for repair Adult mitosis baby embryo mitosis 38 Comparing Mitosis and Meiosis 39 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 cells When Throughout life At sexual maturity Growth and repair Sexual reproduction Genetically identical? Chromosome # Role 40