2Facts About MeiosisPreceded by interphase which includes chromosome replicationTwo meiotic divisions --- Meiosis I and Meiosis IICalled Reduction- divisionOriginal cell is diploid (2n)Four daughter cells produced that are monoploid (1n)
3Facts About MeiosisDaughter cells contain half the number of chromosomes as the original cellProduces gametes (eggs & sperm)Occurs in the testes in males (Spermatogenesis)Occurs in the ovaries in females (Oogenesis)
4More Meiosis FactsStart with 46 double stranded chromosomes (2n)After 1 division - 23 double stranded chromosomes (n)After 2nd division - 23 single stranded chromosomes (n) Occurs in our germ cells that produce gametes
5Why Do we Need Meiosis?It is the fundamental basis of sexual reproductionTwo haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote
7Replication of Chromosomes Replication is the process of duplicating a chromosomeOccurs prior to divisionReplicated copies are called sister chromatidsHeld together at centromereOccurs in InterphaseReplication is the process of duplicating chromosome. The new copy of a chromosome is formed by DNA synthesis during S-phase. The chromosome copies are called sister chromatids. Sister chromatids are held together at the centromere.
8A Replicated Chromosome Gene XSister Chromatids(same genes,same alleles)Homologs(same genes, different alleles)Homologs separate in meiosis I and therefore different alleles separate.
9Meiosis Forms Haploid Gametes Meiosis must reduce the chromosome number by halfFertilization then restores the 2n numberfrom momfrom dadchildtoomuch!meiosis reducesgenetic contentThe right number!
10Meiosis: Two Part Cell Division SisterchromatidsseparateMeiosis IMeiosis IIHomologsseparateDiploidHaploidHaploid
11Meiosis I: Reduction Division NucleusSpindlefibersNuclearenvelopeEarly Prophase I(Chromosome number doubled)Late Prophase IMetaphase IAnaphase ITelophase I (haploid)
12Prophase I Late prophase Early prophase Chromosomes condense. Spindle forms.Nuclear envelope fragments.Early prophaseHomologs pair.Crossing over occurs.
13Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids) Join to form a TETRADCalled Synapsis
14Crossing-Over Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchangedProduces Genetic recombination in the offspring
16Crossing-OverCrossing-over multiplies the already huge number of different gamete types produced by independent assortment
17Homologous pairs of chromosomes align along the equator of the cell Metaphase IHomologous pairs of chromosomes align along the equator of the cell
18Anaphase I Homologs separate and move to opposite poles. Sister chromatids remainattached at their centromeres.
19Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.
20Meiosis II Meiosis II produces gametes with Gene XMeiosis II produces gametes withone copy of each chromosome and thus one copy of each gene.Only one homolog of each chromosome is present in the cell.Sister chromatids carryidentical geneticinformation.
21Meiosis II: Reducing Chromosome Number Prophase IIMetaphase IITelophase IIAnaphase II4 Different haploid cells
31Oogenesis Occurs in the ovaries Two divisions produce 3 polar bodies that die and 1 eggPolar bodies die because of unequal division of cytoplasmImmature egg called oocyteStarting at puberty, one oocyte matures into an ovum (egg) every 28 days
35Comparison of Divisions MitosisMeiosisNumber of divisions12Number of daughter cells4Genetically identical?YesNoChromosome #Same as parentHalf of parentWhereSomatic cellsGerm cellsWhenThroughout lifeAt sexual maturityRoleGrowth and repairSexual reproduction