2Human body cells have 46 chromosomes Sexual Reproduction and GeneticsMeiosis - General OverviewHuman body cells have 46 chromosomesEach parent contributes 23 chromosomesTakes place in sex cells (gametes): egg and spermOne cell becomes 4 NON-IDENTICLE cells
3Chromosomes and Chromosome Number Sexual Reproduction and GeneticsMeiosis – Homologous ChromosomesChromosomes and Chromosome NumberSame lengthSame centromere positionCarry genes that control the same inherited traitsOne is from mother, one is from father = a matching set
4Haploid and Diploid Cells Sexual Reproduction and GeneticsMeiosisHaploid and Diploid CellsAn organism produces gametes to maintain the same number of chromosomes from generation to generation.Human gametes contain 23 chromosomes.A cell with n chromosomes is called a haploid cell.A cell that contains 2n chromosomes is called a diploid cell.
5The sexual life cycle in animals involves meiosis. Sexual Reproduction and GeneticsMeiosisMeiosis IThe sexual life cycle in animals involves meiosis.Meiosis produces gametes.When gametescombine in fertilization, the number ofchromosomes is restored.
6Chromosomes replicate. Sexual Reproduction and GeneticsMeiosisMeiosis IInterphaseChromosomes replicate.Chromatin condenses.Interphase
7Pairing of homologous chromosomes occurs. Sexual Reproduction and GeneticsMeiosisMeiosis IProphase IPairing of homologous chromosomes occurs.Each chromosome consists of two chromatids.Prophase IThe nuclear membrane breaks down.Spindles form.
8Crossing over produces exchange of genetic information. Sexual Reproduction and GeneticsMeiosisMeiosis IProphase ICrossing over produces exchange of genetic information.Crossing over—chromosomal segments are exchanged between a pair of homologous chromosomes.
9Chromosome centromeres attach to spindle fibers. Sexual Reproduction and GeneticsMeiosisMeiosis IMetaphase IChromosome centromeres attach to spindle fibers.Metaphase IHomologous chromosomes line up at the equator.
10Homologous chromosomes separate and move Sexual Reproduction and GeneticsMeiosisMeiosis IAnaphase IHomologous chromosomes separate and moveto opposite poles of the cell.Anaphase I
11The spindles break down. Sexual Reproduction and GeneticsMeiosisMeiosis ITelophase IThe spindles break down.Telophase IChromosomes uncoil and form two nuclei.The cell divides.
12A second set of phases begins Sexual Reproduction and GeneticsMeiosisMeiosis IIProphase IIA second set of phases beginsas the spindle apparatus forms and thechromosomes condense.Prophase II
13A haploid number of chromosomes Sexual Reproduction and GeneticsMeiosisMeiosis IIMetaphase IIA haploid number of chromosomesline up at the equator.Metaphase II
14The sister chromatids are Sexual Reproduction and GeneticsMeiosisMeiosis IIAnaphase IIThe sister chromatids arepulled apart at the centromere by spindlefibers and move toward the opposite polesof the cell.Anaphase II
15The chromosomes reach the poles, and Sexual Reproduction and GeneticsMeiosisMeiosis IITelophase IIThe chromosomes reach the poles, andthe nuclear membrane and nuclei reform.Telophase II
16Sexual Reproduction and Genetics MeiosisMeiosis IICytokinesis results in four haploid cells, each with n number of chromosomes.Cytokinesis
17The Importance of Meiosis Sexual Reproduction and GeneticsMeiosisThe Importance of MeiosisMeiosis consists of two sets of divisionsProduces four haploid daughter cells that are not identicalResults in genetic variation
18Meiosis Provides Variation Sexual Reproduction and GeneticsMeiosisMeiosis Provides VariationDepending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result.Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.