Meiosis: The Cellular Basis of Sexual Reproduction Chapter 11

11.1 The Mechanisms of Meiosis  Meiosis is based on the interactions and distribution of homologous chromosome pairs  Meiosis produces four genetically different cells with half the parental chromosomes

Sexual Reproduction  Sexual reproduction produces offspring by union of male and female gametes (sperm and egg) Meiosis produces gametes with half chromosome number Evolutionary advantage: Genetic shuffling of sex

Fertilization  Fertilization fuses nuclei of egg and sperm Zygote cell produced by fertilization Restores parental chromosome number

Homologous Chromosome Pairs  Paternal chromosomes from male parent, maternal from female parent Homologous chromosome pairs Alleles may be different within homologous pairs  Meiosis separates homologous pairs Before meiosis, diploid (2n) After meiosis, haploid (n)

Meiosis and Fertilization Cycle

Meiotic Cell Cycle  Mitosis and meiosis compared Both: Similar cell divisions, meiosis divides twice Mitosis: Two identical daughter cells, diploid Meiosis: Four genetically different cells, haploid  Premeiotic interphases similar to mitotic interphase Chromosomes copied into sister chromatids

Meiosis I  Meiosis I: First meiotic division Recombination exchanges segments between homologues Produces two haploid cells with chromatids attached

Meiosis II  Meiosis II: Second meiotic division Sister chromatids separate into separate cells Produces 4 recombined haploid cells

Meiotic Divisions

Meiotic Cell Cycle (1)  Prophase I Sister chromatids condense to chromosomes Synapsis (pairing of homologs) Tetrads (Fully paired homologs) Recombination mixes alleles across tetrads  Prometaphase I Nuclear envelope breaks down Kinetochores attach to polar spindles

Meiotic Divisions

Meiotic Cell Cycle (2)  Metaphase I and Anaphase Tetrads align on metaphase plate Homologs segregate, move to poles (sister chromatids attached) Nondisjunction creates abnormal chromosome numbers  Telophase I and Interkinesis No change in chromosomes Spindle disassembles

Meiotic Divisions

Meiotic Cell Cycle (3)  Prophase II, Prometaphase II, and Metaphase II Chromosomes condense, spindles form Nuclear envelope breaks, kinetochores form Chromosomes align on metaphase plate  Anaphase II and Telophase II Spindles separate chromatids Spindles disassemble New nuclear envelopes form

Meiotic Divisions

Animation: Meiosis I and II

Comparison of Meiosis and Mitosis

Sex Chromosomes in Meiosis  Sex chromosomes Different in males and females Human females XX, males XY XX fully homologous, XY homologous in short region

Sex Chromosomes in Meiosis  Meiosis and sex chromosome inheritance Gametes produced by females may receive either X chromosome Gametes produce by males may receive either X or Y chromosome

11.2 Mechanisms That Generate Variability  Recombination depends on physical exchanges between homologous chromatids  Segregation of maternal and paternal chromosomes is random  Random joining of male and female gametes in fertilization adds additional variability

Recombination of Chromatids  Recombination (crossing over) Key genetic shuffle of prophase I  Tetrads held together at synaptonemal complex Two of four chromatids exchange alleles Chiasmata or crossovers are points of exchange

Crossing Over

Animation: Crossing over

Synaptonemal Complex

Random Segregation  Random segregation Key genetic shuffle of metaphase I  Each chromosome of a homologous pair may randomly end up at either spindle pole Any combination of maternal and paternal chromosomes can be segregated to gametes 2 X number of possible combinations

Random Spindle Connections

Random Fertilization  Random chance of male and female gamete forming zygote  Meiosis allows randomness necessary for Mendelian laws of inheritance Recombination, random segregation, and random fertilization are mechanisms of randomness

11.3 The Time and Place of Meiosis in Organismal Life Cycles  In animals, diploid phase dominant and meiosis followed directly by gamete formation  In most plants and fungi, generations alternate between haploid and diploid phases  In some fungi and other organisms, haploid phase dominant and diploid phase single cell

Animal Life Cycles  Diploid phase dominates animal life cycles Meiosis followed directly by gamete formation Haploid phase is reduced and short, no mitosis  In males, all four nuclei from meiosis form separate sperm cells  In females, only one nucleus becomes an egg

Animal Life Cycles

Plants and Fungi Life Cycles  Alternation of generations Alternate between haploid and diploid phase  Fertilization produces sporophytes Diploid individuals  Sporophytes produce haploid spores (meiosis) Multicellular gametophytes (mitosis) Gametophytes produce gametes

Plants and Fungi Life Cycles

Other Fungi and Algae Life Cycles  In some organisms, diploid phase limited to single cell zygote  Zygote undergoes meiosis Mitosis only occurs in haploid cells Gametes usually designated + or -

Other Fungi and Algae Life Cycles

Animation: Meiosis overview PLAY ANIMATION