Meiosis and Sexual Reproduction Ch. 11.1-11.3

Meiosis and Sexual ReproductionCh. 11.1-11.3

Why Meiosis?Advantages:Dramatic increase in genetic variationKeeps genetic material from multiplying (ploidy)Replication done with somatic cells (body cells) would double the amount of DNA each timeDisadvantages:Reproduction is more complexMeiosis requires energyMust find other half of genetic material to reproduceSexual Reproduction fusion of two haploid male and female cells (gametes)Fertilization union of male and female nuclei; produces a zygote

The Cells of MeiosisWhat type of cells do meiosis?EukaryotesTypes of meiotic cells:Spermatozoa (sperm) produced by male gonads (testes); contain paternal chromosomesOva (eggs) produces by female gonads (ovaries); contain maternal chromosomesSource of variation though chromosomes are the same (homologous) they have different DNA for the same genes (alleles)

Phases of MeiosisTwo Phase Groups:Meiosis I Prophase I Telophase IHomologous chromosomes separate Meiosis II Prophase II Telophase IISister chromatids separate (like mitosis) Both phases have cytokinesisLeading to Meiosis I normal cell cycle (G1, S, G2)Leading to Meiosis II short interphase but no S phase (interkinesis)

Meiosis IProphase IChromosomes condenseHomologous pairs form tetrads through pairing (synapsis)Crossing-over occursPrometaphase INucleus breaks downSpindle fibers attach to kinetochores (one for each sister chromatids)Metaphase IHomologous pairs line up at metaphase plateAnaphase IHomologous pairs separate to opposite polesTelophase INucleus reforms, spindles break down, and short interkinesis

Meiosis IIFollows mitosis:Prophase IIChromosomes condenseSpindles formPrometaphase IINucleus breaks downSpindles attach to kinetochoresMetaphase IIChromosomes move to metaphase plateAnaphase IISister chromatids separate to opposite polesTelophase IINucleus reforms, chromosomes decondense, spindles disappearEnd result is 4 haploid cells

Producing VariationWhere in meiosis does variation come from?Random separation of paternal and maternal DNA8,388,608 possible combinations with our 23 pairsSwitching of genes during cross-overRandom recombination of haploid material Creates enough variation that no two humans ever look alike (expect to twins)Even in the same family, the chance of offspring being genetically the same is 1:7.0x1013

Crossing-OverProtein complex (synaptonemal complex) carefully exchanges alleles on homologous chromosomesEvent is random and happens in 2-3 places on every chromosome2 (of 4) chromatids exchange alleles results in two recombinant chromosomes and two parental chromosomes

Producing ErrorsSometimes during Anaphase I/II or after crossing over, chromosomes get stuck togetherNondisjunction failure to separate homologous chromosomes/ sister chromatidsOne cell receives extra material and one cell misses a whole chromosomeMostly fetal in humansTrisomy 21 (Downs Syndrome)Produces large amounts of variation in plants

Alternative Life Cycles through MeiosisDiploid Dominate CycleMeiosis cells used for fertilizationNo mitosis for gametesAnimalsHaploid/Diploid Alternation CycleSporophytes (diploid) produce spores (haploid) which under go mitosis to form gametophytesEventually gametophytes form sperm and eggs cells used to make sporophytesBushes, trees, and flowersHaploid Dominate CycleDiploid only at fertilization, then cell produces haploid (+/-) spore cellsSpores produce gametophytesFungi and algae

HomeworkRead Ch. 12 and do Ch. 12 VocabCh. 11 vocabTest Your Knowledge and Interpret the Data for Ch. 11Test on Ch. 11, 12, and 13 next week