Meiosis and Sexual Reproduction Ch. 11.1-11.3

Why Meiosis? Advantages: Dramatic increase in genetic variation Keeps genetic material from multiplying (ploidy) Replication done with somatic cells (body cells) would double the amount of DNA each time Disadvantages: Reproduction is more complex Meiosis requires energy Must find other half of genetic material to reproduce Sexual Reproduction fusion of two haploid male and female cells (gametes) Fertilization union of male and female nuclei; produces a zygote

The Cells of Meiosis What type of cells do meiosis? Eukaryotes Types of meiotic cells: Spermatozoa (sperm) produced by male gonads (testes); contain paternal chromosomes Ova (eggs) produces by female gonads (ovaries); contain maternal chromosomes Source of variation though chromosomes are the same (homologous) they have different DNA for the same genes (alleles)

Phases of Meiosis Two Phase Groups: Meiosis I Meiosis II Prophase I Telophase I Homologous chromosomes separate Meiosis II Prophase II  Telophase II Sister chromatids separate (like mitosis) Both phases have cytokinesis Leading to Meiosis I normal cell cycle (G1, S, G2) Leading to Meiosis II short interphase but no S phase (interkinesis)

Meiosis I Prophase I Prometaphase I Metaphase I Anaphase I Telophase I Chromosomes condense Homologous pairs form tetrads through pairing (synapsis) Crossing-over occurs Prometaphase I Nucleus breaks down Spindle fibers attach to kinetochores (one for each sister chromatids) Metaphase I Homologous pairs line up at metaphase plate Anaphase I Homologous pairs separate to opposite poles Telophase I Nucleus reforms, spindles break down, and short interkinesis

Meiosis II Follows mitosis: Prophase II Prometaphase II Metaphase II Chromosomes condense Spindles form Prometaphase II Nucleus breaks down Spindles attach to kinetochores Metaphase II Chromosomes move to metaphase plate Anaphase II Sister chromatids separate to opposite poles Telophase II Nucleus reforms, chromosomes decondense, spindles disappear End result is 4 haploid cells

Producing Variation Where in meiosis does variation come from? Random separation of paternal and maternal DNA 8,388,608 possible combinations with our 23 pairs Switching of genes during cross-over Random 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-Over Protein complex (synaptonemal complex) carefully exchanges alleles on homologous chromosomes Event is random and happens in 2-3 places on every chromosome 2 (of 4) chromatids exchange alleles results in two recombinant chromosomes and two parental chromosomes

Producing Errors Sometimes during Anaphase I/II or after crossing over, chromosomes get stuck together Nondisjunction failure to separate homologous chromosomes/ sister chromatids One cell receives extra material and one cell misses a whole chromosome Mostly fetal in humans Trisomy 21 (Downs Syndrome) Produces large amounts of variation in plants

Alternative Life Cycles through Meiosis Diploid Dominate Cycle Meiosis cells used for fertilization No mitosis for gametes Animals Haploid/Diploid Alternation Cycle Sporophytes (diploid) produce spores (haploid) which under go mitosis to form gametophytes Eventually gametophytes form sperm and eggs cells used to make sporophytes Bushes, trees, and flowers Haploid Dominate Cycle Diploid only at fertilization, then cell produces haploid (+/-) spore cells Spores produce gametophytes Fungi and algae

Homework Read Ch. 12 and do Ch. 12 Vocab Ch. 11 vocab Test Your Knowledge and Interpret the Data for Ch. 11 Test on Ch. 11, 12, and 13 next week