1 / 11

Meiosis and Sexual Reproduction Ch. 11.1-11.3

Meiosis and Sexual Reproduction Ch. 11.1-11.3. Why Meiosis?. Advantages : Dramatic increase in genetic variation Keeps genetic material from multiplying ( ploidy ) R eplication done with somatic cells ( body cells ) would double the amount of DNA each time Disadvantages :

fran
Download Presentation

Meiosis and Sexual Reproduction Ch. 11.1-11.3

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Meiosis and Sexual ReproductionCh. 11.1-11.3

  2. 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

  3. 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)

  4. Phases of Meiosis • Two Phase Groups: • Meiosis I • 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)

  5. Meiosis I • Prophase 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

  6. Meiosis II • Follows mitosis: • Prophase 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

More Related