Meiosis and Sexual Life Cycles

1. Meiosis and Sexual Life Cycles http://geneticsmadeeasy.com/preguntas/pregunta30.html

2. Heredity • Heredity: the transmission of traits from one generation to the next • Asexual reproduction: clones • Sexual reproduction: variation • Human life cycle: • 23 pairs of homologous chromosomes (46) • 1 pair of sex and 22 pairs of autosomes • gametes = haploid (1N) • Autosomes = diploid (2N) • fertilization (syngamy) results in a zygote • Meiosis: cell division to produce haploid gametes

3. Alternative life cycles • Fungi/some algae • meiosis produces 1N cells that divide by mitosis to produce 1N adults (gametes by mitosis) • Plants/some algae • Alternation of generations: • 2N sporophyte produces 1N spores, by meiosis • spore divides by mitosis to generate a 1N gametophyte • gametes then made by mitosis which then fertilize into 2N sporophyte

4. Meiosis • Occurs only in the reproductive organs (gonads) • Formation of gametes • spermatogenesis in the testes • oogenesis in the ovaries • Allows for variation in offspring • Preceded by chromosome replication (chromatid pairs) • Followed by 2 cell divisions (Meiosis I & Meiosis II) • Results: • 4 daughter cells • 1/2 chromosome number (1N http://www.sinauer.com/cooper/4e/animations1607.html

5. Prophase I (1 cell ) • Nucleus disappears, spindle fibers form, and the chromatid pairs visible. • Chromatid pairs find their homologous pair, forming a tetrad (bivalent). • Homologous chromosomes are 2 of the same type of chromosomes. • Crossing over - 2 homologous chromosomes switch pieces of DNA. • Chiasma site of crossing over

6. Metaphase I (1 cell ) • Tetrads meet at the middle of the cell.

7. Anaphase I (1 cell ) • The tetrads split apart. • Homologous chromosomes split

8. Telophase I (2 cells ) • Two cells form • Each have a full set of chromosomes as chromatid pairs.

9. Prophase II (2 cells ) • Chromatid pairs continue moving to the center of the cell.

10. Metaphase II (2 cells ) • The chromatid pairs meet at the middle.

11. Anaphase II (2 cells ) • The chromatid pairs split apart.

12. Telophase II (4 cells) • Each of the two cells split making a total of 4 cells. • Each cell contains 1/2 the normal number of chromosomes. http://bcs.whfreeman.com/thelifewire/content/chp09/0902002.html http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

13. Meiosis vs. mitosis • Synapsis/tetrad/chiasmata/crossing-over (prophase I) • Homologous vs. individual chromosomes line up(metaphase I) • Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.(anaphase I) http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

14. Origins of Genetic Variation, I • Independent assortment: homologous pairs of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II • Combinations possible: • 2n (n the haploid number of the organism)

15. Origins of Genetic Variation, II • Crossing over (prophase I): • the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes) • Random fertilization: • 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!

16. http://www.mhhe.com/biosci/genbio/biolink/j_explorations/ch10expl.htmhttp://www.mhhe.com/biosci/genbio/biolink/j_explorations/ch10expl.htm Human chromosomal abnormalities • Caused by Nondisjunction • Failure of chromosomes to separate in meiosis • Down syndrome results from extra chromosome 21 • Trisomics an extra copy of an autosome • Monosomics one less copy of an autosome • Inc. maternal age inc. risk of occurence

17. Karyotypes • Pictures of homologous chromosomes lined up together. • Used to determine if there are any chromosomal abnormalities.