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

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

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

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

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

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

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

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

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

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

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

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

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#

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)

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!

Human chromosomal abnormalities http://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

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