Meiosis Sexual Reproduction

Where do you come from?

Sex: Woman, XX Man, XY Gender: Female Male Egg, Ovum Sperm

Fertilization – union of sperm an egg to produce zygote

Sperm and egg cells – gametes Fertilization Sperm and egg cells – gametes

Fetus Development Fertilization Mitosis to Create More Cells Fetus

23 pairs of chromosomes 1 from mom,1 from dad 46 individual chromosomes Body cells have all these chromosomes – diploid (2n) Gametes have ½ the amount because they will combine to eventually form body cells– haploid (n) (hap  half)

Homo sapiens – 46 chromosomes, 23 pairs

Diploid and Haploid Practice

Got it? Questions In humans, the cardiac muscle cells, skin cells, and neurons, of humans all have ____________ chromosomes The sperm and egg cells are called ______________ In humans an ovum (egg cell) would have _____ chromosomes When a cell like a gamete has ½ the amount of chromosomes it is called ___________and symbolized as n When a body cell like a skin cell has the full amount of chromosomes it is called __________, and symbolized as 2n If a sperm cell has 8 chromosomes it came from a male with _____ chromosomes. 46 gametes 23 haploid diploid 16

Meiosis (My-oh-sis) Definition: Process by which 1 diploid eukaryotic cell divides twice to generate 4 haploid gametes Gametes – sperm or cells Produces 4 cells each with 1 copy of each chromosome Purpose: When the egg is fertilized by a sperm the new zygote has the same number of chromosomes as the parents Zygote has 2 of each chromosomes (1 from mom, 1 from dad) Allows multicellular organisms to reproduce sexually Animation: http://www.cellsalive.com/meiosis_js.htm

Special cells that divide in meiosis to become sperm and egg Start off diploid (2n)

Meiosis Phases – watch the chromosomes move Centromeres divide the chromosome short arm and long arm. Chromatids or sister chromatids (two identical copies of DNA) are connected by the centromere Telomeres protect DNA, during DNA replication.

https://www.youtube.com/watch?v=rB_8dTuh73c

Purpose Continued Provides great genetic diversity Offspring have the same # of chromosomes as parents Some of those chromosomes came from each parent Offspring typically have characteristics from both parents

Prophase I Nuclear envelope breaks down DNA condenses into chromosomes Each chromosome has 2 sister chromatids Homologous Chromosomes Pair Different from Mitosis Crossing over occurs, which creates Genetic diversity!!!

Crossing Over Sections of DNA from the chromosome of one parent are switched with the same section from the other parents DNA Allows one chromosome to contain alleles from both parents Occurs homologous chromosomes line up at equator Increases genetic diversity

Metaphase I Homologous Chromosomes line up along equator of the cell 2 Homologous Chromosomes = 4 Sister Chromatids Which side each homolog lines up on is random 50-50 chance a daughter cell will get a chromosome from mother or father

Anaphase I Homologous Chromosomes are separated Each with 2 Sister Chromatids 1 of each homologous chromosome moves to opposite ends of the cell Results in 1 of each chromosome (2 sister chromatids) at each end

Telophase I (Final Phase) Nuclear envelope may reform 2 haploid nuclei are produced Each has 1 of each chromosome

Cytokinesis Not part of Meiosis I Occurs as Meiosis I is ending Cell divides

Result of Meiosis I and Cytokinesis 2 genetically different cells (Remember Crossing Over) Each cell contains 1 copy of each chromosomes 2 Sister Chromatids each

Prophase II Nuclear envelope breaks down DNA condenses into chromosomes Each chromosome has 2 sister chromatids

Metaphase II Sister Chromatids line up along equator of the cell 1 of each Chromosome 2 Sister Chromatids Different than Mitosis and Meiosis I

Anaphase II Sister Chromatids are separated 1 of each chromosome moves to opposite ends of the cell Results in 1 of each chromosome (no sister chromatids) at each end

Telophase II (Final Phase) Chromosomes unwind Nuclear envelope reforms in each new cell 4 haploid nuclei are produced Each has 1 of each chromosome

Cytokinesis Not part of Meiosis II Occurs at the end of Meiosis II Cells divide

Result of Meiosis II and Cytokinesis 4 genetically different haploid cells All contain some chromosomes from mother and some from father Each contains 1 of each chromosomes No Sister Chromatids

Comparing mitosis vs. meiosis Animation: http://highered.mheducation.com/sites/0072495855/student _view0/chapter2/animation__comparison_of_meiosis_and_ mitosis__quiz_1_.html