Mitosis, Meiosis and Heredity: Meiosis Unit 7

What we know: Genes are located on chromosomes in the cell’s nucleus Each organism must inherit a single copy of every gene from both it’s parents When an organism produces its own gametes (sperm/egg) the 2 sets of genes must be separated so each gamete ends up with only 1 set. http://www.pearsonsuccessnet.com/snpapp/iText/products/0-13-181118-5/bm/vasegreg.html

Chromosomes Consider a fruit fly with 8 chromosomes 4 chromosomes from mom 4 chromosomes from dad These 2 sets of chromosomes are homologous because each of the 4 chromosomes from mom match up with each of the 4 from dad

Ploidy A cell that contains both sets of homologous chromosomes is called diploid (2 sets, 2N) Drosophila 2N=8 2 complete sets of chromosomes and 2 complete sets of genes

Ploidy Gametes (sperm/egg), on the other hand, contain only a single set of chromosomes Thus, they have only a single set of genes These cells are called haploid (1 set, N) Drosophila N=4

Meiosis Meiosis is a process of reduction division where the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a 2N cell. 2 divisions: Meiosis I Meiosis II

Meiosis I: Interphase I Remember the steps G1, S, G2 Cells go through a round of replication , forming duplicate chromosomes

Meiosis I: Prophase I Chromosomes have replicated then centrioles migrate to poles Each chromosome from mom pairs with it’s corresponding chromosome from dad This structure is called a tetrad. Tetrads contain 4 chromatids. Crossing-over occurs. This is an exchange in alleles between homologous chromosomes & produces new allele combinations

Meiosis I: Metaphase I Spindle fibers attach to each chromosome in a tetrad The tetrads line up at the metaphase plate

Meiosis I: Anaphase I Spindle fibers pull the homologous chromosomes toward the opposite end of the cell

Meiosis I: Telophase and Cytokinesis Nuclear membranes form around the now separated chromosomes The cytoplasm squeezes in a pinches off into 2 cells The 2 cells have 4 chromatids as in mitosis BUT since there was crossing-over, each cell only has half of the parent cell’s information, this means these new cells are haploid http://www.pearsonsuccessnet.com/snpapp/iText/products/0-13-181118-5/bm/vaameio1.html

Meiosis II: Prophase II There’s NO chromosome replication before prophase II. Centrioles migrate to opposite poles.

Meiosis II: Metaphase II Chromosomes line up in the center of the cell at the metaphase plate (like in mitosis). Spindle fibers attach to the centromeres of each chromatid.

Meiosis II: Anaphase II The spindle fibers pull the sister chromatids apart at the centromeres. Chromatids move toward opposite end of the cell.

Meiosis II: Telophase II and Cytokinesis New nuclear membranes form around each of the groupings of chromatids. The cytoplasm draws in a pinches off forming 2 new cells from each of the 2 original cells. The final result of Meiosis is 4 haploid (N=4) daughter cells with 2 chromosomes each http://www.pearsonsuccessnet.com/snpapp/iText/products/0-13-181118-5/bm/vaameio2.html

Gamete Formation In males the haploid gametes are called sperm. In some plants, pollen grains contain haploid sperm cells

Gamete Formation In females, normally only 1 of the 4 haploid cells becomes an egg (has most of the cytoplasm). The other 3 cells are called polar bodies and do not take part in reproduction http://www.youtube.com/watch?v=D1_-mQS_FZ0

*In both mitosis and meiosis the cell divides. Mitosis vs. Meiosis 1 round of cell division Results in 2 genetically identical daughter cells Diploid= 2N Somatic Cells 2 rounds of cell division Results in 4 different daughter cells Hapliod= N Involved in gamete formation *In both mitosis and meiosis the cell divides.

http://www. youtube. com/watch http://www.youtube.com/watch?v=mr0oiws9ZvM&feature=related&safety_mode=true&persist_safety_mode=1