Unit 5.3 Meiosis

What we need to know: Genes are located on chromosomes in the cell’s nucleus Each organism inherits a single copy of each gene from each of its 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.

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 Ploidy is the number of sets of chromosomes in a cell A cell that contains both sets of homologous chromosomes is called diploid (2 sets, 2N) 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 referred to as haploid (1 set, N)

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 (diploid) 2 divisions: Meiosis I Meiosis II

Meiosis I: Interphase I 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

Crossing over - during meiosis, when homologous chromosomes are paired together, there are points along the chromosomes that make contact with the other pair. These points of contact can allow the exchange of genetic information between chromosomes.

Crossing over Crossing over occurs during prophase I and means that each daughter cell formed in meiosis has chromatids with a different combination of alleles.

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 (cleavage) 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

Meiosis II: Prophase II There’s NO chromosome replication before prophase II. (no interphase for Meiosis 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

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 (body)Cells 2 rounds of cell division Results in 4 different daughter cells Haploid= 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

Additional videos Meiosis vs. Mitosis http://www.youtube.com/watch?v=Ba9LXKH2ztU&feature=related Overview http://www.youtube.com/watch?v=kVMb4Js99tA Square Dance! http://www.youtube.com/watch?v=iCL6d0OwKt8&feature=related

https://www.23andme.com/gen101/genes/ Inheritance http://www.youtube.com/watch?v=lJzZ7p-47P8&feature=related Phenotype: http://www.youtube.com/watch?v=jHWJqzlHl3w&feature=related