Wednesday, 1/16/13 Turn in your HW (the article) Answer the following in your notes: Remember: Why do cells undergo mitosis? What is the end result of a cell undergoing mitosis? Think: We have discussed how somatic cells (body cells other than the gametes – egg/sperm) are formed…what process forms the sex cells (the gametes-sperm/egg)? This process is called MEIOSIS.

The Basics Genes (code for things like eye color, skin pigmentation, hormones, etc) are located on the chromosomes. Each organism must inherit one copy of every gene from both parents. Most organisms have 2 complete sets of genes. Those two sets must be separated in Meiosis so that each gamete produced contains just one set of genes.

Chromosome # Humans have 46 total chromosomes. 23 came from your mother 23 came from your father Each pair of chromosomes are referred to as homologous chromosomes (What did the root word homo mean again? All 46 chromosomes are present in every human body cell (a.k.a. somatic cell). This is the diploid chromosome number (2 sets ). (What did the root word di mean again?) 23 pairs

Gametes Gametes are sex cells —sperm or egg. Gametes contain only 1 copy of each chromosome. These cells are haploid (1 set) sounds like “half”, so they only have half of the number of chromosomes Humans have 23 chromosomes in every gamete. Not 23 pairs, just 23!

Diploid & Haploid Diploid = a cell with two sets of chromosomes, 2n Haploid = a cell with one set of chromosomes, gametes, 1n or n n = number of chromosomes in the organism (23 in humans)

Purpose of Meiosis Meiosis forms the gametes (sperm and eggs) that are necessary for sexual reproduction. Gametes need to be haploid so that when they meet for fertilization, the zygote (fertilized egg) has 2 copies of the genetic material (diploid) Meiosis has 2 stages Meiosis I: Separation of homologous chromosomes (different than mitosis) Meiosis II: Separation of sister chromatids (almost identical to mitosis)

MEIOSIS I Main event: Separation of homologous pairs of chromosomes What do you notice about the names of the stages? What do you notice about the movements of the genetic material during the stages? Homologous Pairs: Sets of same chromosome from different parents

1. Prophase I DNA coils into chromosomes Nuclear envelope & nucleolus break down Synapsis = homologous chromosomes pair up into tetrads (What does the prefix “tetra” mean? Think tetrahedron) Crossing-over = homologous pairs switch pieces of their chromatids Genetic Recombination = maternal & paternal genetic info get shuffled during crossing-over

Tetrad

2. Metaphase I Tetrads line up in the middle of the cell Remember: Metaphase = middle Spindle fibers attach at the centromeres of each homologous chromosome (each chromosome in the tetrad will have one attached)

3. Anaphase I Each homologous chromosome moves to the opposite end of the cell (one member of the pair) Independent Assortment = random separation of chromosomes, allows for genetic variation All of the chromosomes from Mom do not go to one cell, all from dad into the other – they are mixed up between the 2 new daughter cells

4. Telophase I & Cytokinesis I Two newly formed cells are haploid Each chromosome in the new cells is still made up of sister chromatids (which contains 2 identical-ish copies of chromosome; not necessarily identical because of what process?)

 MEIOSIS II Separation of sister chromatids Very similar to how mitosis looks Sister Chromatids = identical halves of a chromosome that has been duplicated

5. Prophase II 2 newly created cells from Meiosis I start to divide

6. Metaphase II Chromosomes line up in the middle

7. Anaphase II Chromatids separate

8. Telophase II & Cytokinesis II 4 new cells, each with half the number of chromosomes of the original cell

Let’s Review What are gametes? Are gametes produced through mitosis or meiosis? What is separated during meiosis II? What is separated during meiosis I? What is one way that genetic diversity of daughter cells is increased during meiosis? (think about it, we covered 2 ways in lecture)

Agenda for today: Finish notes (3 or less slides) Using available resources, complete worksheet. If you are seen not working, points will be deducted. Due tomorrow at the beginning of class. Will be looking at CONTENT, not COMPLETION

Development of Gametes Spermatogenesis = production of sperm cells Oogenesis = production of mature egg cells

Nondisjunction Nondisjunction = failure of homologous pairs to separate in Meiosis I If an organism survives, it may have a genetic disorder

Polyploidy Polyploid = organisms with more than the usual number of chromosome sets Occurs frequently in plants, results in plants that are larger than normal

http://cellsalive.com/meiosis.htm

Mitosis vs. Meiosis Mitosis Body cells 2 cells produced Diploid (2 of each chromosome) Genetically identical to each other AND to the original cell Meiosis Sex cells (sperm/egg) 4 cells produced Haploid (1 of each chromosome) Genetically different