1. MEIOSIS: -the process used by sexually reproducing organisms to create cells responsible for producing offspring. -like does not beget like in meiosis!! -cells have the haploid number of chromos (23 in humans; n=23) (autosomes, produced by mitosis, are identical diploid # = 46 in humans; 2n = 46) -since they have half the chromosomes they are not identical to the parent cell

2. -unlike mitosis, meiosis happens only in cells concerned with reproduction -these cells are called the GAMETES (sperm and egg) -the end result of meiosis are the sperm cells in male organisms and egg cell in female organisms http://scienceline.org/wp-content/uploads/2008/06/ashford_sperms_color.jpg http://powerstates.com/wp-content/uploads/egg-sperm.jpg

3. Meiosis requires 2 nuclear divisions (it goes through prophase, metaphase, anaphase and telophase 2 times) Creates cells with a haploid number of chromosomes. Interphase in meiosis is identical to interphase in mitosis Metaphase, anaphase and telophase are also the same.

4. http://mtwow.org/meiosis.gif Interphase Prophase 1 with crossing over and tetrads Gametes with haploid chromosome number Meiosis 2 with no interphpase to double chromos

5. Meiosis differs from mitosis in prophase I. -2 pairs of sister chromatids will join at the centromere forming a cluster of 4 sister chromatids called a tetrad. When these chromatids lie close together, they will exchange pieces of chromosome (crossing over). This creates 4 unique chromatids. tetrad

6. Meiosis = 2 nuclear divisions, mitosis = one nuclear division. Meiosis cells are all genetically dissimilar, mitosis cells are all genetically identical. Meiosis produces haploid cells. Mitosis produces diploid cells. Meiosis has crossing over. Mitosis has no crossing over. Meiosis = reproduction Mitosis = growth, replacement and repair of worn out cells.

7. What is so important about meiosis??? -First, it creates genetically dissimilar chromatids during crossing over..this allows for variability within species. This can be the difference between survival and extinction. …weeds that can resist herbicides and bacteria that won’t respond to Antibiotics are great examples of this variability!! http://www.niaid.nih.gov/SiteCollectionImages/topics/antimicrobialresistance/1 whatIsDrugResistance.gif

8. -Second, it ensures that the diploid chromosome number in species stays constant during reproduction by creating haploid cells that fertilize each other. 23 + 23 = 46 vs. 46 + 46 = 92 -chromosome number is very important in determining species of organism and function of that organism. http://www.toonpool.com/user/997/files/chromosomes_kid_draw_on_wall_901815.jpg

9. homepages.uel.ac.uk/V.K.Sieber/human.htm www.ucl.ac.uk/%7Eucbhjow/bmsi/bmsi_7.htm Human Chromosomes before being separated and placed in formal karyotype form. Can you count 23 pairs? Normal human karyotype. Is this a male or female? Sex chromosomes are located on the 23 rd pair. XX is a female and XY is a male

10. Meiosis can also be a determining factor in chromosomal disorders. If non-disjunction (or failure of the sister chromatids to separate during anaphase of either meiosis I or II) occurs, then the resulting cell formed after fertilization will contain more or less chromosomes than the normal diploid number. In some cases, this can result in abnormal conditions for the organism.

11. Some examples of Chromosomal Disorders are: -Down’s Syndrome—also called Trisomy 21 happens when non-disjunction occurs on the 21 st pair of chromosomes resulting in 3 chromosomes being here. -affects about 1/700 children born…most common birth defect in the US. www.ucl.ac.uk/%7Eucbhjow/bmsi/bmsi_7.htm

12. Non-disjunction can also occur in the sex chromosomes. --Klinefelters syndrome affects 1/2,000 males XXY results in males with female sex characteristics --Turners Syndrome affects 1/5,000 females XO results in females who never develop secondary sex characteristics Turner’s Syndrome Klinefelter’s Syndrome