November 29, 2012

Reminder: Test on Friday.

Note: 2N: Diploid Cell (N= the number of different chromosomes)

Meiotic Differences Between

Men       

     Spermatogonium :Cells that line the insides of Seminiferous tubules in the testis.       2N

                                                                                                                             Mitosis

     1Degree (1st Division) Spermatocyte                                                                    2N

                                                                                                                             Meiosis 1

     2 Degree (2nd Division) Spermatocytes                                                                N N

                                                                                                                           Meiosis 2

     4 Unique Sperm                                                                                                NNNN

            

Lasts from puberty to death.

Women

Embryonic            Oogonium                                                                                                        2N

                                                                                                                                              Mitosis

               1 Degree (1 Division) Oocytes                                                                                 2N  2N

                                                                                                                                    Stays in Prophase 1

                                                                                                                                                                    

Puberty                                                                                                                                  Meiosis 1

 2 Degree (2 Division) Oocyte: This is the cell that is ovulated. This is also the cell that is fertilized                                                                                                                    N  Polar Body 

The N cell produced from the female has a too much genetic information soonly when the cell is fertilized, Mitosis 2 occurs, then the chromosomes from the nucleus of the sperm cell form the zygote.

Picture Examples of Men

Female

11/28/12 Mieosis Continued

  

(Picture is of overall Process, does not name particular phases named below)

III. Metaphase I - Tetrads line up at the equator.
IV. Anaphase I - Homologous chromosomes split apart and travel toward the poles.
V. Telophase I - Cell divides into 2 "haploid" cells. This division is called the reduction division because the chromatid pairs are still together and this significantly reduces the amount of allelic information.
VI. Prophase II - 2 cells now. No replication, tetrad formation, nor crossing over.
VII. Metaphase II - Chromatid pairs line up at the equators.
    VIII. Anaphase II - Chromatids split apart and move to the poles
    IX. Telophase II - Cells split into 4 genetically unique haploid gametes.
Why is Meiosis So Important?
1. Makes sure that exactly 1/2 of the information comes from a sperm cell and 1/2 from the egg cell.
2. Diploidy- Chromosomes are in pairs. We have backup for "bad" copies.
3. Genetic Variablity
    a. We are 1/2 and 1/2 of 2 uniqure parents.
    b. Crossing over - # 1 source of varablitity
    c. Independent Assortment - in humans there are 2 to the 23 power number of combos of homologues reaching a sperm or egg cell.