Meiosis – Reduction Division in the Sex Cells  

More detailed -  Information

Human cells 23 pairs of chromosomes total = 46

Diploid (2n)  cells have 2 of each kind of chromosomes

Humans (n=23)  2 X 23= 46     Look at page 299

  After Meiosis 2n = 1n

  Gametes ( sex cells) have ½ the number of chromosomes 46 / 2 = 23

23 from each parent  see figure 12.10       page 300

  Meiosis I

Interphase – cell replicates chromosomes as in Mitosis

Prophase 1 – Homologous chromosomes form a tetrad(4 parts) structure

                      Tetrads paired tightly and sometimes leads to crossing over

                       Which means that genetic material may be exchanged

                        See figure 12.12  page 302

Metaphase 1 – Chromosomes line up in pairs  not individually as in Mitosis

Anaphase 1 – Similar to Mitosis but move to opposite ends of cell as pairs

Telophase 1 – Similar to Mitosis but each of the 2 cells has 1 pair of chromosomes

                        Which is has ½ of the genetic material from original cell  

                         Now has to split up sister chromitids from pairs

Meiosis 2 -    Very similar to Mitosis – PMAT

End of Meiosis – end up with 4 haploid cells.

Each contains 1 chromosome  from each  homologous original pair. 

These cells called gametes containing   Genes which will be passed on to offspring.

In humans  46 chromosomes = 2n, the diploid stage, where n = 23

During meiosis, 2n becomes 1n = 23 = reduction

Because each of the 23 pair can line up 2 different ways before splitting

There are 2 ²³  possible arrangements or 8 trillion different kinds of egg or sperm

After fertilization  2 ²³  X   2 ²³   or 70 trillion possible combinations

No wonder we all look different!  Crossing over adds to variations.