SS – G7 Week 04

1. When a sperm-making cell in the testes OR an egg-making cell in the ovary creates sperm OR egg cells, they must first duplicate NOT the number of chromosomes in their nucleus, but instead the number of CHROMATIDS. The duplicated chromatids can be called ‘sister’ chromatids, they it might be even better to call them ‘twin’ chromatids because they are basically identical copies of one another.

2. Just before this ‘parent’ cell divides to make two new cells (which are NOT yet sperm cells), the cell organizes the 46, 2-chromatid chromosomes into pairs according to their size. This process creates 23 pairs of similar-sized, 2-chromatid chromosomes. During this process, the paired 2-chromatid chromosomes come together and actually exchange portions of their DNA–this is called “crossing over” and this process is actually one important reason why human beings are different from one another: crossing over creates a new combination of DNA that never existed before!

3. When the parent cell then divides to make two new cells (again, which are NOT yet sperm cells), one of the 2-chromatid chromosomes from each of the 23 (2-chromatid) chromosome pairs is randomly distributed into the two new cells. This process is actually another important reason why human beings are different from one another: the randomly distributed 2-chromatid chromosomes–one from each similar-sized pair–creates a collection of 2-chromatid chromosomes that never existed before!

4. The two newly made cells each have 23 chromosomes each with 2 chromatids. And so, to make four sperm (or egg) cells each containing 23 chromosomes made of 1 chromatid, you simply need to separate the 2 ‘sister’ chromatids from their shared centromere and place one of the ‘sister’ chromatids in each new sperm cell. This process is actually another important reason why human beings are different from one another: the randomly distributed 1-chromatid chromosomes–one from each side of the 2-chromatid chromosomes–creates a collection of 1-chromatid chromosomes that never existed before!