The cell cycle and mitosis

To multiply, a cell must grow, copy its DNA and split in two, all under genetic control. A typical eukaryotic cell divides about once every 24 hours.

Most of the time is interphase, the preparation:

• G1: the cell grows; no DNA copying.
• S: the DNA is copied, doubling from 2C to 4C. The chromosome number does not change, because each chromosome now just has two identical chromatids. The centrioles also double.
• G2: more growth and preparation for division.

Cells that stop dividing, such as neurons and heart cells, leave G1 and rest in G0.

Then mitosis (M phase):

• Prophase: chromatin condenses into visible chromosomes; centrioles move apart and the spindle forms; the nucleolus, nuclear envelope, ER and Golgi disappear.
• Metaphase: chromosomes (each two sister chromatids joined at the centromere) line up on the metaphase plate at the cell's equator, each chromatid attached by its kinetochore to opposite poles.
• Anaphase: the sister chromatids are pulled apart to opposite poles.
• Telophase: chromosomes uncoil and nuclear envelopes form again.

Cytokinesis divides the cytoplasm. In animal cells, a contractile ring of actin pinches the cell in two. In plant cells, the rigid wall cannot pinch, so a cell plate grows across the middle into a new wall. Mitosis is an equational division: the daughter cells have the same chromosome number as the parent.