Introduction: When cells get too large, they need to divide in order to keep their surface area-to-volume ratios the same. This process, called mitosis, involves the replication of the genetic material found within the cell, and a duplication of all of the organelles (at least in eukaryotic organisms). Mitosis in some cells can take eight hours, while in other types of cells, it can last for days.

Mitosis is consists of a series of events that start with Interphase. Having steps often labeled Growth 1, Synthesis, and Growth 2, the cell spends most of its life in this stage. During the Growth 1 step, the cell in increasing in size. During Synthesis, the genetic material within the nucleus is being copied. This is important because each new cell produced needs to have an identical copy of the DNA. In the Growth 2 stage, the cell is doing a little more growth and preparing to start the nuclear division.

The actual mitotic division is separated into four distinct phases that are outlined in the diagram below. The first, called Prophase, involves the condensing of the DNA into structures called chromosomes. Also, the nuclear membrane dissolves and structures called spindle fibers form, which act as a scaffolding for the chromosomes to attach to.

The next step is called Metaphase. Here, the chromosomes attach to the spindle fibers and slide towards the equator of the cell, where they align.

Metaphase is followed by Anaphase. During this step, the chromosomes separate into chromatids and one half of the pair moves towards opposite poles.

The final step is called Telophase. This is when the spindle fibers disappear, the nuclear membrane reappears, and the chromosomes unwind back into DNA. Telophase is followed by the actual splitting of the cell membrane. This step, called Cytokinesis, involves a pinching off of the cell membrane until two distinct cells are formed. These new daughter cells will then separate and start the process all over again.

This diagram summarizes the movement of the chromosomes during cell division.

Directions for this Lesson: Answer the practice questions and then watch these videos to learn more about how cells divide.