The cell cycle
The cell cycle is the ordered series of events from one cell division to the next. It has two main parts: a long interphase and a shorter M phase (mitosis plus cytoplasmic division).
- Interphase: the cell grows, makes proteins, and copies its DNA. It is divided into G1 (growth), S (DNA synthesis) and G2 (preparation for division).
- M phase: the nucleus divides by mitosis and then the cytoplasm divides (cytokinesis).
The four stages of mitosis
Mitosis divides the nucleus into two identical nuclei. The stages, in order, are PMAT:
- Prophase: chromosomes condense and become visible; the nuclear membrane breaks down.
- Metaphase: chromosomes line up along the middle (equator) of the cell.
- Anaphase: sister chromatids are pulled apart to opposite poles.
- Telophase: two new nuclei form; the chromosomes uncoil.
Key idea
Mitosis produces two daughter cells that are diploid and genetically identical to the parent cell. It is used for growth, repair and asexual reproduction.
Why the cycle is controlled
The cell cycle is tightly regulated so that a cell only divides when it is ready and only when new cells are needed. Before entering the M phase, a cell checks that its DNA has been copied correctly and completely. If the controls fail, a cell may divide again and again without stopping, forming a lump of cells called a tumour. This shows why mitosis, though it produces identical cells for healthy growth and repair, must be switched on and off at the right times. Most body cells spend far longer in interphase, growing and preparing, than in the short M phase itself.
Example
In an onion root tip, cells at the growing region are constantly in mitosis. Under a microscope you can see chromosomes lined up at the equator (metaphase) or being pulled apart (anaphase), which is why root tips are used to study the process.
Remember
Order of mitosis = P M A T (Prophase, Metaphase, Anaphase, Telophase). "DNA is copied in the S phase of interphase, not during mitosis."