The Cell Cycle
From the very earliest moments of life throughout adulthood, cell division is a critical function of cell biology. The rate of division differs between cell types; hair and skin cells divide relatively rapidly (which is why chemotherapy drugs, which target rapidly-dividing cells in an effort to destroy cancerous cells, often cause hair loss), whereas liver cells rarely divide, except in response to injury. Regardless of cell type (with the exception of reproductive cells), the process of cell division follows consistent stages, which make up the cell cycle.
The cell cycle is made up of five stages. Cells at rest, which are not dividing, are considered to be at the G0 (growth phase 0) stage of the cell cycle. Once cell division is triggered (for example, by extracellular signals in response to nearby damage, requiring new cells to replace the damaged cells), cells enter stage G1. In this stage, the organelles of the soon-to-be-dividing cell are duplicated, in order to support both daughter cells upon division. Similarly, in the next stage, S (DNA synthesis) phase, the genetic material of the cell (DNA) is duplicated, to ensure that each cell has the full complement of genetic instructions. Additional growth and protein production occurs in the subsequent stage, G2 phase.
G1, S, and G2 are collectively known as interphase, in which the cell is growing and preparing to divide; the subsequent stages in which the cell is actively dividing are stages of mitosis. The first mitotic stage is prophase, in which the newly replicated DNA condenses into chromosomes. These chromosomes are in pairs (humans have twenty-three pairs of chromosomes), with each pair joined together at the centromere.
Next, in prometaphase, the nuclear membrane breaks down. Kinetochores form on chromosomes, which are proteins that attach to kinetochore microtubules (cellular filaments) anchored at opposite ends of the cell. In metaphase, the chromosomes align along the center of the cell, perpendicular to the poles anchoring the microtubules. The alignment is such that one of each chromosome duplicates is attached to each pole by these microtubules.
In anaphase, the microtubules pull the duplicates apart from each other toward each of the poles. In the final stage of mitosis, telophase, nuclei reform in each pole of the cell, and cellular filaments contract. The process of cytokinesis divides the cell into two daughter cells, both with a full complement of genetic material and organelles.
Following mitosis, both daughter cells return to the G1 phase, either to begin the process of division once again or to rest without dividing (G0).