All cells at some point alternate between periods of cell division (known as 'M' phase) and periods of growth and synthesis (known interphase). This alternation is referred to as the cell cycle, because cells cycle between division and growth. Description of the cell cycle usually refers to mitosis, but the principles are equally applicable to meiosis (although a cell can only go through meiosis once, so it doesn't really cycle).

Interphase is subdivided into three phases, based on what is occurring in the cell at the time. The first phase is called G₁, short for 'first Gap' phase. During this phase, the cell carries out normal metabolic activities associated with growth, such as transcription and protein synthesis. A human cell in culture takes about 20 hours to progress through one turn of the cell cycle; of this, over 9 hours will be spent in G₁. The next phase is called S, which is short for synthesis. The critical event in this phase is the replication of the cell's DNA (duplication of the chromosomes), which in the human cell mentioned above takes about 8 hours. The final stage of interphase is G₂ phase, which is another growth phase like G₁. G₂ lasts about 2 hours in human cultured cells. The actual time these cells spend dividing (i.e. in M phase) is only about 45 minutes.

Not all cells are actively dividing all the time. Most cells, as they differentiate, become mitotically inactive. These cells have diverted from the usual cell cycle into another phase, known as G₀ (pronounced 'G naught'). G₀ is like a holding pattern that cells enter when they no longer need to divide. If they receive the right signals, though, they can reenter the cell cycle from G₀, and resume normal cell division.

Most cells divide only mitotically. We won't be examining mitosis in any detail; if you need to refresh your memory about mitosis, consult Russell, pages 15-19. A small, select population of cells, known as germ cells, can divide by the process of meiosis, which is an important process to consider in genetics, and will be dealt with in another module.

Cell Biology: Summary of Key Points

• There are two basic types of cells: prokaryotic cells such as bacteria that have no nucleus, and eukaryotic cells such as those in plants and animals that have true nuclei.
• Eukaryotic cells also have organelles, each of which is specialized to perform a specific function within the cell.
• The genetic material of cells is organized into discrete bodies called chromosomes, which are a complex of DNA and protein. Most eukaryotic cells are diploid, which means they contain two copies, or homologues, of each chromosome. Gametes, the reproductive cells, are haploid - they have only one copy of each chromosome.
• Most eukaryotic cells divide mitotically, progressing regularly through the cell cycle. The cell cycle consists of a series of stages that prepare the cell for division (primarily by replicating the chromosomes).