Cell Cycle

From a Single Cell to a Whole Organism

The life of every multicellular organism begins from a single cell -- the zygote -- and the growth of any living organism depends fundamentally on that cell, and the cells it produces, dividing again and again. This connects directly back to the cell theory: every cell arises only from a pre-existing cell, so an organism's growth, and later the healing of a wound or the replacement of worn-out tissue, is really the story of cells dividing according to a tightly controlled sequence of events.

Cell Cycle

The sequence of events that a cell goes through, from the moment it is formed until it itself divides to produce new cells, is called the cell cycle. It has two broad phases: interphase, during which the cell either grows and carries out its normal functions or rests, and M-phase (mitosis or meiosis), during which the cell actually divides. These two phases alternate -- a period of division is always followed by another interphase, and so on.

Interphase. Interphase is the interval between two successive divisions, and it is by far the longest part of the cell cycle. Far from being a dormant or resting stage, it is when the cell is at its most metabolically active, quietly preparing everything it will need for the next division. Interphase itself is made up of three sub-phases:

• G1-phase (first growth/gap phase): begins immediately after the previous division ends. The cell synthesises the RNAs it needs (mRNA, rRNA and tRNA), makes new proteins, and builds new membranes as it grows back to a normal working size.
• S-phase (synthesis phase): the cell replicates its DNA, so that by the end of this phase the amount of DNA in the cell has doubled. The histone proteins that DNA wraps around to form chromatin are also synthesised during this phase, alongside the new DNA. …