Summary

Every cell arises from a cell that existed before it — this is the heart of the cell theory, and the event by which one cell gives rise to others is called cell division. The whole life of a sexually reproducing organism begins from a single cell, the zygote, and division does not cease once the adult body has been built; it carries on throughout life, replacing worn-out cells, healing injuries and, in the reproductive organs, making the cells needed for the next generation. Understanding how a cell divides therefore means understanding how living things grow, repair themselves and reproduce.

A dividing cell does not simply split in two on demand; it moves through an orderly, repeating sequence of events called the cell cycle, spanning the interval from one division to the next. This cycle falls into two broad parts. The first is interphase, a long preparatory period during which the cell readies itself for division, and the second is the M phase, or mitosis, the comparatively brief stretch when division actually happens. Although interphase looks quiet under the microscope, it is in fact the busiest time in the life of the cell.

Interphase is itself divided into three sub-stages. In G1 the cell grows, carries out its everyday metabolism and duplicates most of its organelles. In the S phase the genetic material is copied: DNA replication takes place and each chromosome is duplicated. In G2 the cell continues to grow, building up the cytoplasmic materials it will need to carry the division through. Only after these steps are complete does the cell enter mitosis with everything in place.

Mitosis, the division of the nucleus, proceeds through four stages. In prophase the chromosomes condense into visible bodies while the centrioles travel to opposite poles, the nuclear envelope and nucleolus fade away, and the spindle fibres begin to form. In metaphase the chromosomes line up neatly along the equatorial plate. In anaphase the centromeres divide and the sister chromatids are dragged apart toward the two poles. Finally, in telophase, once the chromatids have reached the poles, they uncoil and lengthen, and the nucleolus and nuclear membrane re-form around each group. Nuclear division is then completed by the splitting of the cytoplasm, which is called cytokinesis.

The great importance of mitosis lies in its precision: it is an equational division in which the daughter cells receive exactly the same chromosome number as the parent cell. This faithful copying is what allows a body to grow from a single fertilised egg into countless genetically identical cells, and it is what lets tissues replace cells that are lost or damaged, keeping the organism intact throughout its life.

Meiosis stands in contrast to mitosis. It occurs in diploid cells that are set aside to form gametes, and it is a reduction division, because it cuts the chromosome number in half in the process of making those gametes. The point of this halving becomes clear at sexual reproduction: when two gametes fuse, the chromosome number is restored to the level found in the parent, so the species keeps its characteristic number generation after generation. Meiosis is carried out as two successive divisions, meiosis I and meiosis II. …