A level Biology exam revision resources written by A level Examiners


Cell division is part of the cell cycle. This can be a continuous process of cells growing and dividing, growing and dividing. The figure 1 below shows how the cell cycle is made up of a period of interphase and a period of division.

Figure 1 : The Cell Cycle

The Cell Cycle

Interphase is subdivided into three stages.

  • The G1 phase is the first growth period. During this phase the cell grows and forms new cytoplasmic organelles e.g. ribosomes, mitochondria and chloroplasts.
  • The S phase is when the DNA duplicates (the semi-conservative replication of DNA occurs).
  • The G2 phase is the second growth period. During this phase the cell builds up a sufficient store of energy to carry cell division through and the centrosome divides to form two pairs of centrioles.

Throughout interphase the cell has appearance of a non-dividing cell. It's chromosomes not visible but are present as chromatin. The figure 2 below shows a cell in the G2 phase of interphase

Figure 2 : G2 phase of Interphase

G2 phase pf interphase

The nature of the period of division depends on the type of division which takes place. There are two types of cell division, mitosis and meiosis. In both types the nucleus divides first followed by the actual dividing of the cell (cytokinesis).

  • Mitosis takes place when new cells are added to multicellular organisms as they grow, and when tissues are repaired or replaced. Also asexual reproduction can occur as a result of mitotic division.
  • Meiosis occurs in the production of gametes by organisms that reproduce sexually.

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