Introduction

Both prokaryotic and eukaryotic cells need to replicate to increase populations of cells for growth, repair and also for reproduction. The strategies employed to accomplish these different outcomes can vary quite considerably depending on whether an organism is a prokaryote or eukaryote, unicellular or multicellular and if cell division is for repair or for reproductive purposes. In both prokaryotic and eukaryotic cells, the outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent (somatic) cell.

Prokaryotes such as bacteria achieve this propagation by the process of binary fission which is asexual, as the daughter cells are genetically identical to the parent. Some unicellular eukaryotic organisms also reproduce asexually with one of the more common methods being by budding, which can be external (eg yeast cells see Figure 4.1) or internal. Internal budding (endodyogeny) is found in parasites such as Toxoplasma gondii (protozoal parasite) where the two daughter cells develop internally (Figure 4.2) and when mature, the parent cell is destabilised, releasing daughter cells.

However, in multicellular eukaryotic organisms different strategies are employed to produce daughter cells. Most animals and plants are diploid, containing two sets of chromosomes; in each somatic cell (the non-reproductive cells of a multicellular organism), the nucleus contains two copies of each chromosome that are referred to as homologous chromosomes. Somatic cells are sometimes referred to as “body” cells. Homologous chromosomes are matched pairs containing genes for the same traits in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent; all together, they are considered a full set of chromosomes. In animals, haploid cells containing a single copy of each homologous chromosome are found only within gametes. Gametes fuse with another haploid gamete to produce a diploid cell.