Time period: day 0 to adult
Cell division normally occurs in eukaryotic organisms through the process of mitosis, in which the maternal cell divides to form two genetically identical daughter cells (Figure 6.1). This allows growth, repair, replacement of lost cells and so on. A key process during mitosis is the duplication of DNA to give two identical sets of chromosomes, which are then pulled apart and new cells are formed around each set. The new cells may be considered to be clones of the maternal cell.
A cell dividing by mitosis passes through six phases.
• Interphase: the cell goes about its normal, daily business (Figure 6.2). This is also known as the cell cycle, and includes phases of its own: G1 (gap 1), S (synthesis) and G2 (gap 2). DNA is duplicated (synthesised) during S phase.
• Prophase: DNA condenses to become chromosomes which are visible under a microscope (Figure 6.3). Centrioles move to opposite ends of the cell and extend microtubules out (this is the mitotic spindle). The centromeres at the centre of the chromosomes also begin to extend fibres outwards (Figure 6.4).
• Prometaphase: the nuclear membrane disappears, microtubules attach centrioles to centromeres and start pulling the chromosomes.
• Metaphase: chromosomes become aligned in the middle of the cell.
• Anaphase: chromosome pairs split (centromeres are cut), and one of each pair (sister chromatids) move to either end of the cell.
• Telophase: sister chromatids reach opposite ends of the cell and become less condensed and no longer visible; new membranes form around the new nuclei for the daughter cells.
• Cytokinesis: an actin ring around the centre of the cell shrinks and splits the cell in two.
• Interphase: the cell goes about its normal, daily business (including preparing for and doubling its DNA to form pairs of chromosomes).
Errors in mitotic division, although rare, will be carried into the daughter cells of that division, and onwards to new cells produced from them. Errors in early embryonic development could have catastrophic consequences, as an error in one cell would quickly become an error in a huge number of cells. Chromosomal damage can give small or significant effects, such as trisomy (an extra copy translocation or inversion of a broken section. Trisomy 21, for example, results in Down syndrome.