Mitosis
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).
Clinical relevance
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.
