Page 45 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

The cell (cellula) 27

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1.37 Cell cycle (schematic).

process, known as mitosis, both daughter cells receive an These three consecutive phases typically account for over
identical copy of the genes contained in the nucleus of the 90% of the duration of the cell cycle. In rapidly growing
parent cell. The term mitosis is derived from the threadlike cells, the cell cycle can be completed within 16–24 hours,
appearance of the chromosomes during nuclear division of which 1–2 hours are taken up by mitosis.
(Gk mitos = thread). Division of the nucleus (karyokinesis) The duration of the cell cycle is cell-specific. Epithelial
is followed by division of the cytoplasm (cytokinesis), lead- cells (e.g. those lining deep intestinal crypts) constantly
ing to complete division of the cell. undergo rapid cycles of division, each being completed
Replication of the chromosome set without subsequent within 8–11 hours. Other cell populations are character-
nuclear or cellular division is referred to as endomitosis. ised by cell cycles lasting days, weeks or years (e.g. cartilage
The term amitosis describes the doubling of genetic mate- cells, bone cells and muscle cells).
rial and subsequent division of the nucleus and cytoplasm The G phase is the period in which the cell performs
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without the formation of recognisable chromosomes. the functions that are specific to that cell type. It is domi-
Male and female germ cells undergo a specialised form of nated by RNA production and protein biosynthesis. The
cell division, referred to as meiosis, in which the chromo- duration of this phase is particularly variable, ranging from
some set is halved (haploidy). The full set of chromosomes hours to years, depending on cell type. In preparation for
(diploidy) is re-established only when an ovum is fertilised. the subsequent S phase, the G phase involves the produc-
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tion of enzymes and proteins required for chromosome
Cell cycle assembly. The supply of these molecules results in transi-
For most cells of the body, growth and division are regu- tion to the synthesis phase and the end of the G phase.
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larly recurring processes. The cell cycle begins with division An exception to this sequence of events is observed in
(mitosis), then enters a metabolically active growth phase postmitotic cells that have lost the capacity for division.
(interphase) before ending with another cell division. Each These include mature nerve cells, which, despite being
of these phases can be further subdivided into phases that highly metabolically active, do not progress to the S phase.
differ in their chronological progression (Figure 1.37). The Such cells are described as being in the G phase.
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features of these phases may also vary considerably in dif- The S phase (duration 6–8 hours) is characterised by
ferent cell populations. the duplication of genetic material (replication of the
DNA double helix). The double helix divides asymmetri-
Interphase cally into single strands (Y-shaped replication fork) and,
Interphase encompasses three phases: through complementary base pairing, is copied to form
two new DNA strands. Each chromosome now consists of
· G phase (precedes DNA duplication, G = gap), two identical chromatin strands (sister chromatids). The S
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· S phase (DNA duplication, S = synthesis) and phase concludes when all of the cellular DNA is replicated.
· G phase (between DNA duplication and mitosis). From the end of the S phase until the beginning of mitosis,
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the DNA content of the cell is doubled (4d).

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