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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-
                                                                                         1
                                                                 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.
                                                                                                      1
                  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
                       1
                   ·  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,
                       2
                                                                 the DNA content of the cell is doubled (4d).








       Vet Histology.indb   27                                                                                   16/07/2019   14:53
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