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Biology for Advanced Level Secondary Schools

           Intermediate filaments                     centrioles in the cytoplasm which usually
           These are  structures found between  lie at right angles to each other close to
           microtubules     and     microfilaments.  the  nuclear  membrane.  In cross section,
           They  provide  shapes of cells  and  act  each  centriole  is seen to contain  nine
           as intercellular  tendons that  prevent  groups of microtubules with three tubules
           excessive stretching of the cells.         in each group (Figure 1.18). During cell
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                                                      division, the centrioles divide and migrate
           Centrioles                                 chromosome in opposite poles of the
           A centriole  is a cylindrical  organelle  cell where they act as a focus for spindle
           found in animal  cells,   algal cells and  formation.  Centrioles  also produce  the
           fungal  cells,  but  not  in  cells  of higher  basal bodies from which cilia and flagella
           plants.  Animal cells contain a pair of  develop.




















                                                  (a)                           (b)

                        Figure 1.18: Structure of a centriole (a) side view and (b) cross section

                                                      Comparison of prokaryotic and

           Exercise 1.9                               eukaryotic cells
                                                      Common features which can be found in
           1.  What  do you understand  by the        prokaryotic and eukaryotic cells are as
               term cytoskeleton?                     follows:
                                                      (a) Both  have  cell  membranes  that
           2.  Explain the functions of microtu-         separate  the  cell’s interior  from  its
               bules  and  microfilaments  in  plant     surrounding environment.
               and animal cells.                      (b) Both  have a cytoplasm that consists
                                                         of a jelly-like region within the cell in
           3.  Describe the structure, location and      which other cellular  components are
               functions  of  centrioles  in  animal     suspended.
               cell.                                  (c) Both have ribosomal particles  that
                                                         synthesise proteins,  and genetic
                                                         material of the cell.
                                                      The prokaryotic and eukaryotic cells also
                                                      differ in many  aspects as elaborated  in
                                                      Table 1.1.

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