Figure 2-2. Prokaryotic cell.
d. Class Sporozoa (Obligate Parasites). These organisms are parasites
having no organelles of locomotion. They have a complicated sexual-asexual life cycle
consisting of several morphological stages. In order for the entire cycle to be completed,
an intermediate arthropod host is necessary. These parasites invade and multiply in the
cells of the host. The malaria parasites of man belong to this group. Figure 2-14
illustrates the life cycle of the malarial parasite.
2-3.
TYPES OF CELL REPRODUCTION
a. General. Growth and reproduction are accomplished by means of cell
division. One of the characteristics of cells is their ability to reproduce their own kind of
division. This does not mean, of course, that the cells of a mouse will divide and
produce another mouse. It does mean, however, that tissue cells in a certain part of a
young mouse will divide to form additional cells of the same type, thereby producing
growth in the organ made up of that particular tissue. In the case of microscopic,
unicellular organisms, cell division produces a new organism.
b. Mitosis. Mitosis is a complicated form of cell division found primarily in the
higher forms of life, but which may occur in some of the lower forms, such as certain
protozoa. In mitosis (figure 2-3), the chromatin becomes organized into pairs of
chromosomes and the centrioles (comprising the centrosome) begin to move toward
opposite poles, forming a spindle. The nuclear membrane and nucleolus break down,
the centrioles reach opposite poles, and the pairs of chromosomes separate, moving
along the spindle toward the poles. After reaching the poles, the chromosomes
consolidate each centriole forms a new centriole, two new nucleoli, and nuclear
MD0151
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