The Eukaryotic Cell Cycle by Leesacks

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									                                                                                                                                     Understanding     Inheritance



                                                                               ITHE ELJKARYCYTICCELL CYCLE


                                                                                                                         -Daughter   cell
                                                                                                                     I



                                                     Interphase

                     f-
                                                        A
                                                                                          -v-”-#


                     —-               —                                                               m-
                                                                                                            \
                                                —-     Generation   time                                        ‘%

                     Time   _




 I he term “cell cycle” refers collectively to       Most of the biochemical     produced byacell          few minutes to a few months, The variation
the events that occur within a eukaryoticceli        are synthesized throughout   interphase.              in generation time is due mainly to a varia-
between its birth by mitosis and its division,       DNA is a notable and easily detected ex-              tion in the length of G, and of G,. The mitotic
again by mitosis, into two daughter cells.           ception, and for that reason interphase     is        phase of most species and most cell types
The cell may be either a one-celled organ-           subdivided into the period between cell birth         occupies             only about   10 percent      of the
ism such as baker’s yeast (Saccharomyces             and the onset of DNA synthesis (Gl), the              generation time.
cerevisiae) or a somatic cell of a multicellu-       period of DNA synthesis (S), which ends
lar organism. Early studies of the eukaryotic        when all the nuclear DNA has been repli-              The cell cycle of bacteria,               in addition   to
cell cycle concentrated on the microscopi-           cated and hence the number of chromo-                 being shorter (typically less than an hour), is
cally visible and dramatic physical events of        somes has doubled,        and the period be-          also less complex. In particular, DNA is
the cell-division, or mitotic, phase (M). On-        tween the end of DNA synthesis and the                synthesized continuously, the two copies of
set of the mitotic phase is signaled by the          beginning of the mitotic phase (G,). After a          the single bacterial chromosome do not
appearanceof microscopically visible worm-           cell has entered S, it is committed to com-           undergo extensive condensation before cell
Iike bodies within the nucleus, that is, by the      pleting the cell cycle, even when environ-            division, and a mechanism simpler than the
condensation of duplicated chromosomes               mental conditions are extremely adverse,              one illustrated in “Mitosis” assures parcel-
into a much less diffuse configuration. The                                                                ing out of one chromosome                 copy to each
mitotic phase ends when the cell separates           The length of the cell cycle, the generation          daughter cell.
into two daughter cellls, each of which then         time, varies with environmental    conditions
embarks on its own cycle. (Details of the            and among species and cell types. For
mitotic Iphase are presented in ‘iMitosis.”)         example, epithelial cells, the cells that line
                                                     the interior and exterior surfaces of the
Because the early microscopic studies re-            human body, have relatively short genera-
vealed little physical activity during the por-      tion times (about eight hours); fibroblasts,
tion of the cell cycle that precedes the             cells that assist in healing wounds, com-
mitotic phase (other than a relatively small         pletetheircell cycle onlyon demand; mature
increase in cell size), that portion was inap-       red blood cells never undergo mitosis; and
propriately named the resting phase, or              embryonic cells divide very rapidly, Ob-
interphase. We now know that most of the             served generation times for those cells that
biosynthetic activity required of a cell—both        do have a regular cycle range from about a
for its own maintenance and reproduction
and for its function or functions as a con-
stituent of a multicellular organism—occurs
during interphase.


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