Essential Cell Biology 5th edition

610 CHAPTER 18 The Cell-Division Cycle
the eukaryotic cell manufactures its numerous other components, such
as proteins, membranes, organelles, and cytoskeletal filaments. In this
chapter, we tackle the second and third questions: how a eukaryotic cell
distributes—or segregates—its duplicated contents to produce two genetically
identical daughter cells, and how it coordinates the various steps of
this reproductive cycle.
QUESTION 18–1
Consider the following statement:
“All present-day cells have arisen
by an uninterrupted series of cell
divisions extending back in time to
the first cell division.” Is this strictly
true?
We begin with an overview of the events that take place during a typical
cell cycle. We then describe the complex system of regulatory proteins
called the cell-cycle control system, which orders and coordinates these
events to ensure that they occur in the correct sequence. We next discuss
in detail the major stages of the cell cycle, in which the chromosomes are
duplicated and then segregated into the two daughter cells. At the end of
the chapter, we consider how animals use extracellular signals to control
the survival, growth, and division of their cells. These signaling systems
allow an animal to regulate the size and number of its cells—and, ultimately,
the size and form of the organism itself.
OVERVIEW OF THE CELL CYCLE
The most basic function of the cell cycle is to duplicate accurately the
vast amount of DNA in the chromosomes and then to segregate the DNA
into genetically identical daughter cells such that each cell receives a
complete copy of the entire genome (Figure 18−1). In most cases, a cell
also duplicates its other macromolecules and organelles and doubles in
size before it reproduces; otherwise, each time a cell divided, it would get
smaller and smaller. Thus, to maintain their size, proliferating cells coordinate
their growth with their division. We return to the topic of cell-size
control later in the chapter; here, we focus on cell division.
The duration of the cell cycle varies greatly from one cell type to another.
In an early frog embryo, cells divide every 30 minutes, whereas a mammalian
fibroblast in culture divides about once a day (Table 18−1). In this
section, we describe briefly the sequence of events that occur in proliferating
mammalian cells. We then introduce the cell-cycle control system
that ensures that the various events of the cycle take place in the correct
sequence and at the correct time.
daughter cells
3
CELL
DIVISION
Figure 18−1 Cells reproduce by
duplicating their contents and dividing in
two in a process called the cell cycle. For
simplicity, we use a hypothetical eukaryotic
cell—which has only one copy each of two
different chromosomes—to illustrate how
each cell cycle produces two genetically
identical daughter cells. Each daughter cell
can divide again by going through another
cell cycle, and so on for generation after
generation.
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CHROMOSOME
SEGREGATION
CELL
CYCLE
1
CELL GROWTH
AND CHROMOSOME
DUPLICATION