Biology (Holt)

The Life of a Eukaryotic Cell

Cell division in eukaryotic cells is more complex than cell division
in bacteria because it involves dividing both the cytoplasm and the
chromosomes inside the nucleus. Many internal organelles must be
correctly rearranged before the eukaryotic cell can properly divide
and form two fully functioning cells.

The Cell Cycle

The life of a eukaryotic cell is traditionally shown as a cycle, as
illustrated in Figure 6.The is a repeating sequence of cel-
lular growth and division during the life of an organism. A cell
spends 90 percent of its time in the first three phases of the cycle,
which are collectively called .A cell will enter the last
two phases of the cell cycle only if it is about to divide. The five
phases of the cell cycle are summarized below:

1. First growth (G 1 ) phase. During the G 1 phase, a cell grows
   rapidly and carries out its routine functions. For most organ-
   isms, this phase occupies the major portion of the cell’s life. Cells
   that are not dividing remain in the G 1 phase. Some somatic cells,
   such as most muscle and nerve cells, never divide. Therefore, if
   these cells die, the body cannot replace them.


2. Synthesis (S) phase. A cell’s DNA is copied during this phase.
   At the end of this phase, each chromosome consists of two chro-
   matids attached at the centromere.


3. Second growth (G 2 ) phase. In the G 2 phase,
   preparations are made for the nucleus to
   divide. Hollow protein fibers called micro-
   tubules are assembled. The microtubules are
   used to move the chromosomes during mitosis.


4. Mitosis.The process during cell division in
   which the nucleus of a cell is divided into two
   nuclei is called (mie TOH sihs). Each
   nucleus ends up with the same number and
   kinds of chromosomes as the original cell.


5. Cytokinesis.The process during cell division
   in which the cytoplasm divides is called
   (SIET oh kih nee sihs).
   Mitosis and cytokinesis produce new cells
   that are identical to the original cells and allow
   organisms to grow, replace damaged tissues,
   and, in some organisms, reproduce asexually.

cytokinesis

mitosis

interphase

cell cycle

The Cell Cycle Section 2

Objectives

Identifythe major

events that characterize

each of the five phases of

the cell cycle.

Describehow the cell

cycle is controlled in

eukaryotic cells.

Relatethe role of the

cell cycle to the onset

of cancer.

Key Terms

cell cycle

interphase

mitosis

cytokinesis

cancer

G 1

(Cell growth)

G 2

(Growth and

preparation for

mitosis)

Mitosis

Cytokinesis

S

(DNA synthesis)

INTER

PHA

SE

SECTION 2 The Cell Cycle 125

Figure 6 The eukaryotic

cell cycle.The cell cycle

consists of phases of growth,

DNA replication, preparation

for cell division, and division of

the nucleus and cytoplasm.

4B 6E

4B 6E

4B 6C