Chapter 10 Cell Growth and Division

In most cases, living things grow by producing more cells. There
are two main reasons why cells divide:
1. The larger a cell gets, the more demands it places on its DNA.
2. As a cell gets larger, it has more trouble moving enough
nutrients (food) and wastes across its cell membrane. The
rates at which materials move through the cell membrane
depend on the cell’s surface area—the total area of its cell
membrane. However, the rate at which food and oxygen are
used up and waste products are formed depends on the cell’s
volume. As a cell grows, its volume increases faster than its
surface area. That is, as a cell becomes larger, its ratio of surface
area to volume decreases.
Before a cell gets too large, it divides, forming two “daughter”
cells. Cell division is the process by which a cell divides into two
new daughter cells.

10–2 Cell Division
Acell must copy its genetic information before cell division begins.
Each daughter cell then gets a complete copy of that information.
• In most prokaryotes, the rest of cell division is a simple
matter of separating the contents of the cell into two parts.
• In eukaryotes, cell division occurs in two main stages,
mitosis and cytokinesis. Mitosis is the division of the
nucleus. Cytokinesis is the division of the cytoplasm.
The cell cycle is a series of events cells go through as they grow
and divide. During the cell cycle, a cell grows, prepares for
division, and divides to form two daughter cells. Each daughter
cell then begins the cycle again. The phases of the cell cycle
include interphase and cell division.
• Interphase is divided into three phases: G1, S, and G2.
▪ During the G1 phase, cells increase in size and make
new proteins and organelles.
▪ In the S phase, replication (copying) of chromosomes
takes place.
▪ During the G2 phase, many of the organelles and molecules
needed for cell division are produced.
• The M phase, or cell division includes mitosis and
cytokinesis.

Biologists divide the events of mitosis into four phases:
prophase, metaphase, anaphase, and telophase.
1. Prophase. During prophase, the chromosomes condense and
become visible. There are two tiny structures located in the
cytoplasm near the nuclear envelope. These structures are
called centrioles. The centrioles separate and move to opposite
sides of the nucleus. The spindle is a structure that helps move
chromosomes apart. During prophase, the chromosomes attach
to fibers in the spindle. At the end of prophase, the nuclear
envelope breaks down.
2. Metaphase. During metaphase, chromosomes line up across
the center of the cell. The centromere of each chromosome
attaches to the spindle.
3. Anaphase. During anaphase, the centromeres joining the sister
chromatids split. The sister chromotids become individual
chromosomes. The two sets of chromosomes move apart.
4. Telophase. During telophase, the chromosomes move to
opposite ends of the cell. They lose their distinct shapes. Two
new nuclear envelopes form.
Cytokinesis usually occurs at the same time as telophase. In
most animal cells, the cell membrane pinches the cytoplasm into
two nearly equal parts. In plant cells, a cell plate forms midway
between the divided nuclei. A cell wall then begins to form in the
cell plate.

10–3 Regulating the Cell Cycle
In a multicellular organism, cell growth and cell division are carefully
controlled. For instance, when an injury such as a cut in the
skin occurs, cells at the edge of the cut divide rapidly. When the
healing process is nearly complete, the rate of cell division slows
and then returns to normal.
Cyclins—a group of proteins—regulate the timing of the
cell cycle in eukaryotic cells. Cyclins are one group of proteins
involved in cell cycle regulation. Other proteins, called regulatory
proteins, regulate the cell cycle in different ways.
Controls on cell growth can be turned on and off by the body.
Cancer is a disorder in which some of the body’s cells lose the
ability to control growth. Cancer cells do not respond to the
signals that control the growth of most cells. As a result, cancer
cells divide uncontrollably. Cancer cells do not stop growing when
they touch other cells. Instead, they continue to grow and divide
until their supply of nutrients is used up.
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