The larger a cell becomes, the more demands
the cell places on its DNA. As a cell increases
in size, it usually does not make copies of
DNA. If a cell were to grow without limit, an
âinformation crisisâ would occur. In addition, as a cell increases in size, the more trouble it has moving enough nutrients (food)
and wastes across its cell membrane. The
rate at which materials move through the
cell membrane depends on the surface area
of the cellâthe total area of its cell membrane. However, the rate at which food and
oxygen are used up and waste products are
produced depends on the volume of the cell.
If a cell were a cube, you could determine surface area by multiplying length !
width ! number of sides. You could determine volume by multiplying length !
width ! height. You then could determine
the cellâs ratio of surface area to volume by
dividing the surface area by the volume. As
a cell grows, its volume increases more
rapidly than its surface area. That is, as a
cell becomes larger, its ratio of surface area
to volume decreases.
Before a cell becomes too large, a growing cell divides, forming two âdaughterâ
cells. The process by which a cell divides into
two new daughter cells is called cell division.
10â2 Cell Division
Each cell has only one set of genetic information. For that reason, a cell must first
copy its genetic information before cell division begins. Each daughter cell then gets a
complete copy of that information. In most
prokaryotes, 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. The first stage is
division of the nucleus, called mitosis. The
second stage is division of the cytoplasm,
called cytokinesis.
In eukaryotes, genetic information is
passed on by chromosomes. Well before cell
division, each chromosome is replicated
(copied). When copying occurs, each chromosome consists of two identical âsisterâ
chromatids. Each pair of chromatids is
attached at an area called a centromere.
The cell cycle is a series of events that
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 of which then begins the cycle
again. The cell cycle consists of four phases.
The M phase includes mitosis and cytokinesis. The other three phases are sometimes
grouped together and called interphase.
Interphase is divided into three phases: G1
, S,
and G2
. During the G1 phase, cells increase in
size and make new proteins and organelles.
During the next phase, the S phase, the replication (copying) of chromosomes takes
place. When the S phase is complete, the cell
enters the G2 phase. During the G2 phase,
many of the organelles and molecules
required for cell division are produced.
Mitosis consists of four phases: prophase,
metaphase, anaphase, and telophase. The
first and longest phase is prophase. During
prophase, the chromosomes condense and
become visible. The centrioles separate and
take up positions on opposite sides of the
nucleus. Centrioles are two tiny structures
located in the cytoplasm near the nuclear
envelope. The centrioles lie in a region
called the centrosome that helps to organize
the spindle, a fanlike microtubule structure
that helps separate the chromosomes.
Summary
.
During the second phase, called
metaphase, chromosomes line up across the
center of the cell. During the third phase,
called anaphase, the centromeres that join the
sister chromatids split and the sister chromatids become individual chromosomes. The
two sets of chromosomes move apart. During
the fourth and final phase, called telophase,
the chromosomes gather at opposite ends of
the cell and 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 is drawn inward until the
cytoplasm is pinched into two nearly equal
parts. In plant cells, a structure known as a
cell plate forms midway between the divided nuclei. A cell wall then begins to
appear in the cell plate.
10â3 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 will
divide rapidly. When the healing process
nears completion, the rate of cell division
slows down and then returns to normal.
Cyclinsâa group of proteinsâregulate
the timing of the cell cycle in eukaryotic
cells. There are two types of these regulatory proteins: internal regulators and
external regulators.
Internal regulators are proteins that
respond to events inside the cell. They
allow the cell cycle to proceed only when
certain processes have happened inside the
cell. External regulators are proteins that
respond to even
