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Chapter Introduction
The Life of a Eukaryotic Cell
8.1 Cell Division in Eukaryotes
8.2 The Phases of the Cell Cycle
DNA Replication
8.3 DNA Structure
8.4 DNA Synthesis
8.5 DNA Repair
Mitosis and Cell Division
8.6 The Stages of Cell Division
8.7 Differences in Mitosis
Regulation of the Cell Cycle
8.8 Control of the Cell Cycle
8.9 Checkpoints
Chapter Highlights
Chapter Animations
Learning Outcomes
By the end of this chapter you will be able to:
A Compare the processes of cell division in
prokaryotes and eukaryotes.
B Describe the four phases of the cell cycle and how
they are controlled.
C Summarize the events of DNA replication and
evaluate the importance of correcting DNA
replication errors.
D Describe the states of mitosis and compare and
contrast mitosis in plant and animal cells.
E Describe how the cell cycle is regulated.
Transport Systems
 What is happening to
these cells?
 How is this event
important in the growth and
development of multicellular
organisms?
You can see the root tip cells of an onion
plant at various stages of the cell cycle in
this light micrograph (x400).
Transport Systems
• Eukaryotic cells cycle
through a series of ordered
processes that result in
duplication of the cell.
• Errors in either DNA
replication or mitosis can
seriously damage or kill a
cell or even a multicellular
organism.
You can see the root tip cells of an onion
plant at various stages of the cell cycle in
this light micrograph (x400).
The Life of a Eukaryotic Cell
8.1 Cell Division in Eukaryotes
• Eukaryotic cell division is
part of a more complex
series of stages called the
cell cycle.
• Unicellular eukaryotes,
such as yeast and Amoeba,
divide to produce two new
identical organisms.
A scanning electron micrograph of the
cilia-covered Tetrahymena, x6000, is
shown here. This single-celled eukaryote
is shown in a late stage of cell division.
The Life of a Eukaryotic Cell
8.1 Cell Division in Eukaryotes (cont.)
• Multicellular organisms usually develop from a
single fertilized egg cell.
• Plants have specialized regions at the tips of their
roots and stems where, through repeated cell
divisions, they produce the new cells that develop
into the mature tissues of growing roots, stems,
leaves, and other organs.
• During animal development, cell division produces
many different types of cells that form the nerves,
skin, and other organs.
The Life of a Eukaryotic Cell
8.1 Cell Division in Eukaryotes (cont.)
• Eukaryotic cell division requires accurate replication
and equal division of the genetic information
encoded in the cell’s DNA.
• Cell division also replaces cells that simply wear out
or are damaged during the life of an organism.
• The cell cycle is remarkably similar in all eukaryotes.
The Life of a Eukaryotic Cell
8.2 The Phases of the Cell Cycle
• As a single cell completes the cell cycle, it becomes
two new daughter cells.
• When a eukaryotic cell divides, its nuclear
membrane breaks down, the individual
chromosomes separate, and they are distributed to
the daughter cells in a process called mitosis.
• The period between divisions is called interphase.
The Life of a Eukaryotic Cell
8.2 The Phases of the Cell Cycle (cont.)
• Cells pass in order through, and are always in one
of, the five phases of the cell cycle known as:
– G1 (Gap 1 or prereplication)
– S (DNA Synthesis)
– G2 (Gap 2 or premitosis)
– M (Mitosis)
– G0 (Gap 0 or nondividing cells).
The Life of a Eukaryotic Cell
8.2 The Phases of the Cell Cycle (cont.)
• When a cell in G0 or G1 receives a signal to divide,
it passes through the restriction point (R).
• Once a cell passes the restriction point, it cannot
return to G1 or G0 without completing a full cell cycle.
• Different types of cells
vary in their ability to
leave G0 and commit to
a cell-division cycle.
The Life of a Eukaryotic Cell
8.2 The Phases of the Cell Cycle (cont.)
• During the S phase, the DNA of each chromosome
replicates to form a new identical set of
chromosomes.
• During G2, the cell
prepares for mitosis by
synthesizing specific
types of RNA and
proteins.
• During interphase, the
chromosomes spread out
and fill up the nucleus.
The Life of a Eukaryotic Cell
8.2 The Phases of the Cell Cycle (cont.)
• Mitosis, sometimes called nuclear division, is a
series of events that ensures that each new
daughter cell receives one copy of each
chromosome.
• The division of the whole
cell which occurs after
mitosis is called
cytokinesis.
• After cytokinesis, each
daughter cell enters G1.
DNA Replication
8.3 DNA Structure
• Mitosis provides each daughter cell with a complete
set of chromosomes that are the same type and
number as those of the parent cell.
• The process of DNA replication depends on the
molecular shapes of DNA and its nucleotide bases.
• Base pairing depends on how many hydrogen
bonds each nitrogen base can form with its
counterpart.
DNA Replication
8.3 DNA Structure (cont.)
• Adenine (A) pairs only with
thymine (T) because these
two bases can make two
hydrogen bonds.
• Guanine (G) pairs only with
cytosine (C) because three
hydrogen bonds hold them
together.
A short section of a DNA molecule, as it would appear
if uncoiled and flattened, is depicted here. Sugarphosphate bonds connect the nucleotides along each
strand. Hydrogen bonds between the nitrogen bases
connect the two strands.
DNA Replication
8.3 DNA Structure (cont.)
• DNA strands are parallel but
run in opposite directions in an
antiparallel arrangement.
A short section of a DNA molecule, as it would appear
if uncoiled and flattened, is depicted here. Sugarphosphate bonds connect the nucleotides along each
strand. Hydrogen bonds between the nitrogen bases
connect the two strands.
DNA Replication
8.4 DNA Synthesis
• The synthesis of new DNA during the S phase of the
cell cycle is a multistep process that can be divided
into three major parts:
1. binding of enzymes to existing DNA
2. unwinding of the double helix
3. synthesis of a new matching strand for each
existing strand
The three major parts of DNA replication
DNA Replication
8.4 DNA Synthesis (cont.)
• First, enzymes and other proteins involved in DNA
synthesis bind to specific regions of chromosomes
called replication origins.
• The proteins include an enzyme that unwinds the
double helix, an RNA-synthesizing enzyme, and
DNA polymerase, the enzyme that catalyzes the
formation of the new DNA strands.
• The combination of DNA and proteins is called a
replisome.
DNA Replication
8.4 DNA Synthesis (cont.)
• Prokaryotes have one origin of replication while in
eukaryotes there are multiple origins.
• DNA polymerase can add nucleotides only to the end
of an existing nucleic-acid strand.
DNA Replication
8.4 DNA Synthesis (cont.)
• Synthesis of the new
matching strand occurs
continuously on only one
of the original strands
called the leading strand.
• On the other original
strand, called the lagging
strand, synthesis occurs in
short segments.
DNA Replication
8.4 DNA Synthesis (cont.)
• This type of replication is
known as semiconservative
(half conservative)
replication because each of
the two new doublestranded DNA molecules
conserves one strand
(half) of the original DNA,
but adds one strand of
new DNA.
DNA Replication
8.4 DNA Synthesis (cont.)
• Proteins are involved in wrapping the DNA into the
tightly condensed structure called the chromosome.
A DNA molecule wraps around histone proteins to form
nucleosomes, the basic packing unit of eukaryotic chromosomes.
The coiled, beaded chain of DNA with its nucleosomes forms still
thicker coils that make up the chromosome.
DNA Replication
8.5 DNA Repair
• Any change in the sequence of a cell’s DNA is
known as a mutation.
• Mutations can be nonharmful (silent), harmful, or
lethal to the cell.
• Mutations in human cells that persist to the next cell
division are inherited by the daughter cells and
cause many diseases.
DNA Replication
8.5 DNA Repair (cont.)
• Cells have processes to detect and correct errors in
replication as well as damage to DNA by
environmental factors such as mutagenic chemicals.
• The DNA polymerase that produces the DNA itself
proofreads its own work and replaces any
mismatched neucleotides.
• Most mutations consist of base pairs that cannot
form hydrogen bonds and are repaired through an
excision repair.
Excision repair
Mitosis and Cell Division
8.6 The Stages of Cell Division
• When DNA replication is complete, the cell passes
from the S phase to the G2 phase.
• The two copies of each chromosome made during
the S phase are called sister chromatids.
This scanning electron micrograph,
x87,000, shows a replicated chromosome
in metaphase with its pair of sister
chromatids joined at their centromere.
Mitosis and Cell Division
8.6 The Stages of Cell Division (cont.)
• As a cell enters the M phase,
sister chromatids are still
attached by proteins at a
narrow point called the
centromere.
This drawing illustrates the
chromosome’s structures.
Mitosis and Cell Division
8.6 The Stages of Cell Division (cont.)
• If segregation occurs correctly, each new nucleus
receives one copy of each chromosome.
• A mistake at this stage will result in daughter cells
with abnormal numbers of chromosomes called
aneuploid cells.
Mitosis and Cell Division
8.6 The Stages of Cell Division (cont.)
• The process of mitosis, once begun, is continuous
but is considered to have four distinct steps.
• Individual chromosomes are not visible during
interphase (a).
All photos in this sequence are of the
interphase and stages of mitosis in the
blood lily Haemanthus. Chromosomes
are stained blue, and microtubules
are stained red.
Mitosis and Cell Division
8.6 The Stages of Cell Division (cont.)
• Prophase begins when the nuclear membrane
breaks down into small vesicles
and the chromosomes condense.
• Microtubules begin to form
around the nucleus (b)
and join to form a mitotic
spindle (c).
• The microtubules are
anchored to protein
structures that surround
the centrioles (if present),
called the spindle poles.
Mitosis and Cell Division
8.6 The Stages of Cell Division (cont.)
• Within each centromere is a protein complex called
the kinetochore.
• Some of the microtubules
in the spindle bind to the
kinetochores of each
chromatid so that a chain
of microtubules connects
each chromatid to a
spindle pole.
• Sister chromatids move to
opposite poles.
Mitosis and Cell Division
8.6 The Stages of Cell Division (cont.)
• Metaphase is the second step of mitosis.
• By this time, motor proteins
in the kinetochores have
pulled the chromosomes
into a ring between the two
poles, forming the
metaphase plate (d and e).
Mitosis and Cell Division
8.6 The Stages of Cell Division (cont.)
• In the third step, anaphase,
enzymes break down the
protein holding sister
chromatids together.
• The sisters separate, and the
motor proteins of their
kinetochores pull them along
the spindle microtubules to
opposite spindle poles (f and g).
Mitosis and Cell Division
8.6 The Stages of Cell Division (cont.)
• In telophase (h and i), the
chromosomes begin to expand,
and the nuclear envelope
re-forms around them,
producing two new nuclei.
• Soon after this, cytokinesis
divides the cell in two, as the
plasma membrane constricts
between the nuclei and
completes cell division.
Mitosis and Cell Division
8.7 Differences in Mitosis
• Although the major events and molecular players of
cell division are similar in all eukaryotic cells, there
are some subtle differences.
– Cytokinesis begins during anaphase in most animal cells.
– At cytokinesis in plants, vesicles containing cellulose begin
to congregate and fuse between the two nuclei, forming the
plasma membranes and completing the cell wall between
the two new cells.
– In some fungi, such as yeast, the nuclear envelope forms a
bud instead of breaking down, and the spindle poles are
embedded in the nuclear membrane.
Regulation of the Cell Cycle
8.8 Control of the Cell Cycle
• The controls that regulate the order and timing of
cell-cycle events are of major interest to scientists
who study the eukaryotic cell cycle.
– Something in S-phase and M-phase cells can
cause G1 and G2 nuclei to advance to the next
phase (S or M).
– A factor or factors in S-phase cells can enter G1
nuclei and initiate DNA replication.
– M-phase cells can move G2 nuclei into mitosis.
Regulation of the Cell Cycle
8.8 Control of the Cell Cycle (cont.)
• Proteins called cyclins regulate progression through
the cell cycle.
• The most important cyclins are the G1 cyclins and
the mitotic cyclins.
G1 cyclins peak at S phase, and mitotic cyclins peak at metaphase
in M phase.
Regulation of the Cell Cycle
8.8 Control of the Cell Cycle (cont.)
• Cyclins act by binding to various kinases, which are
enzymes that transfer a phosphate group from ATP
to other enzymes.
• The phosphate group activates these enzymes.
• The quantity of these kinases in the cell remains
steady throughout the cycle but they are active only
when bound to the appropriate cyclin.
Regulation of the Cell Cycle
8.8 Control of the Cell Cycle (cont.)
• As the amount of a particular cyclin rises, it activates
more kinases which activate various enzymes
needed for progress through the cell cycle.
Regulation of the Cell Cycle
8.8 Control of the Cell Cycle (cont.)
The abundance of different cyclins varies
during the cell cycle. Each cyclin activates
specific kinases. The kinases activate some
enzymes directly at (a) and signal the cell to
synthesize other proteins needed to progress
to the next phase of the cycle at (b).
Regulation of the Cell Cycle
8.9 Checkpoints
• Eukaryotic cells have an elaborate system called
checkpoint control that monitors the condition of the
DNA, the chromosomes, and the mitotic spindle.
• Checkpoint controls consist of proteins that detect
mistakes and damage and put the cell into
cell-cycle arrest until the damage is fixed.
• Without checkpoint controls, mitosis could produce
daughter cells with damaged or missing
chromosomes.
Regulation of the Cell Cycle
8.9 Checkpoints (cont.)
• Checkpoints throughout
the cell cycle ensure that
problems are corrected
before the cycle
progresses preventing
the production of
daughter cells with
genetic damage.
Regulation of the Cell Cycle
8.9 Checkpoints (cont.)
• When the regulators involved in preventing cells
from leaving the G0 stage are inactivated, cells may
divide at the wrong time.
• Mutations in the genes that encode these proteins
can lead to uncontrolled growth known as cancer.
Summary
• The eukaryotic cell cycle forms two offspring cells from a
parent cell.
• There are five phases of the cell cycle: G0, G1, S, G2, and M.
• Interphase consists of G1, S, and G2.
• DNA synthesis occurs in S phase and begins at
replication origins.
• Replisomes catalyze the synthesis of two new strands of
DNA that are complements of the two parental strands. The
leading strand is synthesized in a continuous process. The
lagging strand replicates in short stretches of DNA that are
then joined.
• Eukaryotic DNA replication is semiconservative; the resulting
chromosomes each contain an old strand of DNA paired with a
newly synthesized strand.
Summary (cont.)
• Errors in replication are repaired by the DNA polymerase itself,
while damaged segments may be repaired by the excisionrepair system.
• The newly replicated sister chromatids are segregated to the
daughter nuclei during mitosis.
• In mitosis, arrays of microtubules form a mitotic spindle.
Microtubules emanating from the spindle poles link to others
that attach to the chromosomes at their centromeres. Motor
proteins in the kinetochores then transport the chromosomes
toward the metaphase plate.
• At anaphase, the sister chromatids separate and migrate to
the spindle poles.
• The nuclear envelope re-forms around the new daughter
nuclei in telophase. Cytokinesis usually follows mitosis.
Summary (cont.)
• The timing and sequence of events are linked to the synthesis
and disappearance of various cyclins throughout the cell cycle.
• Errors that occur during this process are closely monitored by
checkpoint control.
• Failure of the checkpoint-control system is one important step
in the development of cancer.
Reviewing Key Terms
Match the term on the left with the correct description.
___
mitosis
d
___
cyclins
a
___
kinetochore
e
___
telophase
c
___
aneuploid
b
___
G0
f
a. group of proteins that regulate the
progression of the cell cycle
b. a condition of having an abnormal
number of chromosomes
c. stage in the cell cycle
characterized by new nuclei
forming at opposite ends of the
cell
d. the phase of the cell cycle when
nuclear division occurs
e. links chromosomes to the mitotic
spindle
f.
a resting stage in the cell cycle
Reviewing Ideas
1. What is the basic, most common cause
of cancer?
The basic, most common cause of cancer is a
mutation in the genes that encode the
checkpoint proteins, or tumor-suppressor genes.
Reviewing Ideas
2. What is an aneuploid cell and how do
they form?
Aneuploid cells are daughter cells with abnormal
numbers of chromosomes. These cells are
formed as a result of a mistake during
chromosome segregation.
Using Concepts
3. Why do prokaryotes only require one replication
origin while eukaryotes require many?
In prokaryotes, one replication origin is sufficient
because bacteria contain only one small
chromosome, which can replicate quickly.
Eukaryotes have multiple chromosomes, each
containing much more DNA than bacteria
chromosomes do.
Using Concepts
4. Could you study cell division in yeast to help
understand human cell division? Explain.
Scientists have gained much of their knowledge of
the cell cycle from studies of yeast. This is possible
because the cell cycle is remarkably similar in all
eukaryotes. Studies have found that yeast and
human cells perform the cell cycle in a comparable
fashion using very similar proteins.
Synthesize
5. Suppose you spent too much time in the sun
and developed a severe sunburn. Undoubtedly,
some cells were damaged by the excessive UV
exposure. What prevents this damage from
becoming cancerous automatically?
Your cells have the ability to detect and repair
mutations caused by environmental factors such
as UV radiation. Proteins in the cell can detect
DNA damage and put the cell into cell-cycle
arrest until the damage is fixed, preventing the
mutation from being passed on to daughter cells.
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Chapter Animations
The three major parts of DNA replication
Stages of the cell cycle
Excision repair
The three major parts of DNA replication
Excision repair
End of Custom Shows
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