How Cells Reproduce
Chapter 8
8.1 Cell Division Mechanisms
 Reproduction
• Produces a generation of individuals like parents
 Cell division
• Bridges two generations
 Each daughter cell receives
• A required number of DNA molecules
• Some cytoplasm
Dividing HeLa Cells
Eukaryotes and Prokaryotes
 Eukaryotic cells undergo mitosis and/or meiosis
• Separates duplicated chromosomes of parent cell
into two daughter nuclei
• Another mechanism divides cytoplasm
 Prokaryotic cells divide by a different process
Mitosis and Meiosis
 Mitosis
• Basis of growth, cell replacements, and tissue
repair in multicelled species
• Basis of asexual reproduction in many singlecelled and multicelled species
 Meiosis
• Basis of sexual reproduction
• Precedes formation of gametes or sexual spores
Cell Division Mechanisms
Chromosome Structure
 Eukaryotic chromosome
• Association of DNA, histones, and other proteins
• Proteins structurally organize the chromosome
and affect access to its genes
 Nucleosome
• Smallest unit of organization
• Double-stranded DNA looped twice around a
spool of histones
Structure of a Condensed Chromosome
Fig. 8.4a, p.127
centromere
a One duplicated human chromosome in its most condensed form.
Fig. 8.4a, p.127
Fig. 8.4b, p.127
multiple levels of coiling
of DNA and proteins
b When a chromosome
is most condensed, the
proteins associated with
it interact in ways that
package loops of DNA,
which are already coiled,
into higher order levels
of coiling.
Fig. 8.4b, p.127
Fig. 8.4c, p.127
c At a deeper level of
structural organization, the
chromosomal proteins and
DNA are organized as a
cylindrical fiber.
fiber
d Immerse a chromosome in
saltwater and it loosens to a
beads-on-a-string organization.
What appears to be a “string” is
one DNA molecule. Each “bead”
is a nucleosome.
beads on
a string
Fig. 8.4c, p.127
Fig. 8.4d, p.127
DNA
double
helix
core of
histone
nucleosome
e A nucleosome consists
of part of a DNA molecule
looped twice around a core
of histone proteins.
Fig. 8.4d, p.127
Animation: Chromosome structural
organization
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Sister Chromatids
 A duplicated
chromosome consists
of two sister
chromatids, each with
a kinetochore
• Sister chromatids
remain attached at
their centromere until
late in mitosis (or
meiosis)
one
chromatid
one chromosome
(unduplicated)
its sister
chromatids
one chromosome
(duplicated)
Fig. 8.3, p.126
Key Concepts:
CHROMOSOMES AND DIVIDING CELLS
 Individuals of a species have a characteristic
number of chromosomes in each cell
 Chromosomes differ in length and shape, carry
different portions of cell’s hereditary information
 Mechanisms divide information between
daughter cells, along with enough cytoplasm for
each cell to operate on its own
8.2 Introducing the Cell Cycle
 Cell cycle
• Starts when a new cell forms
• Runs through interphase
• Ends when cell reproduces by nuclear and
cytoplasmic division
Interphase
 Most cellular activities occur in interphase
• G1: Cell grows in mass, doubles number of
cytoplasmic components
• S: DNA replication duplicates chromosomes
• G2: Cell prepares for division
Eukaryotic Cell Cycle
G1
Interval of cell growth before
DNA replication (chromosomes
unduplicated)
S
Interval of cell growth when
the DNA is replicated (all
chromosomes duplicated)
cytoplasmic
division; each
daughter cell
enters interphase
G2
Interval after DNA
replication; the cell
prepares to divide
Interphase ends for parent cell
Fig. 8.5, p.128
Chromosome Number
 Sum of all chromosomes in cells of a given type
 In human body cells, chromosome number is 46
 Body cells are diploid (have two of each kind of
chromosome)
Human Chromosomes: 23 Pairs
Mitosis and Chromosome Number
 Mitosis maintains parental chromosome number
from one generation to the next
• Bipolar spindle divides sister chromatids
pole
microtubule of
bipolar spindle
chromosomes
pole
p.129
mitosis,
cytoplasmic
division
One of the
unduplicated
chromosomes
in a parent cell
at interphase
The same two
chromosomes,
(duplicated) at
interphase,
prior to mitosis
After mitosis
and cytoplasmic
division, the two
daughter cells
each have one
(unduplicated)
chromosome.
Both daughter
cells start life
in interphase.
Fig. 8.6b, p.129
mitosis,
cytoplasmic
division
One of the
unduplicated
chromosomes
in a parent cell
at interphase
The same two
chromosomes,
(duplicated) at
interphase,
prior to mitosis
After mitosis
and cytoplasmic
division, the two
daughter cells
each have one
(unduplicated)
chromosome.
Both daughter
cells start life
in interphase.
Stepped Art
Fig. 8-6b, p.129
Key Concepts:
MITOSIS IN THE CELL CYCLE
 Cell cycle starts when a daughter cell forms and
ends when that cell completes its own division
 A typical cycle goes through interphase, mitosis,
and cytoplasmic division
 In interphase, a cell increases its mass and
number of components, and copies its DNA
Animation: The cell cycle
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8.3 A Closer Look at Mitosis
 Mitosis
• A nuclear division mechanism that maintains the
chromosome number
 Mitosis proceeds in four stages:
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
Prophase
 Duplicated chromosomes become threadlike as
they start to condense
 Microtubules form a bipolar spindle
 Nuclear envelope starts to break apart
Transition to Metaphase
 Microtubules from one spindle pole harness one
chromatid of each chromosome
• Microtubules from the opposite spindle pole
harness its sister chromatid
 Other microtubules extend from both poles and
grow until they overlap at the spindle’s midpoint
Metaphase
 All chromosomes become aligned midway
between the two spindle poles
• Chromosomes in most condensed forms
Anaphase
 Sister chromatids detach from each other
• Spindles move them toward opposite poles
 Microtubules that overlap at spindle’s midpoint
slide past each other, push poles farther apart
 Motor proteins drive movements
Telophase
 Two identical clusters (one chromosome of each
type) reach opposite spindle poles
 Nuclear envelope forms around each cluster
 Both new nuclei have the parental chromosome
number
Mitosis
Mitosis
Fig. 8.7a, p.130
a Cell at
Interphase
A diploid cell
duplicates
its DNA and
prepares for
mitosis.
Fig. 8.7a, p.130
Fig. 8.7b, p.130
nuclear
envelope
chromosome
b Early Prophase
Mitosis begins. DNA and its associated
proteins have started to condense. Two
chromosomes (color-coded purple) were
inherited from the female parent. The
other two (blue) are their counterparts,
inherited from the male parent.
Fig. 8.7b, p.130
Fig. 8.7c, p.130
pair of centrioles
c Late Prophase
The duplicated chromosomes continue
to condense. New microtubules move
one of two pairs of centrioles to the
opposite side of the nucleus. The
nuclear envelope starts to break up.
Fig. 8.7c, p.130
Fig. 8.7d, p.130
microtubule
d Transition to Metaphase
Microtubules penetrate the nuclear region and
collectively form a bipolar spindle. Some tether
one sister chromatid of each chromosome
to a spindle pole. Others overlap at the spindle
equator (not shown).
Fig. 8.7d, p.130
e Metaphase
All of the chromosomes have
become lined up midway
between the spindle poles.
At this stage of mitosis, the
chromosomes are in their
most tightly condensed form.
Fig. 8.7e, p.130
Fig. 8.7f, p.130
f Anaphase
Sister chromatids separate as
motor proteins moving along
spindle microtubules drag
them to opposite spindle
poles. Other microtubules
push the poles farther apart.
Fig. 8.7f, p.130
Fig. 8.7g, p.130
g Telophase
There are two clusters of
chromosomes, which now
decondense. Patches of
new membrane fuse to form
a new nuclear envelope.
Mitosis is over.
Fig. 8.7g, p.130
Fig. 8.7h, p.130
h Two Daughter
Cells at Interphase
After cytoplasmic division,
there are two daughter
cells. Each is diploid: Its
nucleus has two of each
type of chromosome, just
like the parent cell.
Fig. 8.7h, p.130
Key Concepts:
STAGES OF MITOSIS
 Mitosis divides the nucleus, not the cytoplasm
• Mitosis has four sequential stages: prophase,
metaphase, anaphase, and telophase
 A microtubular spindle forms
• Moves cell’s duplicated chromosomes into two
parcels, end up in two genetically identical nuclei
Animation: Mitosis step-by-step
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8.4 Cytoplasmic Division Mechanisms
 Mechanisms of cytoplasmic division differ in
plant and animal cells
 In animal cells
• A contractile ring of microfilaments (part of cell
cortex) contracts and pulls the cell surface inward
until the cytoplasm is divided
Cytoplasmic Division in Animal Cells
Fig. 8.8a1, p.132
1 Mitosis is completed, and
the bipolar spindle is starting
to disassemble.
Fig. 8.8a1, p.132
Fig. 8.8a2, p.132
2 At the former spindle equator,
a ring of actin filaments attached to
the plasma membrane contracts.
Fig. 8.8a2, p.132
Fig. 8.8a3, p.132
3 The diameter of the contractile
ring continues to shrink and pull the
cell surface inward.
Fig. 8.8a3, p.132
Fig. 8.8a4, p.132
4 The contractile mechanism
continues to operate until the
cytoplasm is partitioned.
Fig. 8.8a4, p.132
Cytoplasmic Division in Plant Cells
 In plant cells
• A band of microtubules and microfilaments forms
around the nucleus before mitosis starts
• Marks site where cell plate will form
 Cell plate becomes a cross-wall that partitions
the cytoplasm
Cytoplasmic Division in Plant Cells
Fig. 8.8b1, p.132
1 The plane of division and of a
future cross-wall was established
by a band of microtubules and
actin filaments that formed and
broke up before mitosis. Vesicles
cluster here when mitosis ends.
Fig. 8.8b1, p.132
Fig. 8.8b2, p.132
cell
plate
forming
2 The vesicles fuse with each
other and with endocytic vesicles
bringing cell wall components
and plasma membrane proteins
from the cell surface. The fused
materials form a cell plate along
the plane of division.
Fig. 8.8b2, p.132
Fig. 8.8b3, p.132
3 The cell plate grows
outward along the plane
of division until it reaches
and attaches to the plasma
membrane. The cell plate
has now partitioned the
cell’s cytoplasm.
Fig. 8.8b3, p.132
Fig. 8.8b4, p.132
4 The cell plate matures
as two new primary cell walls
surrounding middle lamella
material. The new walls join
with the parent cell wall, so
each daughter cell becomes
enclosed by its own wall.
Fig. 8.8b4, p.132
Animation: Cytoplasmic division
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Results of Cell Division:
A Human Embryo
Key Concepts:
HOW THE CYTOPLASM DIVIDES
 After nuclear division, the cytoplasm divides and
typically puts a nucleus in each daughter cell
 The cytoplasm of an animal cell is simply
pinched in two
 In plant cells, a cross-wall forms in the
cytoplasm and divides it
8.5 Controls Over Cell Division
 Products of checkpoint genes control cell cycle
• Kinases
• Growth factors
When Control is Lost
 Mutant checkpoint genes can cause tumors by
disrupting normal controls
Cancers
 Altered cells grow and divide abnormally
• Malignant cells may metastasize (break loose
and colonize distant tissues)
Neoplasms: Benign and Malignant
benign tumor
Fig. 8.12a, p.135
malignant tumor
Fig. 8.12b, p.135
a Cancer cells
break away from
their home tissue.
2 The metastasizing
cells become attached
to the wall of a blood
vessel or lymph vessel.
They release digestive
enzymes onto it. Then
they cross the wall at
the breach.
3 Cancer cells creep or
tumble along inside blood
vessels, then leave the
bloodstream the same way
they got in. They start new
tumors in new tissues.
Fig. 8.12c, p.135
Key Concepts:
THE CELL CYCLE AND CANCER
 Built-in mechanisms monitor and control the
timing and rate of cell division
 On rare occasions, surveillance mechanisms
fail, and cell division becomes uncontrollable
 Tumor formation and cancer are the outcome
Animation: Cancer and metastasis
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Video: Henrietta's Immortal Cells
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