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How Cells Reproduce
Chapter 9
Impacts, Issues
Henrietta’s Immortal Cells
 Henrietta Lacks died of cancer at age 31, but her
cells (HeLa cells) are still growing in laboratories
9.1 Overview of
Cell Division Mechanisms
 Individual cells or organisms produce offspring
by the process of reproduction
 When a cell reproduces, each descendent
receives information coded in DNA, and enough
cytoplasm to begin operating
Mitosis, Meiosis, and the Prokaryotes
 Eukaryotic cells
• Mitosis copies DNA and divides a nucleus,
producing two identical nuclei
• Meiosis is a nuclear division that produces
haploid gametes for sexual reproduction
 Prokaryotic cells reproduce asexually by
prokaryotic fission
Comparison of Cell Division Mechanisms
Key Points About Chromosome Structure
 Each species has a characteristic number of
chromosomes that differ in length and shape
• Each consists of one double strand of DNA
• After duplication, each consists of two double
strands (sister chromatids) that remain attached
to each other at a centromere until late in
nuclear division
A Chromosome and Sister Chromatids
one chromosome
(unduplicated)
one chromatid
its sister chromatid
one chromosome
(duplicated)
Fig. 9-2, p. 142
Key Points About Chromosome Structure
 A chromosome consists of DNA that is wrapped
around proteins (histones) and condensed
 Each histone and the DNA wrapped around it
make up a nucleosome, the smallest unit of
structural organization in chromosomes
Chromosome Structure
Fig. 9-3a, p. 143
centromere
A Duplicated human
chromosome in its most
condensed form. If this
chromosome were actually
the size shown in the
micrograph, its two DNA
strands would stretch out
about 800 meters (0.5 miles).
Fig. 9-3a, p. 143
Fig. 9-3 (b-e), p. 143
B When a chromosome
is at its most condensed,
the DNA is packed into
tightly coiled coils.
multiple levels of coiling
of DNA and proteins
fiber
beads on
a string
DNA
double
helix
C When the coiled coils
unwind, a molecule of
chromosomal DNA and
its associated proteins
are organized as a
cylindrical fiber.
D A loosened fiber
shows a “beads-on-astring” organization.
The “string” is the DNA
molecule; each “bead”
is one nucleosome.
core of
histones
nucleosome
E A nucleosome
consists of part of a
DNA molecule looped
twice around a core of
histone proteins.
Fig. 9-3 (b-e), p. 143
B When a chromosome
is at its most condensed,
the DNA is packed into
tightly coiled coils.
centromere
A Duplicated human
chromosome in its most
condensed form. If this
chromosome were
actually the size shown in
the micrograph, its two
DNA strands would
stretch out about 800
meters (0.5 miles).
multiple levels of
coiling of DNA and
proteins
fiber
beads on
a string
DNA
double
helix
core of
histones
nucleosome
C When the coiled coils
unwind, a molecule of
chromosomal DNA and
its associated proteins
are organized as a
cylindrical fiber.
D A loosened fiber
shows a “beads-on-astring” organization.
The “string” is the DNA
molecule; each “bead”
is one nucleosome.
E A nucleosome
consists of part of a
DNA molecule looped
twice around a core of
histone proteins.
Stepped Art
Fig. 9-3 (b-e), p. 143
Animation: Chromosome structural
organization
G1
S
Interval of cell growth before
DNA replication (chromosomes
unduplicated)
Interval of cell growth when
the DNA is replicated (all
chromosomes duplicated)
Telophase
Anaphase
cytoplasmic division;
each descendant cell
enters interphase
Metaphase
Prophase
G2
Interphase
ends for
parent cell
Interval after DNA
replication; the cell
prepares to divide
Stepped Art
Fig. 9-4, p. 144
Animation: The cell cycle
9.1 Key Concepts:
Chromosomes and Dividing Cells
 Individuals have a characteristic number of
chromosomes in each of their cells
 The chromosomes differ in length and shape,
and they carry different portions of the cell’s
hereditary information
 Division mechanisms parcel out the information
into descendent cells
9.2 Introducing the Cell Cycle
 Cell cycle
• A sequence of three stages (interphase, mitosis,
and cytoplasmic division) through which a cell
passes between one cell division and the next
Interphase
 Interphase consists of three stages, during
which a cell increases in size, doubles the
number of cytoplasmic components, and
duplicates its DNA
• G1: Interval of cell growth and activity
• S: Interval of DNA replication (synthesis)
• G2: Interval when the cell prepares for division
Interphase and the Life of a Cell
 Most cell activities take place during G1
 Control mechanisms work at certain points in the
cell cycle; some can keep cells in G1
 Loss of control may cause cell death or cancer
Mitosis and the Chromosome Number
 Mitosis produces two diploid nuclei with the same
number and kind of chromosomes as the parent
 Chromosome number
• The sum of all chromosomes in a type of cell
• Human cells have 46 chromosomes paired in 23
sets (diploid number)
• Pairs have the same shape and information about
the same traits (except sex chromosomes XY)
Mitosis and the Chromosome Number
 Bipolar spindle
• A dynamic network of microtubules that forms
during nuclear division
• Grows into the cytoplasm from opposite poles of
the cell and attaches to duplicated chromosomes
• Microtubules from opposite poles attach to
different sister chromatids and separate them
Bipolar Spindle
Separates Sister Chromatids
Mitosis Maintains Chromosome Number
Fig. 9-5a, p. 145
Fig. 9-5b, p. 145
mitosis,
cytoplasmic
division
An unduplicated chromosome
in a cell in G1 of
interphase.
The same
chromosome,
duplicated in S.
The cell is now in
G2 of interphase.
After mitosis
and cytoplasmic
division, the two
new cells each have
one (unduplicated)
chromosome. Both
new cells start life
in G1 of interphase.
Fig. 9-5b, p. 145
9.2 Key Concepts:
Where Mitosis Fits in the Cell Cycle
 A cell cycle starts when a new cell forms by
division of a parent cell, and ends when the cell
completes its own division
 A typical cell proceeds through intervals of
interphase, mitosis, and cytoplasmic division
9.3 A Closer Look at Mitosis
 When a nucleus divides by mitosis, each new
nucleus has the same chromosome number as
the parent cell
 There are four main stages of mitosis: prophase,
metaphase, anaphase, and telophase
Prophase
 Prophase
•
•
•
•
Chromosomes condense
Microtubules form a bipolar spindle
Nuclear envelope breaks up
Microtubules attach to the chromosomes
 Centrosome
• A region near the nucleus that organizes spindle
microtubules; usually includes two centrioles
Metaphase and Anaphase
 Metaphase
• All duplicated chromosomes line up midway
between the spindle poles
 Anaphase
• Microtubules separate the sister chromatids of
each chromosome and pull them to opposite
spindle poles
Telophase
 Telophase
• Two clusters of chromosomes reach the spindle
poles
• A new nuclear envelope forms around each
cluster
 Two new nuclei are formed, each with the same
chromosome number as the parent cell
Mitosis
Fig. 9-6 (1a), p. 146
Fig. 9-6 (1b), p. 146
Fig. 9-6 (2), p. 147
Fig. 9-6 (2a), p. 147
A Early Prophase
Mitosis begins. In the nucleus,
the chromatin begins to
appear grainy as it organizes
and condenses. The
centrosome is duplicated.
Fig. 9-6 (2a), p. 147
Fig. 9-6 (2b), p. 147
B Prophase
The chromosomes become
visible as discrete structures
as they condense further.
Microtubules assemble and
move one of the two
centrosomes to the opposite
side of the nucleus, and the
nuclear envelope breaks up.
Fig. 9-6 (2b), p. 147
Fig. 9-6 (2c), p. 147
C Transition to Metaphase
The nuclear envelope is gone,
and the chromosomes are at
their most condensed.
Microtubules of the bipolar
spindle assemble and attach
sister chromatids to opposite
spindle poles.
Fig. 9-6 (2c), p. 147
Fig. 9-6 (2d), p. 147
D Metaphase
All of the chromosomes are
aligned midway between the
spindle poles. Microtubules
attach each chromatid to one
of the spindle poles, and its
sister to the opposite pole.
Fig. 9-6 (2d), p. 147
Fig. 9-6 (2e), p. 147
E Anaphase
Motor proteins moving along
spindle microtubules drag
the chromatids toward the
spindle poles, and the sister
chromatids separate. Each
sister chromatid is now a
separate chromosome.
Fig. 9-6 (2e), p. 147
Fig. 9-6 (2f), p. 147
F Telophase
The chromosomes reach
the spindle poles and
decondense. A nuclear
envelope begins to form
around each cluster; new
plasma membrane may
assemble between them.
Mitosis is over.
Fig. 9-6 (2f), p. 147
Animation: Mitosis-step-by-step
9.3 Key Concepts:
Stages of Mitosis
 Mitosis divides the nucleus, not the cytoplasm
 Mitosis has four sequential stages: prophase,
metaphase, anaphase, and telophase
 A bipolar spindle forms; it moves the cell’s
duplicated chromosomes into two parcels, which
end up in two genetically identical nuclei
9.4 Cytoplasmic Division Mechanisms
 In most kinds of eukaryotes, the cell cytoplasm
divides between late anaphase and the end of
telophase, but the mechanism of division differs
 Cytokinesis
• The process of cytoplasmic division
Cytoplasmic Division
in Animal and Plant Cells
 Animal cells
• A contractile ring partitions the cytoplasm
• A band of actin filaments rings the cell midsection,
contracts, and pinches the cytoplasm in two
 Plant cells
• A cell plate forms midway between the spindle
poles; it partitions the cytoplasm when it reaches
and connects to the parent cell wall
Cytoplasmic Division
in Animal and Plant Cells
Fig. 9-7a, p. 148
Fig. 9-7a (1), p. 148
1 Mitosis is completed, and the
bipolar spindle is starting to
disassemble.
Fig. 9-7a (1), p. 148
Fig. 9-7a (2), p. 148
2 At the former spindle equator, a
ring of actin filaments attached to
the plasma membrane contracts.
Fig. 9-7a (2), p. 148
Fig. 9-7a (3), p. 148
This contractile ring pulls
the cell surface inward as it
continues to contract.
3
Fig. 9-7a (3), p. 148
Fig. 9-7a (4), p. 148
4 The contractile ring contracts
until the cytoplasm is partitioned
and the cell pinches in two.
Fig. 9-7a (4), p. 148
Fig. 9-7b, p. 148
Fig. 9-7b (1), p. 148
1 The plane of division
(and of the future crosswall) was established by
microtubules and actin
filaments that formed and
broke up before mitosis
began. Vesicles cluster
here when mitosis ends.
Fig. 9-7b (1), p. 148
Fig. 9-7b (2), p. 148
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. 9-7b (2), p. 148
Fig. 9-7b (3), p. 148
3 The cell plate expands
outward along the plane of
division until it reaches the
plasma membrane. When
the cell plate attaches to
the plasma membrane, it
partitions the cytoplasm.
Fig. 9-7b (3), p. 148
Fig. 9-7b (4), p. 148
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. 9-7b (4), p. 148
A Contractile Ring Formation
1 Mitosis is completed,
and the bipolar spindle
is starting to
disassemble.
2 At the former spindle
equator, a ring of actin
filaments attached to the
plasma membrane
contracts.
3 This contractile ring pulls
the cell surface inward as it
continues to contract.
4 The contractile ring
contracts until the
cytoplasm is partitioned and
the cell pinches in two.
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.
3 The cell plate expands
outward along the plane of
division until it reaches the
plasma membrane. When
the cell plate attaches to
the plasma membrane, it
partitions the cytoplasm.
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.
B Cell Plate Formation
1 The plane of division
(and of the future
cross-wall) was
established by
microtubules and actin
filaments that formed
and broke up before
mitosis began. Vesicles
cluster here when
mitosis ends.
Stepped Art
Fig. 9-7, p. 148
Animation: Cytoplasmic division
The Importance of Timing
and Completion of Cell Cycle Events
9.4 Key Concepts:
How the Cytoplasm Divides
 After nuclear division, the cytoplasm divides
 One nucleus ends up in each of two new cells
 In animal cells, the cytoplasm pinches in two
 In plant cells, a cross-wall forms in the
cytoplasm and divides it
9.5 When Control is Lost
 Sometimes, controls over cell division are lost
• Cancer may be the outcome
Cell Cycle Controls
 Checkpoints in the cell cycle allow problems to
be corrected before the cycle advances
 Proteins produced by checkpoint genes interact
to advance, delay, or stop the cell cycle
• Kinases can activate other molecules to stop the
cell cycle or cause cells to die
• Growth factors can activate kinases to start
mitosis
Protein Products of
Checkpoint Genes in Action
Checkpoint Failure and Tumors
 When all checkpoint mechanisms fail, a cell
loses control over its cell cycle and may form a
tumor (abnormal mass) in surrounding tissue
 Usually one or more checkpoint gene products
are missing in tumor cells
• Tumor suppressor gene products inhibit mitosis
• Protooncogene products stimulate mitosis
Neoplasms
 Neoplasms
• Abnormal masses of cells that lack control over
how they grow and divide
• Benign neoplasms (such as ordinary skin moles)
stay in one place and are not cancerous
• Malignant neoplasms are cancerous
Characteristics of Cancer Cells
 Cancers (malignant neoplasms)
• Cells grow and divide abnormally; capillary blood
supply to the cells may increase abnormally
• Cells may have altered plasma membrane and
cytoplasm; metabolism may shift toward
fermentation
• Cells have altered recognition proteins and
weakened adhesion; may break away and invade
distant tissues (metastasis)
Benign and
Malignant Tumors
benign tumor
malignant tumor
A Cancer cells
break away from
their home tissue.
B 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 resulting breach.
C 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. 9-11, p. 151
Animation: Cancer and metastasis
Skin Cancers
Fig. 9-12a, p. 151
Fig. 9-12b, p. 151
Fig. 9-12c, p. 151
9.5 Key Concepts:
The Cell Cycle and Cancer
 Built-in mechanisms monitor and control the
timing and rate of cell division
 On rare occasions, the surveillance mechanisms
fail, and cell division become uncontrollable
 Tumor formation and cancer are the outcome
Animation: Mechanisms for chromosome
movement
Animation: Mitosis
ABC video: Blood test for lung cancer
ABC video: Genetically modified cancer
fighters
Video: Henrietta's immortal cells