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Chapter 11
How Cells
Reproduce
© Cengage Learning 2016
11.1 Henrietta’s Immortal Cells
• Henrietta Lacks died of cancer in 1951, at
age 31, but her cells (HeLa cells) are still
growing in laboratories
– HeLa cells are widely used to investigate
cancer, viral growth, and other processes
important in medicine and research
• Understanding the structures and
mechanisms that cells use to divide help us
understand why cancer cells are immortal
and we are not
© Cengage Learning 2016
HeLa Cells
© Cengage Learning 2016
11.2 Multiplication by Division
• Division of a eukaryotic cell typically occurs
in two steps: nuclear division followed by
cytoplasmic division
• The sequence of stages through which a
cell passes during its lifetime is called the
cell cycle
– A sequence of three stages (interphase,
mitosis, and cytoplasmic division) through which
a cell passes between one cell division and the
next
© Cengage Learning 2016
The Eukaryotic Cell Cycle
G1
cytoplasmic division
telophase
anaphase
metaphase
prophase
mitosis
S
G2
A cell spends most of its life in interphase, which includes three stages: G1, S, and G2.
G1 is the phase of growth before DNA replication. The cell’s chromosomes are unduplicated.
S is the phase of synthesis, during which the cell makes copies of its chromosome(s) by DNA replication.
G2 is the phase after DNA replication and before mitosis. The cell prepares to divide during this stage.
The nucleus divides during mitosis, the four stages of which are detailed in the next section.
After mitosis, the cytoplasm may divide. Each descendant cell begins the cycle anew, in interphase.
Built-in checkpoints stop the cycle from proceeding until certain conditions are met.
© Cengage Learning 2016
Interphase
• Interphase consists of three stages, during
which a cell increases in size, doubles the
number of cytoplasmic components, and
replicates its DNA
– G1: Interval of cell growth and activity
– S: Interval of DNA replication (synthesis)
– G2: Interval when the cell prepares for division
© Cengage Learning 2016
Interphase and the Life of a Cell
• Most cell activities take place during G1
• Control mechanisms work at certain points
in the cell cycle
– Most cell cycle checkpoints are in the stages of
interphase
– Some can keep cells in G1
• Loss of control may cause cell death or
cancer
© Cengage Learning 2016
Mitosis and Asexual Reproduction
• Mitosis is a nuclear division mechanism that
maintains the chromosome number
• In multicelled species, mitosis and
cytoplasmic division increase cell number
during development, and replace damaged
or dead cells later in life
• In asexual reproduction, a single individual
can reproduce by mitosis and cytoplasmic
division
© Cengage Learning 2016
Homologous Chromosomes
• Pairs of chromosomes having the same
length, shape, and genes
• Typically, one member of a pair is inherited
from the mother, the other from the father
• Human cells have 46 chromosomes (23
pairs)
– Except for a pairing of sex chromosomes (XY)
in males, the chromosomes of each pair are
homologous
© Cengage Learning 2016
A An unduplicated pair of
chromosomes in a cell in G1.
B By G2, each chromosome
has been duplicated.
C Mitosis and cytoplasmic division package
one copy of each
chromosome into each
of two new cells.
Stepped Art
© Cengage Learning 2016
How Mitosis Maintains the
Chromosome Number
2 Pair of homologous chromosomes in a cell
during G1. Both are unduplicated.
4 By G2, each chromosome has been
duplicated.
6 Mitosis and cytoplasmic division package
one copy of each chromosome into each of
two new cells.
© Cengage Learning 2016
Chromosomes During Division
• A cell cannot function without a full
complement of DNA
– Each cell needs to have one copy of each
chromosome
• The cell replicates its DNA in preparation for
mitosis
© Cengage Learning 2016
Stages of Mitosis
• Four stages of mitosis parcel sister
chromatids into separate nuclei
– Prophase
– Metaphase
– Anaphase
– Telophase
• Cytoplasmic division results in two diploid
cells, each with the same number and kind
of chromosomes as the parent
© Cengage Learning 2016
Controls Over the Cell Cycle
• Most differentiated human cells are in G1 –
some types never progress past that stage
• When a cell divides is determined by gene
expression controls
– Some cause the cell cycle to advance
– Others stop the cycle from proceeding
• These built-in checkpoints allow problems to
be corrected before the cycle proceeds
© Cengage Learning 2016
11.3 A Closer Look at Mitosis
• When a nucleus divides by mitosis, each
new nucleus has the same chromosome
number as the parent cell
• What are the four stages of mitosis?
© Cengage Learning 2016
Preparing for Mitosis
• During interphase, a cell’s chromosomes
are loosened to allow transcription and DNA
replication
• In preparation for nuclear division,
chromosomes condense into their most
compact “X” forms
• Tight condensation keeps the chromosomes
from getting tangled and breaking during
nuclear division
© Cengage Learning 2016
Interphase and Early Prophase
Plant cell nucleus
Animal cell nucleus
4 Metaphase
All of the chromosomes are aligned
midway between the spindle poles.
5 Anaphase
Spindle microtubules separate the sister
chromatids and move them toward
opposite spindle poles. Each sister
chromatid has now become an
individual, unduplicated chromosome.
© Cengage Learning 2016
Prophase
• What happens during 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
© Cengage Learning 2016
Mitosis – Prophase
Plant cell nucleus
Animal cell nucleus
6 Telophase
The chromosomes reach opposite
sides of the cell and loosen up.
Mitosis ends when a new nuclear
envelope forms around each cluster of
chromosomes.
© Cengage Learning 2016
The 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
© Cengage Learning 2016
The Spindle – Illustrated
spindle
pole
© Cengage Learning 2016
© Conly L Rieder
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
© Cengage Learning 2016
Mitosis – Metaphase and Anaphase
Plant cell nucleus
Animal cell nucleus
4 Metaphase
All of the chromosomes are aligned
midway between the spindle poles.
5 Anaphase
Spindle microtubules separate the sister
chromatids and move them toward
opposite spindle poles. Each sister
chromatid has now become an
individual, unduplicated chromosome.
© Cengage Learning 2016
left, Michael Clayton/University of Wisconsin, Department of Botany;
right, ISM/Phototake; far right, © Cengage Learning.
Telophase
• During 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
© Cengage Learning 2016
Mitosis – Telophase
Plant cell nucleus
Animal cell nucleus
6 Telophase
The chromosomes reach opposite
sides of the cell and loosen up.
Mitosis ends when a new nuclear
envelope forms around each cluster of
chromosomes.
© Cengage Learning 2016
left, Michael Clayton/University of Wisconsin, Department of Botany;
right, ISM/Phototake; far right, © Cengage Learning.
11.4 Cytokinesis: Division of
Cytoplasm
• In most kinds of eukaryotes, the cell
cytoplasm divides between late anaphase
and the end of telophase, but the
mechanism of division differs between
plants and animals
• Cytokinesis
– The process of cytoplasmic division
© Cengage Learning 2016
Cytokinesis in Animal and Plant Cells
• Animal cells
• A cleavage furrow 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
• Partitions the cytoplasm when it reaches and
connects to the parent cell wall
© Cengage Learning 2016
Cytoplasmic Division in Animal Cells
Animal cell cytokinesis
1 In a dividing animal cell, the
spindle disassembles as mitosis
ends.
2 At the midpoint of the former
spindle, a ring of actin and myosin
filaments attached to the plasma
membrane contracts.
3 This contractile ring pulls the
cell surface inward, forming a
cleavage furrow as it shrinks.
4 The ring contracts until it
pinches the cell in two.
© Cengage Learning 2016
Cytoplasmic Division in Plant Cells
Plant cell cytokinesis
5 In a dividing plant cell, vesicles
cluster at the future plane of division
before mitosis ends.
6 The vesicles fuse with each
other, forming a cell plate along
the plane of division.
7 The cell plate expands out- ward
along the plane of division. When it
reaches and attaches to the plasma
membrane, it parti- tions the
cytoplasm.
8 The cell plate matures as two
new cell walls. These walls join
with the parent cell wall, so each
descendant cell becomes
enclosed by its own wall.
© Cengage Learning 2016
11.5 Marking Time With Telomeres
• The ends of eukaryotic DNA strands consist
of noncoding sequences called telomeres
– Telomeres protect eukaryotic chromosomes
from losing genetic information at their ends
– Vertebrate telomeres have a short DNA
sequence, 5′-TTAGGG-3′, repeated thousands
of times
• Shortening telomeres are associated with
aging
© Cengage Learning 2016
Telomeres
• Telomeres are important because a
polymerase can’t copy the last hundred or
so bases of the 3′ end of a chromosome
• When telomeres get too short, checkpoint
gene products stop the cell cycle, and the
cell dies
© Cengage Learning 2016
Stem Cells and Telomerase
• In an adult, only stem cells retain the ability
to divide indefinitely
• What are stem cells immortal?
– They retain the ability to make telomerase, a
molecule that reverses the telomere shortening
that normally occurs after DNA replication
• Research suggests that the built-in limit on
cell divisions may be part of the mechanism
that sets an organism’s life span
© Cengage Learning 2016
Telomeres – Illustrated
© Cengage Learning 2016
11.6 When Mitosis Becomes
Pathological
• When enough checkpoint mechanisms fail,
a cell loses control over its cell cycle forms a
neoplasm – a group of cells that lost control
over how they grow and divide
• A neoplasm that forms a lump (abnormal
mass) in the body is called a tumor
© Cengage Learning 2016
Oncogenes and Proto-Oncogenes
• An oncogene is any gene that helps
transform a normal cell into a tumor cell
– Genes encoding proteins that promote mitosis
are called proto-oncogenes – mutations can
turn them into oncogenes
• Growth factors are molecules that stimulate
a cell to divide and differentiate
– A gene that encodes the epidermal growth
factor (EGF) receptor is an example of a protooncogene
© Cengage Learning 2016
Oncogenes and Overactive EGF Receptors
© Cengage Learning 2016
Tumor Suppressors
• Checkpoint gene products that inhibit
mitosis are called tumor suppressors
because tumors form when they are missing
– The products of the BRCA1 and BRCA2 genes
are examples of tumor suppressors – they
regulate the expression of DNA repair enzymes
• Viruses such as HPV (human
papillomavirus) cause cells to make proteins
that interfere with their own tumor
suppressors
© Cengage Learning 2016
Checkpoint Genes in Action
A
© Cengage Learning 2016
B
© Phillip B. Carpenter, Department of Biochemistry and Molecular Biology,
University of Texas - Houston Medical School.
Cancer
• Benign neoplasms (such as ordinary skin
moles) grow slowly, stay in one place, and
are not cancerous
• Malignant neoplasms (cancers) disrupt body
tissues, both physically and metabolically
– Get progressively worse
– Dangerous to health
• Cancer causes 15-20% of all human deaths
in developed countries
© Cengage Learning 2016
Three Characteristics of Cancer Cells
• Cancer cells grow and divide abnormally
– Capillary blood supply to the cells may increase
abnormally
• Cytoplasm and plasma membrane are
altered
– Malignant cells typically have an abnormal
chromosome number
– Metabolism may shift toward fermentation
– Altered or missing proteins impair the function
of the plasma membrane
© Cengage Learning 2016
Three Characteristics of Cancer Cells
(cont’d.)
• Cancer cells break loose and invade other
parts of the body (metastasis)
– Due to altered recognition proteins and
weakened adhesion, they do not stay anchored
properly in tissues
– Can slip easily into and out of circulatory and
lymphatic systems vessels
• Use these vessels to migrate elsewhere in the body
© Cengage Learning 2016
Neoplasms and Malignancy
Benign neoplasms
grow slowly and stay in
their home tissue.
Cells of a malignant
neoplasm can break away
from their home tissue.
The malignant cells become
attached to the wall of a lymph
vessel or blood vessel (as shown
here). They release digestive
enzymes that create an opening in
the wall, then enter the vessel.
The cells creep or tumble along
inside vessels, then exit the same
way they got in. Migrating cells may
start growing in other tissues, a
process called metastasis.
© Cengage Learning 2016
Reducing the Risk of Cancer
• Life style choices can reduce the risk of
acquiring mutations that lead to cancer
– Not smoking, avoiding exposure of unprotected
skin to sunlight, etc.
• Some neoplasms can be detected with
periodic screening procedures such as
gynecology or dermatology exams
© Cengage Learning 2016
Skin Cancers
A Basal cell carcinoma is the most
common type of skin cancer. This slowgrowing, raised lump may be
uncolored, reddish-brown, or black.
B Squamous cell carcinoma is the second
most common form of skin cancer. This
pink growth, firm to the touch, grows
under the skin’s surface.
C Melanoma spreads fastest. Cells form
dark, encrusted lumps that may itch or
bleed easily.
© Cengage Learning 2016
(A) Dr. Allan Harris/Phototake; (B) Biophoto Associates/Science Source; (C) James Stevenson/Science Source.
Points to Ponder
• Exactly where and when does mitosis
occurs in any organism?
• What would happen if cytokinesis did not
occur in plants and animals?
• Many of the drugs used in chemotherapy
cause loss of hair in the individual being
treated. Why is this?
© Cengage Learning 2016