Fig. 12-1
The Cell Cycle
Chapter 12 AP Bio
You should now be able to:
1. Describe the structural organization of the prokaryotic genome and
the eukaryotic genome
2. List the phases of the cell cycle; describe the sequence of events
during each phase
3. List the phases of mitosis and describe the events characteristic of
each phase
4. Draw or describe the mitotic spindle, including centrosomes,
kinetochore microtubules, nonkinetochore microtubules, and asters
5. Compare cytokinesis in animals and plants
6. Describe the process of binary fission in bacteria and explain how
eukaryotic mitosis may have evolved from binary fission
7. Explain how the abnormal cell division of cancerous cells escapes
normal cell cycle controls
8. Distinguish between benign, malignant, and metastatic tumorsCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: The Key Roles of Cell Division
• The ability of organisms to REPRODUCE best
distinguishes living things from nonliving
matter
• The continuity of life is based on the
reproduction of cells, or cell division
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Why do cells divide?
• In unicellular organisms, division of one cell
reproduces the entire organism
• Multicellular organisms depend on cell
division for:
– Development from a fertilized cell
– Growth
– Repair
• Cell division is an integral part of the cell
cycle, the life of a cell from formation to its
own division
▪As the cell grows, its volume increases much more
rapidly than the surface area.
▪ The cell might have difficulty supplying nutrients
and expelling enough waste products.
▪Diffusion over large distances is slow and
inefficient.
▪Substances move by diffusion or by motor
proteins.
▪Small cells maintain more efficient transport
systems.
Cellular Growth
Transport of Substances
Cellular Growth
Cellular Communications
▪ The need for signaling proteins to move
throughout the cell also limits cell size.
▪Cell size affects the ability of the cell to
communicate instructions for cellular functions.
▪Cell division prevents the cell from becoming too
large.
▪ It also is the way the cell reproduces so that you
grow and heal certain injuries.
▪Cells reproduce by a cycle of growing and dividing
called the cell cycle.
The Cell Cycle
Cellular Organization of the Genetic Material
• All the DNA in a cell constitutes the cell’s genome
• A genome can consist of a single DNA molecule
(common in prokaryotic cells) or a number of
DNA molecules (common in eukaryotic cells)
• DNA molecules in a cell are packaged into
chromosomes
Fig. 12-3
20 µm
• Eukaryotic
chromosomes
consist of
chromatin, a
complex of
DNA and
protein that
condenses
during cell
division
• Every eukaryotic species has a
characteristic number of chromosomes in
each cell nucleus – not related to the
complexity of the organism
• Somatic cells (nonreproductive cells) have
two sets of chromosomes - diploid (2N).
The members of the pair are called
homologous chromosomes.
• Gametes (reproductive cells: sperm and
eggs) have half - haploid (1N) as many
chromosomes as somatic cells. They only
have one member of each pair.
23 pairs of homologous chromosomes in humans
karyotype
in somatic cells
• In preparation for cell division, DNA is
replicated and the chromosomes
condense
• Each duplicated chromosome has two
sister chromatids (identical DNA), which
separate during cell division
• The centromere is the narrow “waist” of
the duplicated chromosome, where the
two chromatids are most closely attached
Fig. 12-40.5 µm Chromosomes
Chromosomeduplication(including DNAsynthesis)
Chromo-some arm
Centromere
Sisterchromatids
DNA molecules
Separation ofsister chromatids
Centromere
Sister chromatids
• Most cell division (mitosis) results in
daughter cells with identical genetic
information, DNA, and used for
growth, development, and repair
• A special type of division (meiosis)
produces nonidentical daughter cells
(gametes, or sperm and egg cells)
with half the number of chromosomes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Eukaryotic cell division consists of:
– Mitosis, the division of the nucleus
– Cytokinesis, the division of the cytoplasm
▪ Interphase is the stage during which the cell
grows, carries out cellular functions, and
replicates.
▪ Mitosis is the stage of the cell cycle during which
the cell’s nucleus and nuclear material divide.
▪ Cytokinesis is the method by which a cell’s
cytoplasm divides, creating a new cell.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Stages of Interphase
▪The first stage of interphase, G1
▪The cell is growing, carrying out
normal cell functions, and preparing to
replicate DNA.
Cellular Growth
The Second Stage of Interphase, S
▪The cell copies its DNA in
preparation for cell division.
The Third Stage of Interphase, G2
▪The cell prepares for the division of its nucleus.
• Interphase (about 90%
of the cell cycle) can be
divided into subphases:
– G1 phase (“first
gap”)
– S phase (“synthesis
of DNA”)
– G2 phase (“second
gap”)
• The cell grows during all
three phases, but
chromosomes are
duplicated only during
the S phase.
What is the Go stage?
• Cells that do not normally divide or for various
reasons, are not preparing to divide, enter a state
of arrest.
Ex: - nerve cells, muscle cells, rbc’s
- cells that are starved of nutrients, density-
inhibited, or treated with growth inhibitors
The Stages of Mitosis
• Mitosis is conventionally divided into five
phases:
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
• Cytokinesis (division of the cytoplasm) is
well underway by late telophase.
Fig. 12-UN1
Telophase andCytokinesis
Anaphase
Metaphase
Prometaphase
Prophase
MITOTIC (M) PHASE
Cytokinesis
Mitosis
SG1
G2
Prophase 1st step in Mitosis
• Mitosis begins (cell begins to divide)
• Centrioles (or poles) appear and begin to move to opposite end of the cell.
• Spindle fibers form between the poles.
•Centrioles•Sister
chromatids
•Spindle fibers
The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis
During prophase, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center
The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them
An aster (a radial array of short microtubules) extends from each centrosome
• The spindle includes the centrosomes, the spindle microtubules, and the asters
Prophase•Animal Cell •Plant Cell
•Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
•Spindle fibers
•Centrioles
PROMETAPHASE AND METAPHASE
• During prometaphase, some
spindle microtubules attach to the
kinetochores of chromosomes and
begin to move the chromosomes
• At metaphase, the chromosomes
are all lined up at the metaphase
plate, the midway point between
the spindle’s two polesCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 12-7
Microtubules Chromosomes
Sisterchromatids
Aster
Metaphaseplate
Centrosome
Kineto-chores
Kinetochoremicrotubules
Overlappingnonkinetochoremicrotubules
Centrosome 1 µm
0.5 µm
Metaphase•Animal Cell •Plant Cell
•Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
ANAPHASE
• Sister chromatids separate and
move along the kinetochore
microtubules toward opposite
ends of the cell
• The microtubules shorten by
depolymerizing at their
kinetochore ends
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 12-8a
Kinetochore
Spindlepole
Mark
EXPERIMENT
RESULTS
Notice
the shortening of
the spindle fibers
Anaphase•Animal Cell •Plant Cell
•Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
TELOPHASE
• In telophase, genetically identical daughter
nuclei form at opposite ends of the cell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Telophase
•Animal Cell •Plant Cell
•Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Mitosis
CytokinesisCell plate:
In plant cells, a new cell wall made of cellulose forms between the 2 new nuclei
Cleavage furrow: In animals, a indentation begins from the outside, pinching inwards.
Cleavage furrow
Fig. 12-9a
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
Contractile ring ofmicrofilaments
Fig. 12-9b
Daughter cells
(b) Cell plate formation in a plant cell (TEM)
Vesiclesformingcell plate
Wall ofparent cell
New cell wallCell plate
1 µm
Fig. 12-UN2
•1 2 34 5 6 7
8
9
10
1112
1314
15
16
Fig. 12-UN5
Interphase
The mitotic phase alternates with interphase in the cell cycle
• In 1882, the German anatomist Walther
Flemming developed dyes to observe
chromosomes during mitosis and cytokinesis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
What is the purpose of mitosis?
• To produce two genetically
identical cells – ie, have the same
number of chromosomes as the
original cell.
• Occurs in eukaryotic multicellular
somatic cells for growth,
development, and repair.
• In unicellular organisms, it is a
method of asexual reproduction.
Binary Fission
• Prokaryotes (bacteria and archaea)
reproduce by a type of cell division
called binary fission
• In binary fission, the chromosome
replicates (beginning at the origin of
replication), and the two daughter
chromosomes actively move apart
• No mitosis is involved
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 12-11-4
Origin ofreplication
Two copiesof origin
E. coli cellBacterialchromosome
Plasmamembrane
Cell wall
Origin Origin
The Evolution of Mitosis
• Since prokaryotes evolved
before eukaryotes, mitosis
probably evolved from binary
fission
• Certain protists exhibit types of
cell division that seem
intermediate between binary
fission and mitosisCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The eukaryotic cell cycle is regulated by a molecular control system
• The frequency of cell division varies with the
type of cell
• These cell cycle differences result from
regulation at the molecular level
• The cell cycle appears to be driven by specific
chemical signals present in the cytoplasm
• Some evidence for this hypothesis comes from
experiments in which cultured mammalian
cells at different phases of the cell cycle were
fused to form a single cell with two nuclei
The Cell Cycle Control System
• The sequential events of the cell cycle are
directed by a distinct cell cycle control
system, which is similar to a clock
• The cell cycle control system is regulated by
both internal and external controls
• The clock has specific checkpoints where
the cell cycle stops until a go-ahead signal is
received
• For many cells, the G1 checkpoint seems
to be the most important one
• If a cell receives a go-ahead signal at the G1
checkpoint, it will usually complete the S, G2, and
M phases and divide
• If the cell does not receive the go-ahead signal, it
will exit the cycle, switching into a nondividing
state called the G0 phase
The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
• Two types of regulatory proteins are
involved in cell cycle control: cyclins and
cyclin-dependent kinases (Cdks)
• Cyclin + Cdk = MPF
• The activity of cyclins and Cdks fluctuates
during the cell cycle
• MPF (maturation-promoting factor) pushes
the cell past the G2 checkpoint into the M
phaseCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Stop and Go Signs: Internal and External Signals at the Checkpoints
• An example of an internal signal is that
kinetochores not attached to spindle microtubules
send a molecular signal that delays anaphase until
they all have attached; making sure that the cell
doesn’t divide until it has all its parts
• Some external signals are growth factors,
proteins released by certain cells that stimulate
other cells to divide
• For example, platelet-derived growth factor
(PDGF) stimulates the division of human fibroblast
cells in cultureCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Internal Signals
• An example of an internal signal is that
kinetochores not attached to spindle
microtubules send a molecular signal that
delays anaphase
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
External Signals:
• Growth Factors – example platelet
derived
growth factor PDGF
• Density-dependent inhibition, in which
crowded cells stop dividing
• Most animal cells also exhibit anchorage
dependence, in which they must be
attached to a substratum in order to
divideCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Loss of Cell Cycle Controls in Cancer Cells
Cancer cells exhibit neither
density-dependent inhibition nor
anchorage dependence
Cancer cells do not respond
normally to the body’s control
mechanisms
Cancer cells may not need
growth factors to grow and
divide:
They may make their own
growth factor
They may convey a growth
factor’s signal without the
presence of the growth
factor
They may have an abnormal
cell cycle control system
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• A normal cell is converted to a cancerous
cell by a process called transformation
• Cancer cells form tumors, masses of
abnormal cells within otherwise normal
tissue
• If abnormal cells remain at the original site,
the lump is called a benign tumor
• Malignant tumors invade surrounding
tissues and can metastasize, exporting
cancer cells to other parts of the body,
where they may form secondary tumorsCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
p53 genes and p27 genes in cancer
• p53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe this protein can cause apoptosis (cell death).
• p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle.
• A p53 mutation is the most frequent mutation leading to cancer.
• p27 is a protein that binds to cyclin and cdkblocking entry into S phase. Recent research suggests that breast cancer prognosis is determined by p27 levels. Reduced levels of p27predict a poor outcome for breast cancer patients.
Fig. 12-20
Tumor
A tumor grows
from a single
cancer cell.
Glandulartissue
Lymphvessel
Bloodvessel
Metastatictumor
Cancercell
Cancer cells
invade neigh-
boring tissue.
Cancer cells spreadto other parts ofthe body.
Cancer cells maysurvive and
establish a new
tumor in another
part of the body.
1 2 3 4
Unfortunately, the majority of drugs currently
on the market are not specific, which leads to
the many common side effects associated with
cancer chemotherapy.
Since the drugs are not specific to recognize
normal cells from cancerous cells, the side
effects are seen in bodily systems that
naturally have a rapid turnover of cells
including skin, hair, gastrointestinal, and bone
marrow. These healthy, normal cells, also end
up damaged by the chemotherapy program.
Case Study: The Immortal Cells of Henrietta Lacks
•http://www.cbsnews.com/8301-
3445_162-6300824/the-immortal-
henrietta-lacks/
• Part I: The HeLa Cells
• Part II: The Family
• Part III: Henrietta’s Cancer Cells
• Part IV: The Continuing Story:
Henrietta’s Genome
•http://www.cbsnews.com/8301-3445_162-
6300824/the-immortal-henrietta-lacks/
•http://www.foxnews.com/opinion/2013/08/26/scientific-
breakthroughs-vs-your-privacy-lessons-from-henrietta-
lacks-saga/
Recommended