Chapter 10
Cell Growth and Division
10-1 Cell Growth
• Objectives:
– Explain the problems that growth causes for cells
– Describe how cell division solves the problems of
cell growth
Limits to Cell Growth
• Cells are small for a reason
• As cells grow
1. Greater demand is placed on the DNA
-DNA is responsible for all cell
functions, so a larger cell means
more cellular reactions that need to
take place
2. Moving materials in and out of the cell
becomes more difficult
Ratio of Surface Area to Volume
Cell Size
5 cm
10 cm
Surface
Area
(l×w×6)
Volume
(l×w×h)
SA to
Volume
Ratio
150 cm2
600 cm2
125 cm3
1000 cm3
150:125
= 1.2:1
600:1000
= .6:1
• Volume increases more rapidly than surface area
• Becomes difficult for cells to bring in necessary
nutrients and eliminate waste produced
Division of the Cell
• Process by which a cell divides in two
• Reasons:
–
–
–
–
Growth (multicellular organism)
Repair (multicellular)
Reproduction (unicellular, asexual)
Maintenance
• Before cells can divide, DNA must be copied
– Each daughter cell receives a complete copy of the cell’s
genetic information
• Resulting cells are small enough (increased ratio of
surface area to volume) to allow for efficient materials
exchange with environment
10-2 Cell Division
• Objectives:
– List and explain the main events of the cell cycle
– Describe the 4 stages of mitosis
Cell Division in Prokaryotes vs.
Eukaryotes
• Prokaryotes- copy the single chromosome and
split contents into two cells
– Called binary fission
• Eukaryotes, more complex process
– Mitosis-division of nucleus, followed by
– Cytokinesis- division of cytoplasm
Chromosomes
• Chromosomes are made of DNA that is highly
organized
Chromosome Structure
• You are looking at a chromosome and
its copy
• The result is identical sister chromatids
attached by a centromere
• One chromatid goes to each of the
daughter cells
• A human body cell entering cell
division contains 46 chromosomes,
each of which contains 2 sister
chromatids
chromosome
(condensed form)
How many chromosomes do we have
now?
Fight the urge. This is still considered a
SINGLE chromosome since they are
identical and attached. After replication of
the chromosome, it is still considered one
chromosome!
Diploid Number
• Each organism has a specific number of
chromosomes, for humans it is 46
– 23 pairs of homologous chromosomes
• The total number is referred to as the diploid
number (2n)
Chromosome Numbers
The Cell Cycle
• The Cell Cycle is a series of events that cells go
through as they grow and divide
• It’s a combination of 2 main phases
• Interphase- the period of growth between
divisions
– Includes 3 subphases G1, S, and G2
– (accounts for ~90% of time spent in cell cycle)
• M phase- consisting of mitosis and cytokinesis
Cell Cycle
• Actively dividing cells take
about 24 hours to
complete this cycle
• Cells that don’t divide are
“stuck” in G1
M phase
Interphase
Interphase
• Consists of
– G1 phase (growth, really gap)
– S phase (synthesis of DNA)
– G2 phase (growth, really gap)
G1 Phase
• Cell grows in size
• Protein and organelle production increased
• Gather and synthesize nutrients
– ex. Make the 6 billion nucleotides needed to
replicate the DNA. Acquire/synthesize enough
amino acids to build all the required proteins to
divide the cell, etc…
Cells can hang in this subphase for a very
long time like certain muscle cells or
forever like cardiac (heart) muscle or
neurons, which do not divide at all…
S phase
• Enzymes make an exact copy of
the DNA
– Every chromosome has a copy,
DNA has doubled, but there are
still only 46 chromosomes
– Why?? Remember that sister
chromatids (original DNA and the
copy) only equal one
chromosome
• Centrosomes replicate
• Centrosomes, also called
asters, hold the centriole pair
Centrosome Structure
• Microtubules from the centrioles are what pull sister chromatids apart
during mitosis
G2 phase
• More growth and organelle production
– Shortest of the Interphase subphases
– Once finished, mitosis begins
M phase
Mitosis
• Four Stages:
– Prophase (pro- means first)
– Metaphase (meta- means middle/after)
– Anaphase (ana- means apart)
– Telophase (telo- means far away/end)
Prophase
• Longest phase of mitosis
• Chromosomes become visible
• Centrioles (animal cells only)
migrate to opposite ends of the
cell
• Mitotic spindle starts to form
– Spindle fibers form in foot ball
shape across cell
• Spindle attaches to each
chromosome near the
centromere
• Chromosomes coil more tightly
• Nuclear envelope and nucleolus
disintegrate
Prophase
Chromatin condenses into
chromosomes
Metaphase
• Chromosomes line up
in the middle
(equator) of the cell
• Spindle fibers attach
to centromeres
Metaphase
Anaphase
• Centromere holding sister
chromatids together splits
separating chromosomes
• Spindle fibers shorten,
pulling chromatids apart to
opposite ends of cell
• Animal cells begin to pinch
in
• Plant cells begin to form
cell plate in the middle
Anaphase
Telophase
• Nuclear membrane built
from ER around each
set of chromosomes
• Nucleolus reforms in
each nucleus
• Spindle breaks down
• Chromosomes become
mass of chromatin again
Telophase
Two cells dividing into
four
Cleavage
furrow
forming
(animal cells)
Cytokinesis
• Final division of cytoplasm resulting in two
daughter cells
– Animals – cell membrane pinches together
– Plants – cell plate forms new cell membrane
dividing the daughter cells
• Often happens at the same time as telophase
10-3 Regulating the Cell Cycle
• Objectives:
– Explain how the cell cycle is regulated
– Explain how normal cells and cancerous cells are
different
Regulating the Cell Cycle
The controls on cell growth and division can be turned on
and off.
• For example, broken bone stimulates cells to divide rapidly and
start the healing process.
• Rate of cell division slows when the healing process nears
completion.
Regulating the Cell Cycle
– Internal regulators are proteins that respond to
events inside a cell.
• They allow the cell cycle to proceed only once certain
processes have happened inside the cell.
• Ex: cyclins
– External regulators are proteins that respond
to events outside the cell.
• They direct cells to speed up or slow down the cell
cycle.
• Growth factors are external regulators that stimulate
the growth and division of cells.
– Important during embryonic development and wound
healing.
Internal Regulation
• Cyclins- family of proteins that regulate the cell cycle
in eukaryotic cells
• Discovered in the 1980s
– When injected into a nondividing cell, a mitotic
spindle will form
Internal Regulation
• This graph shows how cyclin levels change
throughout the cell cycle in fertilized clam eggs.
External Regulation
Apoptosis is a process of programmed cell
death.
– Can be caused by growth factor deprivation
– Plays a role in development by shaping the
structure of tissues and organs in plants and
animals.
• Ex: Foot of a mouse is shaped the way it is partly
because the toes undergo apoptosis during tissue
development.
Uncontrolled Cell Growth
• Cancer occurs when cells lose the
ability to control division
• Don’t respond to regulatory signals
and form tumors as a result
– Tumors are masses of cells
• Benign tumors do not spread to healthy
tissue or other parts of the body
• Malignant tumors spread to other parts
of the body (i.e. cancer)
– The spread of cancer cells is called
metastasis. Cancer cells absorb nutrients
needed by other cells, block nerve
connections, and prevent organs from
functioning
Cancer
• Cells placed in a petri dish filled with a
nutrient broth will cover the surface in a thin
layer
• They stop growing when they come in contact
with each other
• Molecules on neighboring cells can have an inhibiting
effect on cell division
• External regulation
- Initially a couple of cells are put into
the flask coated with collagen or some
other extracellular matrix material
- The cells will start dividing
immediately until they cover the entire
plate and then they STOP.
- Initially a couple of cells are put into
the flask
- The cells will start dividing
immediately until they cover the entire
plate and then they STOP.
What do you think would happen if you
were to scrape away some of the cells?
- Initially a couple of cells are put into
the flask
- The cells will start dividing
immediately until they cover the entire
plate and then they STOP.
What do you think would happen if you
were to scrape away some of the cells?
Cell division turns on in the cells next to
the cells that were removed and they fill
in the empty space.
Cancer
• Commonality is control over cell cycle has broken
down
• Often a defect in p53 gene
– p53s job is to halt cell cycle until all chromosomes
have all successfully replicated
– When defected causes cells to lose the information
needed to respond to growth signals.
• Chromosomal damage results in cells that have
lost vital information that regulates cell division
Cancer Causes and Treatment
Causes include:
– tobacco use
– radiation exposure
– viral infection
Treatment:
– Some localized tumors can be removed by surgery.
– Many tumors can be treated with targeted
radiation.
– Chemotherapy is the use of compounds that kill or
slow the growth of cancer cells.
From One Cell to Many
– How do cells become specialized for different
functions?
– During the development of an organism, cells
differentiate into many types of cells.
– All organisms start life as just one cell.
– Most multicellular organisms pass through an
early stage of development called an embryo,
which gradually develops into an adult
organism.
– During development, an organism’s cells
become specialized for particular functions.
– For example, a plant has specialized cells in its
roots, stems, and leaves.
Defining Differentiation
– The process by which cells become
specialized is known as differentiation.
– During development, cells differentiate into
many different types and become specialized
to perform certain tasks.
– Differentiated cells carry out the jobs that
multicellular organisms need to stay alive.
Mapping Differentiation
– In some organisms, a cell’s role is
determined at a specific point in development.
– In the worm C. elegans, daughter cells from
each cell division follow a specific path toward
a role as a particular kind of cell.
Differentiation in Mammals
– Cell differentiation in mammals is controlled
by a number of interacting factors in the
embryo.
– Adult cells generally reach a point at which
their differentiation is complete and they can
no longer become other types of cells.
Stem Cells and Development
– What are stem cells?
Stem Cells and Development
– What are stem cells?
– The unspecialized cells from which
differentiated cells develop are known as stem
cells.
– One of the most important questions in biology is
how all of the specialized, differentiated cell types in
the body are formed from just a single cell.
– Biologists say that such a cell is totipotent,
literally able to do everything, to form all the tissues
of the body.
– Only the fertilized egg and the cells produced by
the first few cell divisions are truly totipotent.
Human Development
– After about four days of development, a
human embryo forms into a blastocyst, a
hollow ball of cells with a cluster of cells inside
known as the inner cell mass.
– The cells of the inner cell mass are said to
be pluripotent, which means that they are
capable of developing into many, but not all,
of the body's cell types.
Stem Cells
– Stem cells are unspecialized cells from
which differentiated cells develop.
– There are two types of stem cells: embryonic
and adult stem cells.
Embryonic Stem Cells
– Embryonic stem cells are found in the inner
cells mass of the early embryo.
– Embryonic stem cells are pluripotent.
– Researchers have grown stem cells isolated
from human embryos in culture. Their
experiments confirmed that embryonic stem
cells have the capacity to produce most cell
types in the human body.
Adult Stem Cells
– Adult organisms contain some types of stem
cells.
– Adult stem cells are multipotent. They can
produce many types of differentiated cells.
– Adult stem cells of a given organ or tissue
typically produce only the types of cells that
are unique to that tissue.
Frontiers in Stem Cell Research
– What are some possible benefits and issues
associated with stem cell research?
Frontiers in Stem Cell Research
– What are some possible benefits and issues
associated with stem cell research?
– Stem cells offer the potential benefit of using
undifferentiated cells to repair or replace badly
damaged cells and tissues.
Frontiers in Stem Cell Research
– What are some possible benefits and issues
associated with stem cell research?
– Stem cells offer the potential benefit of using
undifferentiated cells to repair or replace badly
damaged cells and tissues.
– Human embryonic stem cell research is
controversial because the arguments for it and
against it both involve ethical issues of life and
death.
Potential Benefits
– Stem cell research may lead to new ways to
repair the cellular damage that results from
heart attack, stroke, and spinal cord injuries.
– One example is the approach to reversing
heart attack damage illustrated below.
Ethical Issues
– Most techniques for harvesting, or gathering,
embryonic stem cells cause destruction of the
embryo.
– Government funding of embryonic stem cell
research is an important political issue.
– Groups seeking to protect embryos oppose such
research as unethical.
– Other groups support this research as essential to
saving human lives and so view it as unethical to
restrict the research.