Transcript Ch 10 Notes - Mitosis.ppt

Chapter 10 Cell Growth and Division
• This liver cell has almost completed the process of cell
division. During cell division, a cell splits into two roughly
equal daughter cells (magnification: 11,500×).
10-1 Cell Growth, Division,
and Reproduction
Limits to Cell Size
What are some of the difficulties a cell
faces as it increases in size?
•The larger a cell becomes, the more
demands the cell places on its DNA and
the more trouble the cell has moving
enough nutrients and wastes across the
cell membrane.
Limits to Cell Size
• DNA Overload
– As a cell grows in size, its DNA does not
– “information crisis”
• Exchanging Materials
– getting food into and wastes out of the
cell
• Ratio of Surface Area to Volume
– volume increases more rapidly than
surface area
Division of the Cell
• Before a cell becomes too large, it
divides into two new “daughter cells”
• This process is called “cell division”
• Cell division solves all 3 problems
Solutions:
• DNA Overload
– Before cell division occurs, the cell
replicates (copies) all of its DNA so that
each daughter cell will get a copy of
genetic information
• Exchanging Materials
– Reduces cell volume
• Ratio of Surface Area to Volume
– Increases Surface Area to Volume Ratio
Cell Division and Reproduction
How do asexual and sexual
reproduction compare?
• Reproduction (the formation of new
individuals) is one of the most
important characteristics of living
things.
• Asexual Reproduction
– Offspring are produced from a single
parent cell
– Simple, efficient, effective
– Enables populations to increase in number
very quickly
– The two daughter cells are genetically
identical to the parent cell (in most cases)
– Exs. – Bacteria
– Single-celled organisms
• Sexual Reproduction
– Offspring are produced by inheriting some
of their genetic information from each
parent cell
– Involves the fusion of genetic information
from two separate parent cells
– Allows for genetic diversity in populations
– The daughter cells are genetically different
from the parent cell
– Exs. – Most animals and plants
Comparing Methods of Reproduction
Asexual
• Faster Reproduction
– when conditions are right
• Lack of Genetic Diversity
Sexual
• Slower Reproduction
• More Genetic Diversity
– Able to survive changes in
environmental conditions
Comparing Asexual &
Sexual Reproduction
Asexual
Sexual
Reproduction
Reproduction
Parent Cells
Offspring
10-2 The Process of
Cell Division
Prokaryotic Chromosomes
• Prokaryotes do not have a nucleus
• Their DNA is found in the cytoplasm
• Most Prokaryotes contain a single,
circular DNA chromosome
Eukaryotic Chromosomes
• DNA is contained in the nucleus
• Chromosomes are made up of DNA
and proteins.
• Chromosomes are not visible except
during division.
• Before division, each chromosome is
replicated (copied).
• Chromosomes become visible at the
beginning of cell division.
• Each chromosome consists of two
identical “sister” chromatids.
• Each pair of chromatids is attached at an
area called the centromere. Centromeres
are usually located near the middle of the
chromatids, some lie near the ends.
A Human Chromosome
• This is a human chromosome shown as it appears through
an electron microscope. Each chromosome has two sister
chromatids attached at the centromere.
The Prokaryotic Cell Cycle
• Takes place very rapidly under ideal
conditions
• DNA is replicated when bacteria
reach a certain size
• Cell Division begins when replication
is complete
• The 2 DNA molecules
attach to different regions
of the cell membrane
• The cell is pinched inward,
dividing the cytoplasm and
chromosomes between the
two new cells
• This results in a form of
asexual reproduction
called binary fission
The Eukaryotic Cell Cycle
• The cell cycle consists of 4 Phases:
G1, S, G2, and M.
• The length of the cell cycle and the length
of each phase depends on the type of cell.
• Interphase –
– a period of growth between
cell divisions
– Divided into 3 parts:
• G1 - Cell Growth
– Cells do most of their growing in this phase
• S – DNA Replication (Synthesis)
– DNA is replicated (copied)
– The cell contains 2 copies of its DNA
• G2 – Preparing for Cell Division
– Usually the shortest phase
– When completed, cell division begins
• M Phase – Cell Division
– Produces 2 identical daughter cells
– Two Stages:
• Mitosis- Division of the Nucleus
• Cytokinesis – Division of the Cytoplasm
Events of the Cell Cycle
•
During the cell cycle, the cell grows, replicates its DNA, and
divides into two daughter cells.
Mitosis
• Mitosis – the part of cell division during
which the nucleus divides.
•
Biologists divide the events of mitosis
into four phases:
–
–
–
–
prophase
metaphase
anaphase
telophase
• Mitosis may last anywhere from a few
minutes to several days.
Prophase
• Longest phase
• Chromosomes
become visible
• Centrioles move to
opposite sides of
the nucleus
• Spindles form
• Nuclear membrane
breaks down
Metaphase
• Lasts only a few
minutes
• Chromosomes
line up in the
center of the cell
• Microtubules
connect the
centromere to
the spindles
Anaphase
• Centromeres joining sister chromatids
separate to become individual
chromosomes
• Chromosomes move apart
• Ends when chromosomes are at the poles
of the spindle.
Telophase
• Chromosomes begin
to fade into a tangle
of dense material
• Nuclear envelope
reforms
• Spindle breaks apart
• Nucleolus becomes
visible
• Last phase of
mitosis
Cytokinesis
• Last phase of the M phase
• Division of the cytoplasm occurs
• Cell plate is formed in plants
10-3 Regulating the Cell Cycle
• Controls on Cell Division
– Cells grown in the lab will continue to
divide until they come into contact with
other cells. Then they stop growing.
– If you remove cells, the cells will divide
again until they touch other cells.
– This shows that controls on cell growth
and division can be turned on and off.
Cell Growth
The Discovery of Cyclins
• For many years, biologists searched for a
signal that would regulate the cell cycle –
something that would tell cells when it was
time to divide, replicate their chromosomes,
or enter another phase of the cell cycle.
• In the 1980’s, a protein was discovered that
when injected, would cause a nondividing
cell to form a mitotic spindle.
• They named this protein Cyclin.
• Cyclins are proteins that regulate the
timing of the cell cycle.
• Scientists have discovered a family
of cyclins that regulate the timing of
the cell cycle in eukaryotic cells.
Cell Cycle
Regulation
Regulatory Proteins
•
The cell cycle is controlled by
regulatory proteins both inside and
outside the cell.
• Internal Regulators – proteins that
monitor and respond to events inside
the cell.
– Examples:
• Making sure the a cell does not enter mitosis
until its chromosomes have replicated
• Preventing a cell from entering anaphase
until the spindle fibers have attached to the
chromosomes
• External Regulators
– proteins that respond to events outside
the cell.
– Can direct the cell to speed up or
slow down their cell cycles.
• Examples:
– Growth Factors
• Stimulate the growth and division of cells
• Important during embryonic
development and wound healing
Cell Growth and Healing
Apoptosis
• A process of programmed cell death
• Once triggered, a cell undergoes a
series of controlled steps leading to
its self-destruction:
– The cell and its chromatin shrink
– Then parts of the cell’s membranes
break off
– Neighboring cells then quickly clean up
the cell’s remains
• Apoptosis plays a key role in development
by shaping the structure of tissues and
organs in plants and animals.
• Example – the embryonic development of a
mouse’s foot
– The space between the toes is caused by
cell death through Apoptosis
• When Apoptosis does not occur as it
should, a number of diseases can
result
– Examples: Cell loss in AIDS and Parkinson’s
disease from too much Apoptosis
Apoptosis
Cancer: Uncontrolled Cell Growth
• Cancer is a disorder in which body
cells lose the ability to control cell
growth and division
• Cancer cells do not respond to the
signals that regulate the growth of
most cells.
• As a result, most cancer cells divide
uncontrollably.
• Cancer cells form a mass of cells
called a tumor
• Not all tumors are cancerous
• Some tumors are benign, or
noncancerous
• A benign tumor does not spread to
surrounding healthy tissue or to
other parts of the body.
• Cancerous tumors are malignant
• Malignant tumors invade and destroy
surrounding healthy tissue
• As cancer cells spread:
– They absorb the nutrients needed by other
cells
– Block nerve connections
– Prevent organs from functioning properly
• This disrupts the delicate balances of the
body, and life-threatening illness results
Lung Cancer
What Causes Cancer?
• Cancer is caused by defects in the
genes that regulate cell growth and
division
• Sources of defects:
– Smoking or chewing tobacco
– Radiation exposure
– Defective genes
– Viral Infection
• All cancers have one thing in
common
The control over the
cell cycle has broken down.
• Many cells have a defect in the
gene p53. This gene normally stops
the cell cycle until all of the
chromosomes have properly
replicated.
• Cells lose the information they need
to be able to respond to the signals
that normally control cell growth.
Treatments for Cancer
• When a cancerous tumor is located, it
can often be removed by surgery
– Example – Melanomas (skin cancer)
• Cancer cells grow rapidly so they
must copy their DNA quickly.
• This makes them vulnerable to
damage from radiation
• Chemical compounds that would kill
cancer cells (or at least slow their
growth) are used in chemotherapy.
• Great advances in chemotherapy has
made it possible to cure some forms
of cancer.
• However, because chemotherapy
compounds target rapidly dividing
cells, they also interfere with cell
division in normal, healthy cells.
• Chemotherapy produces some
serious side effects in some patients
• Researchers are searching to find
highly specific ways in which cancer
cells can be targeted for destruction
while leaving healthy cells unaffected
• Cancer is a serious disease. It is a
disease of the cell cycle and
conquering it will require a deeper
understanding of the processes that
control cell division
10-4 Cell Differentiation
From One Cell to Many
How do cells become specialized for
different functions?
• Multicellular organisms start life as just
one cell
• Living things pass through a
developmental stage called an embryo
from which the adult organism is gradually
produced
• During the development process, an
organism’s cells become more and more
differentiated and specialized for particular
functions
Differentiation
• Differentiation is the process by which
cells become specialized
•
During the development of an organism,
cells differentiate into many types of cells
• A differentiated cell has become different
from the embryonic cell that produced it
• The cell is specialized to perform certain
tasks
– Exs. – contraction, photosynthesis,
protection
Mapping Differentiation
• The process of differentiation
determines a cell’s ultimate identity
• In some organisms, a cell’s role is
determined at a specific point in
development
• Each time an organism develops, the
process is the same
Differentiation in Mammals
• In mammals and other organisms, cell
differentiation is a flexible process that
is controlled by a number of interacting
factors in an embryo
• Adult cells generally reach a point at
which their differentiation is complete
(they can no longer become other types
of cells)
• How are all of the specialized,
differentiated types of cells in the
body formed from just a single cell?
• Such a cell is called “totipotent”
• It is literally able to do everything and
to develop into any type of cell in the
body
Stem Cells
What are Stem Cells?
• Stem cells are unspecialized cells from
which differentiated cells develop
• They are at the base of a branching
“stem” of development from which
different cell types form
• They have the potential to develop into
other cell types
Human Development
• After about 4 days of development, a human embryo
forms into a blastocyst, a hollow ball of cells with a
cluster of cells inside called the inner cell mass
• Even at this early stage, the cells of the blastocyst
begin to specialize.
• The cells of the inner cell mass are
pluripotent
• Pluripotent cells can develop into
most (but not all) of the body’s cell
types
Embryonic Stem Cells
• These are pluripotent cells found in
the early embryo
• These cells can be grown in culture
and coaxed to differentiate into nerve
cells, muscle cells, and even into
sperm and egg cells
• Typically, stem cells of a given organ
or tissue produce only the types of
cells that are unique to that tissue
• Examples:
– Adult stem cells in bone marrow can
develop into several different types of
blood cells
– Stem cells in the brain can produce
neurons (nerve cells)
Adult Stem Cells
• These are groups of cells that
differentiate to renew and replace
cells in the adult body
• They have limited potential and are
called multipotent, meaning that they
can develop into many types of
differentiated cells
Potential Benefits
•
Stem cells offer the potential benefit of
using undifferentiated cells to repair or
replace badly damaged cells and tissues
• Stem cells may have an important impact
on human health
• Stem cells may be able to repair the cellular
damage of some conditions such as:
– Heart muscle cells following a heart attack
– Brain cell damage caused by a stroke
– Paralysis from spinal cord injuries
Ethical Issues
•
Human embryonic stem cell research is
controversial because the arguments for it
and against it both involve ethical issues
of life and death
• Most techniques for harvesting embryonic
stem cells cause the destruction of the
embryo
• Unlike embryonic stem cells, adult stem
cells have raised few ethical questions as
they can be obtained from the body of a
willing donor
• In the future, it may be possible to address
these concerns with a technological
solution
• Recent experiments suggest that it may be
possible to extract a small number of
embryonic stem cells without damaging
the embryo itself
• Other experiments have shown that it is
possible to reprogram adult cells by
switching “on” a few genes, causing them
to function like embryonic stem cells
• In this way, there would be no need
to involve embryos at all
• It could also make it possible to tailor
specific therapies to fit the needs of
each individual patient
• If successful, methods like these
might allow research to go forward
while avoiding any destruction of
embryonic life