Transcript Cell Growth and Division

Cell Growth and Division
The Cell Cycle
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The cycle of growth, DNA
synthesis, and division is
essential for an organism
to grow and heal.
If it goes out of control,
abnormal cell growth may
occur, resulting in cancer
cells.
Aveoli in the lung
Lung cancer cell
The Cell Cycle
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The cell cycle is the
regular pattern of growth,
DNA duplication, and cell
division that occurs in
eukaryotic cells.
Cell Cycle: Four Main Stages
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The four main stages of
the cell cycle are:
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Gap 1
Synthesis
Gap 2
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Gap 1, Synthesis, Gap 2 =
Interphase
Mitosis
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Scientists were limited by the
microscopes of the time and
named the “gaps” because
they weren’t able to see
activity in the cell at these
times.
Gap 1 (G1)
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During G1, the cell carries
out its normal functions.
The cell will increase in
size, and its organelles will
increase in number.
A cell spends most of it’s
time in this stage, but the
length of the stage depends
on the cell type.
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This is the stage where the
cell determines if it will
undergo apoptosis, or divide.
Synthesis (S)
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The cell makes a copy of
its nuclear DNA during
this stage.
By the end of this stage,
the cell contains two
complete sets of DNA.
DNA as seen from an SEM
Gap 2 (G2)
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Cells will continue to carry
out their normal functions.
Additional growth occurs.
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The cell is checked a final
time for damage and
adequate size before
undergoing mitosis.
Mitosis (M)
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Mitosis includes two
processes: mitosis and
cytokinesis.
Mitosis is the division of the
cell nucleus and its contents.
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The nuclear membrane
dissolves, duplicated DNA
condenses around proteins
and separates, and two new
nuclei form.
Cytokinesis is the process
that divides the cell
cytoplasm.
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The result is two daughter
cells that are genetically
identical to the original cell.
Cells Divide At Different Rates
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The rate at which your cells divide is linked to your
body’s need for those cells.
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The internal lining of your digestive track receives a lot of
wear and tear. As a result, cells that line your stomach and
intestine are replaced every few days.
Cells that make up the smooth muscle of the intestine, the
lungs, kidney, and liver, divide only occasionally in response to
injury or death.
G0
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Cells that divide only rarely
are thought to enter a stage
called G0.
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G0 cells are unlikely to divide,
although they continue to
carry out their normal
functions.
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Some cells, such as neurons,
appear to stay permanently in
the G0 stage.
Lymphocytes, a type of white
blood cell, may remain in the
G0 stage for years until they
recognize an invader.
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Nerve cell communicating via
neurotransmitters
Once the invader binds to a
lymphocyte receptor, the
lymphocyte goes through rapid
cell divisions to help fight
infection.
Lymphocyte
Cell Size Is Limited
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Cells have upper and lower size
limits.
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If cells were too small, they could
not contain all the necessary
organelles and molecules.
The upper limit of cell size is due
to the ratio of cell surface area to
volume.
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Oxygen, nutrients, and wastes move
across the cell membrane, or the
surface of a cell.
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These materials must be transported in
adequate amounts and with adequate
speed to keep the inside of the cell
functioning.
As a cell increases in size, its volume
increases faster than its surface area.
Therefore, a further increase in size
could result in a surface area too small
for the adequate exchange of materials.
A neuron running down a giraffe’s neck
to its legs may be several meters long,
but it is not shaped like a cube or
sphere. It is extremely long and thin.
This structure gives the neuron a large
surface area with a relatively small
increase in volume.
Mitosis and Cytokinesis
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Mitosis is an amazing
process that efficiently
sorts two sets of DNA
and divides them
between two nuclei.
Chromosomes Condense
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DNA is a double-stranded
molecule made of four different
subunits called nucleotides.
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A chromosome is one long
continuous thread of DNA that
consists of numerous genes along
with regulatory information.
Your body cells have 46
chromosomes each.
If stretched out straight and laid
end to end, the DNA in just one
of your cells would be about 10
feet long.
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During interphase, DNA is loosely
organized (like spaghetti).
During mitosis, DNA has to be
condensed so it can be divided
accurately.
Chromosomes Condense
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Each of your
chromosomes is associated
with a group of proteins
called histones.
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DNA wraps around
histones at regular intervals
like beads on a string.
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When DNA is loosely
wrapped (during interphase)
it is called chromatin.
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As a cell progresses into
mitosis, chromatin further
condenses, until it forms small
thick rods.
Chromosomes Condense
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The chromosome now looks
like an ‘X’.
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One half of the duplicated
chromosome is called a
chromatid.
Each chromosome is held
together in the center by a
centromere.
The ends of the DNA
molecule form telomeres.
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Telomeres are sequences of
repeating nucleotides that do
not form genes.
They prevent the ends of
chromosomes from accidently
attaching to each other, and
they prevent the loss of genes.
Interphase
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Interphase prepares the
cell to divide.
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It provides critical time for
the duplication of organelles
and for DNA replication.
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By the end of interphase, an
individual cell has two full sets
of DNA, or chromosomes,
and is large enough to divide.
Mitosis
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Mitosis divides a cell’s
nucleus into two
genetically identical
nuclei, each with its own
single, full set of DNA.
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The four main stages of
mitosis are:
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Prophase
Metaphase
Anaphase
Telophase
Mitosis: Prophase
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Chromatin condenses into
tightly coiled chromosomes.
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Each consists of two identical
sister chromatids.
The nuclear envelop breaks
down, the nucleolus
disappears, and the
centrosomes and centrioles
begin to migrate to opposite
sides of the cell.
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Organized microtubules called
spindle fibers grow from the
centrioles and radiate toward
the center of the cell.
Mitosis: Metaphase
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The spindle fibers attach
to a protein structure on
the centromere of each
chromosome and align the
chromosomes along the
cell equator, around the
middle of the cell.
Mitosis: Anaphase
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Sister chromatids separate
from each other.
Spindle fibers begin to
shorten, which pulls the
sister chromatids away
from each other and
toward opposite sides of
the cell.
Mitosis: Telophase
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A complete set of identical
chromosomes is
positioned at each pole of
the cell.
The nuclear membranes
start to form, the
chromosomes begin to
uncoil, and the spindle
fibers fall apart.
Cytokinesis
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Cytokinesis divides the cell
into two cells and completes
a full stage of the cell cycle.
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In animal cells, the membrane
forms a furrow, or trench, that
is pulled inward by tiny
filaments, like a drawstring.
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Cleavage
furrow in
animal cell
Gradually, the membrane
pinches closed, forming a
separate cell around each
nucleus.
In plant cells, a cell plate is
formed between the two
nuclei.
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It is made by the golgi apparatus,
which supplies the new plasma
membrane.
Cell
plate in
plant cell
Regulation of the Cell Cycle
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How does your body regulate all the millions of cells
divisions happening in your body?
External Factors
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External factors that help regulate the
cell cycle include physical and chemical
signals.
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One physical factor is cell-cell contact.
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Once a cell touches another cell, it will stop
dividing.
A chemical factor is growth factor.
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Growth factors are a broad group of
proteins that stimulate cell division. Growth
factors bind to receptors that activate
specific genes to trigger cell growth. There
are many different types of growth factor.
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Platelets let out a growth factor that stimulates
all types of cells around a wound to begin the
healing process.
Erythropoietin stimulate the production of red
blood cells.
Growth hormone results in bone growth and
affects your protein and fat metabolism.
Internal Factors
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Kinases and cyclins are
internal factors that regulate
cell division.
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A kinase is an enzyme that
energizes a molecule and
forces it to change shape or to
become energized.
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Kinases that help the cell cycle
are activated by cyclins.
Cyclins are a group of proteins
that are rapidly made and
destroyed at certain points in
the cell cycle.
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These two factors help cells
advance to different stages of
the cell cycle.
Apoptosis
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Apoptosis is programmed cell
death.
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This occurs when internal or
external signals activate genes
that help produce selfdestructive enzymes.
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Cells in the immune system
recognize apoptopic cells and
eat them.
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For example: in early stages of
development, human embryos
have webbing between their
fingers and toes. Before a baby is
born, those cells typically go
through apoptosis, and most
babies are born with clearly
defined, un-webbed fingers and
toes.
Uncontrolled Division
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Cancer is the common
name for a class of
diseases characterized by
uncontrolled cell division.
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It arises when regulation of
the cell cycle breaks down.
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Cancer cells continue to
divide even when they come
into contact with other cells.
They will also divide in the
absence of growth factors. As
a result, they divide much
more often then healthy cells.
Breast cancer cells
Uncontrolled Division
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Cancer cells form disorganized
clumps called tumors.
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Benign tumors are when cancer
cells typically remain clustered
together.
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This means that the tumor may be
relatively harmless and can
probably be cured by removing it.
Malignant tumors are when
cancer cells break away from the
main tumor. This is called
metastasizing.
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Benign tumor
The breakaway cells can be carried
in the bloodstream or lymph
system to other parts of the body
where they can form more tumors.
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These are very difficult to remove.
Malignant tumor
Causes of Uncontrolled Division
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Some cancers are caused by cells
that have suffered damage to the
genes that help make proteins
involved in cell-cycle regulation.
Most canter cells carry mutations, or
errors, in two types of genes.
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One type, oncogenes, accelerate the
cell cycle, the other type acts as cellcycle brakes.
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Mutations in these genes can be
inherited.
Some mutations can be caused by
exposure to radiation (UV light) or
chemicals.
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Substances known to produce or
promote the development of cancer
are called carcinogens.
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These include tobacco smoke and
certain air pollutants.
Other cancers can be caused by
viruses.
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HPV is known to cause cervical cancer.
Treatments for Cancer
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Standard treatment often
involves both radiation and
chemotherapy.
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Radiation therapy is the use of
radiation to kill cancer cells and
shrink tumors. It works by
damaging a cell’s DNA so much
that the cell cannot divide.
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Radiation is usually localized
because it can also hurt healthy
cells.
Chemotherapy uses certain drugs,
often in combination, to kill
actively dividing cells.
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Chemotherapy is systemic—drugs
travel throughout the entire body.
Susan G. Komen was diagnosed with breast
cancer at the age of 33. She succumbed to the
disease three years later in 1980. Her sister
made a promise to increase research and
development of a cure for breast cancer, and
founded the Susan G. Komen Breast Cancer
Foundation in 1982.