Download Cell Size Limitations

Cell Size Limitations
• Cells come in a wide variety of sizes and shapes.
• Considering this wide range of cells sizes, why then
can’t most organisms be just one giant cell?
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Diffusion limits cell size
• Although diffusion is a fast and efficient process over
short distances, it becomes slow and inefficient as
the distances become larger.
• Because of the slow rate of diffusion, organisms can’t
be just one giant-sized cell.
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DNA limits cell size
• The cell cannot survive unless there is enough DNA
to support the protein needs of the cell.
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Surface area-to-volume ratio
1 mm
4 mm
2 mm
1 mm
2 mm
1 mm
SA = 6 mm2
Vol = 1 mm3
Ratio of 6:1
4 mm
2 mm
SA = 24
mm2
Vol = 8 mm3
SA = 96
mm2
4 mm
Vol = 64 mm3
Ratio of 1.5:1
Ratio of 3:1
• As a cell’s size increases, its volume increases much faster
than its surface area (SA).
• If cell size doubled, the cell would require eight times
more nutrients and would have eight times more waste
to excrete.
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Surface area-to-volume ratio
1 mm
4 mm
2 mm
1 mm
2 mm
1 mm
SA = 6 mm2
Vol = 1 mm3
Ratio of 6:1
4 mm
2 mm
SA = 24
mm2
Vol = 8 mm3
SA = 96
mm2
4 mm
Vol = 64 mm3
Ratio of 1.5:1
Ratio of 3:1
• The surface area (SA), however, would increase by a
factor of only four.
• The cell would either starve to death or be poisoned
from the buildup of waste products.
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Cell Division
• Cell division is the process by which new cells are
produced from one cell.
• Cell division results in two cells that are identical to
the original, parent cell.
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Chromosomes
• Structures, which contain
DNA and become darkly
colored when stained, are
called chromosomes.
• Chromosomes are the
carriers of the genetic
material that is copied
and passed from
generation to generation
of cells.
• Accurate transmission of
chromosomes during cell
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division is critical.
The large, complex chromosomes of eukaryotes
duplicate with each cell division
• Chromosomes contain Sister chromatids
a very long DNA
molecule with
thousands of genes
Centromere
– Individual
chromosomes are
only visible
during cell division
– They are packaged
as chromatin when
visible
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Centromere
The large, complex chromosomes of eukaryotes
duplicate with each cell division
• Before a cell starts
dividing, the
chromosomes are
duplicated
– This process
produces sister
chromatids
Sister chromatids
Centromere
Centromere
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double-stranded
human chromosomes
ready for mitosis
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The Cell Cycle
• The cell cycle is the
sequence of growth and
division of a cell.
• The majority of a cell’s
life is spent in the
growth period known as
interphase.
Interphase
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The Cell Cycle
• Following interphase,
a cell enters its period
of nuclear division
called mitosis.
• Following mitosis, the
cytoplasm divides,
separating the two
daughter cells.
Mitosis
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Interphase: A Busy Time
• Interphase, the busiest phase of the cell cycle, is
divided into three parts: G1,S, & G2
Interphase
DNA synthesis
and replication (S)
Rapid
growth and
metabolic
activity (G1)
Cell prepares
for division (G2)
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Interphase: A Busy Time
• During the first part of interphase, (G1),the cell
grows and protein production is high.
Interphase
(G1)
Rapid
growth and
metabolic
activity (G1)
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Interphase: A Busy Time
• In the second part of interphase, (S),the cell
copies its chromosomes.
Interphase
(S)
DNA synthesis and
replication (S)
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Interphase: A Busy Time
• After the chromosomes have been duplicated, the
cell enters another shorter growth period, (G2),in
which mitochondria and other organelles are
manufactured and cell parts needed for cell division
are assembled.
Interphase
(G2)
Cell prepares for
division (G2)
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Biology is the only subject in which
multiplication is the same thing as
division…
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MITOSIS:
Making New Cells
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Where it all began…
You started as a cell smaller than a
period at the end of a sentence…
.
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Getting from there to here…
• Going from egg to fetus….
the original fertilized ova (egg) has to divide…
and divide…
and divide…
and divide…
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Why do cells divide…
 Single-celled organisms
for reproduction
 asexual reproduction
(identical clones)
amoeba

starfish
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Why do cells divide…
 Multi-celled organisms
for growth & development
 from fertilized egg to adult
 for repair & replacement
 replace cells that die from
normal wear & tear or
from injury
amoeba

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starfish
The Phases of Mitosis
• The four phases of mitosis are Prophase,
Metaphase, Anaphase, and Telophase.
•PMAT
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Prophase: The first phase of mitosis
• During prophase, the chromosomes coil to form
visible chromatins.
Spindle fibers
Disappearing
nuclear
envelope
Doubled
chromosome
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Prophase: The
st
1
phase of mitosis
• The two halves of the doubled structure are called
sister chromatids.
• Sister chromatids are held together by a structure
called a centromere, which plays a role in
chromosome movement during mitosis.
Sister chromatids
Centromere
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Metaphase: The
nd
2
stage of mitosis
• During metaphase, the chromatids move to the
equator (middle) of the spindle.
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Anaphase: The 3rd phase of mitosis
• During anaphase, the centromeres split and the
sister chromatids are pulled apart to opposite
poles of the cell.
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Telophase: The 4th phase of mitosis
• During telophase, two distinct daughter cells are
formed.
• The cells separate as the cell cycle proceeds into
the next interphase.
Nuclear
envelope
reappears
Two daughter cells are formed
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DNA must be duplicated…
chromosomes in cell
DNA in chromosomes
cell
4 single-stranded chromosomes
duplicated chromosomes
nucleus
duplicated
chromosomes
4 double-stranded chromosomes
cell
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nucleus
Mitosis: Dividing DNA & cells
• Stage 1: cell copies DNA
• Making new DNA during S phase of interphase
Copy DNA!
DNA
cell
Copying of DNA occurs
during interphase S
nucleus
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(interphase S)
Mitosis: Dividing DNA & cells
• Stage 2: DNA winds into chromosomes
– DNA is wound up into chromosomes to keep it
organized.
duplicated chromosomes
Wind up!
cell
Organizing of DNA occurs
during prophase
nucleus
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(prophase)
Mitosis: Dividing DNA & cells
• Stage 3: Chromosomes line up
– chromatids line up in middle
– attached to protein “cables” that will help
them move
Line up!
Lining of DNA along equator
occurs during metaphase
duplicated chromosomes
line up in middle of cell
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(metaphase)
Mitosis: Dividing DNA & cells
• Stage 4: Chromosomes separate
– chromatids divide, separating pairs
– start moving to opposite ends
Separate!
Dividing & separating of
DNA occurs during
anaphase
chromosomes divide &
move to opposite ends
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(anaphase)
Mitosis: Dividing DNA & cells
• Stage 5: Cell starts to divide
– cell start to divide
– nucleus re-forms
Divide!
Cell division begins to
occur during telophase
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(telophase)
Mitosis: Dividing DNA & cells
• Stage 6: DNA unwinds again
– Identical ‘daughter’ cells separate
– now they can do their every day jobs
The End!
New cells are formed during
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the process Website
of cytokinesis.
(cytokinesis)
New “daughter” cells
• Get 2 exact copies of original cells
– same DNA
– “clones”
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Cytokinesis
• Following telophase, the cell’s cytoplasm divides
in a process called cytokinesis.
• Cytokinesis differs between plants and animals.
• Toward the end of telophase in animal cells, the
plasma membrane pinches in along the equator.
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Cytokinesis
• Plant cells have a rigid cell wall, so the plasma
membrane does not pinch in.
• A structure known as the cell plate is laid down across
the cell’s equator.
• A cell membrane forms around each cell, and new cell
walls form on each side of the cell plate until separation
is complete.
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Results of Mitosis
• When mitosis is complete, unicellular organisms
remain as single cells.
• In multicellular organisms, cell growth and
reproduction result in groups of cells that work
together as tissue to perform a specific function.
• Tissues organize in various combinations to form
organs that perform more complex roles within the
organism.
• Multiple organs that work together form an organ
system.
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Mitosis in whitefish blastula
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Mitosis in plant cell
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onion root tip
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Overview of mitosis
interphase
I.P.M.A.T.C.
prophase
Please Make Another Two Cells
cytokinesis
metaphase
anaphase
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telophase
Mitosis & Cancer:
When Making New Cells Goes Wrong!
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When is mitosis a good thing?
• When you have to add or replace cells
– growth & development
– repair
– replacement
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When is mitosis a BAD thing
• When cells reproduce & they are not needed
– these cells take over organs, but don’t do the right
job
– they just keep making copies
– cancer
• Uncontrolled cell growth
• Damages organs
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Why would cells just make bad copies?
• If DNA gets damaged, cells stop listening to
correct instructions
– Mutations (wrongly constructed DNA strands)
• Some causes of mutations:
UV radiation
 chemical exposure
 radiation exposure
 heat

cigarette smoke
 pollution
 age
 genetics

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Normal Control of the Cell Cycle
Proteins and enzymes control the cell cycle
• The cell cycle is controlled by proteins called cyclins
and a set of enzymes that attach to the cyclin and
become activated.
• Occasionally, cells lose control of the cell cycle.
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Normal Control of the Cell Cycle
• This uncontrolled dividing of cells can result from the
failure to produce certain enzymes, the
overproduction of enzymes, or the production of
other enzymes at the wrong time.
• Cancer is a malignant growth resulting from
uncontrolled cell division.
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Normal Control of the Cell Cycle
• Enzyme production is directed by genes located on
the chromosomes.
• A gene is a segment of DNA that controls the
production of a protein.
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Cancer: A mistake in the Cell Cycle
• Currently, scientists consider cancer to be a result of
changes in one or more of the genes that produce
substances that are involved in controlling the cell
cycle.
• Cancerous cells form masses of tissue called tumors
that deprive normal cells of nutrients.
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Tumors
• Benign tumor
– abnormal cells remain at original site as a lump
– most do not cause serious problems &
can be removed by surgery
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Tumors
• Malignant tumor
– cells leave original site
• carried by circulatory systems to other tissues
• start more tumors
– damage functions of organs throughout body
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Cancer: A mistake in the Cell Cycle
• In later stages, cancer cells enter the circulatory
systems and spread throughout the body, a
process called metastasis, forming new tumors
that disrupt the function of organs, organ
systems, and ultimately, the organism.
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The causes of cancer
• The causes of cancer are
difficult to pinpoint
because both genetic
and environmental
factors are involved.
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The causes of cancer
• Environmental factors,
such as cigarette smoke,
air and water pollution,
and exposure to
ultraviolet radiation from
the sun, are all known to
damage the genes that
control the cell cycle.
• Cancer may also be
caused by viral infections
that damage the genes.
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Cancer prevention
• Physicians and dietary experts
agree that diets low in fat and
high in fiber content may
reduce the risk of many kinds
of cancer.
• Vitamins and minerals may
also help prevent cancer.
• In addition to diet, other
healthy choices such as daily
exercise and not using
tobacco also are known to
reduce the risk of cancer.
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