Transcript Cell Growth and Reproduction

Cell Growth and
Reproduction
The Cell Cycle
Questions you will be able to
answer?
What problems does growth cause for
cells?
 When an organism becomes larger what
happens to the cells?
 How do organisms grow?
 Why do cells stay so small?
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Cells Limitations
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Cells come in a variety of
sizes
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Red blood cell diameter of
8 micrometers
Nerve Cells Length up to 1
meter
Ostrich egg yolk diameter
of 8 cm
With so many varieties of
cells, why can’t we be
made of just once Giant
Cell?
Most cells are
between 2µm and
200µm
 A micrometer is 1
millionth of a
meter!
 Too small to be
seen with naked
eye
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Cell Limitations
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Diffusion
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Surface area to volume ratio
DNA
Limits
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Diffusion limits cell size
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Movement from higher concentration to lower
concentration
Larger the distance, slower the diffusion rate
 A cell 20 cm would require months for
nutrients to get to the center
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Diffusion limits Cell Size
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Diffusion lets a variety of materials through
the membrane such as glucose and
oxygen. (in and out)
Diffusion works efficiently over small areas
but when the area gets larger it becomes
slow and inefficient
 Cells and organelles would die before any
nutrients could reach them.
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Surface Area to Volume Ratio
Limiting Factor
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When size increases the volume increases
faster than the surface area.
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In your book on page 243.
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The volume increases drastically while the
surface area increases in small
increments.
Surface area to volume ratio limits size
Surface Area to Volume
Explanation
If cell size doubled the cell would require
8X more nutrients and have 8X more
waste to excrete.
 The surface area would only increase by a
factor of 4.
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This means the membrane would not have
enough surface area to through which O2,
nutrients, and wastes can diffuse.
The Cell will either starve or poison itself.
What is the function of DNA?
DNA limits Cell Size
DNA is in the Nucleus and is dependent
on certain proteins to perform critical cell
functions.
 There is a limit on how quickly that the
proteins can be copied in the nucleus and
made into proteins in the cytoplasm.
 The cell will die if there is not enough DNA
to support proteins needs of the cell.
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DNA limits cell size
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larger cells need more
DNA. Needs more of
everything
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Most cells have just
one nucleus
So.. What happens when they get
too big?
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Cells Divide before they become too large
to function properly.
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Cell division has a lot of benefits.
Why do cells divide?
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Replacement
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Repair
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Growth
The Cell Cycle
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Cell Theory says all cells come from preexisting
cells.
Once a cell reaches its optimum size it has to do
one of 2 things
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Divide
Stop Growing
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Most cells divide
Dividing allows the cell the to
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Reproduce
Heal injuries
New Cells
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Cell are reproducing by
the second.
Right now in your
intestine there are
millions of cells being
reproduced by the
second.
While on the palm of your
hand and the bottom of
your feet you are
shedding and replacing
cells
Cell Cycle
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All cells have life cycles.
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They grow and develop.
All organisms grow and change
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Worn out tissues are repaired or are replaced
by newly produced cells.
Important definitions:
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Chromosomes:
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Carriers of the genetic material that is copied
and passed from generation to generation of
cells.
Before they become chromosomes they are
CHROMATIN.
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Long strands of DNA wrapped around proteins.
 Looks
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like tangled up spaghetti
Must organize before division.
The Cell Cycle
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Many things in life follow patterns.
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Cell are no different.
The Cell Cycle is the growth and division
of a cell.
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Goes through 2 periods
 Growth
 Division
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There are 3 main Stages of the Cell Cycle
Majority of Cells Life are in the 1st
Phase
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Interphase
Cell grows in size
 Carries out cellular functions
 Replicates DNA
 Prepares for Division
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Divided into 3 substages
Mitosis
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Stage of the Cell Cycle during which the
cell’s nucleus and nuclear material divide.
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4 Substages
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Towards the end cytokinesis begins.
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The method which a cell’s cytoplasm divides
creating a new cell.
Time it takes to Divide
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Depends on the cell
can take minutes to a
year.
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For most normal cells,
it takes 12-24 hours
Stages of Interphases
The cell grows
 Develops into a mature, functioning cell
 Duplicates DNA
 Prepares for Division
 Three Stages
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G1, S, and G2
 Gap
1, Synthesis, and Gap 2
G1 or 1st Stage of Interphase
Stage immediately after a cell divides.
 Functions
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Growing and carrying out normal cell
functions
 Preparing to replicate DNA
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2nd Stage or Synthesis Interphase
A cell copies DNA to prepare for division
 This where the chromosomes and
chromatin start to organize themselves for
division.
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Gap 2 or G2 Interphase
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3rd and final stage of Interphase
Cell prepares for the division of nucleus
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Proteins are also synthesized in this stage
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The cell takes inventory and makes sure it is
ready for mitosis.
http://www.cellsalive.com/cell_cycle.htm
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Questions for Homework
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Relate cell size to cell functions, explain why cell
size is limited
Hypothesize what the result would be if a large
cell managed to divide, despite the fact that it
had grown beyond an optimum size.
Summarize the Primary stages of Cell Cycle
Describe what happens to DNA during the S
stage of interphase.
Make a Diagram of the stages of Interphase and
describe what happens in each.
Game
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http://nobelprize.org/educational_games/m
edicine/2001/
Two Mitotic Phases of
Eukaryotic Cell Division
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Mitosis – the
nucleus & its
content, including
chromosomes,
divide & are evenly
distributed to form 2
daughter nuclei.
Cytokinesis – the
cytoplasm is divided
in two.
Prophase
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Chromatin fibers
become more tightly
coiled & folded
Nucleoli disappear
Sister chromatids joined
at the centromere
Mitotic spindle begins to
form; microtubules grow
from centrosomes
which are moving apart
Nuclear envelope
breaks in to fragments
Metaphase
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Chromosomes on
metaphase plate
The microtubules
attach to a particular
chromatid all from
one pole of spindle;
those attached to
sister chromatids
from the other pole
Anaphase
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Two centromeres of each
chromosome come apart
separating sister chromatids
Motor proteins, powered by
ATP, “walk” daughter
chromosomes centromere
toward opposite poles
Spindle microtubules
attached to kinetochores
shorten
Spindle microtubules not
attached lengthen elongating
the cell
Telophase
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Reverse of
prophase
Cell elongation
continues
Nuclear envelopes
form
Chromatin uncoils
Mitotic spindle
disappear
Nucleoli reappear
Cytokinesis
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Division of cytoplasm
In animal cells, cytokinesis
involves a cleavage furrow in
which contracting
microfilaments pinch the cell
in two.
In plants, it involves
formation of a cell plate, a
fusion of vesicles that forms
new membrane & cell walls
between the cells.
Anchorage Dependence
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Most animal & plant
cells do not divide
unless they are in
contact with a solid
surface.
Most cells of the animal
body are normally
anchored to an
extracellular matrix or to
other cells of same
tissue.
Cell Cycle Control System
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Cyclically operating
set of proteins in the
cell that both trigger
& coordinates the
major events in the
cell cycle.
3 key checkpoints:
G1, G2 & M phase
Importance of G1 Checkpoint
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If a cell receives a go ahead
signal (for example, from a
growth factor) at the G1
checkpoint, it will usually go
through its cycle & divide.
If it does not receive the go
ahead signal at G1, it will
switch into a non-dividing
mode.
Our non-dividing nerve cells
& muscle cells are
permanently arrested at the
G1 checkpoint.
Density-Dependent Inhibition & Its
Relationship to Growth Factor
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Cells usually stop dividing
when they touch one
another.
Growth factor is a protein
secreted by certain body
cells that stimulates cells in
the vicinity to divide
Most cells require growth
factors to begin dividing &
they stop dividing when they
run out of these substances.
Cancer
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Cancer, which currently
claims the lives of 1 out of
every 5 people in the United
States & other developed
nations is a disease of the
cell cycle.
Cancer cells do not have a
properly functioning cell
cycle control system; They
divide excessively & can
invade other tissues of the
body.
This excessive growth can
result in an abnormal mass
of cells called a tumor.
Terms related to Cancer
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Tumor – abnormal mass of cells
Benign tumor – an abnormal mass of essentially normal cells always
remain at their original site.
Malignant tumor – a mass of cancer cells which are capable of
spreading into neighboring tissues & often to other parts of the body.
Metastasis – the spread of cancer cells beyond their original site.
Carcinomas – Cancers which originate in the external or internal
coverings of the body, such as skin or the lining of the intestine.
Sarcomas – cancers which arise in tissues that support the body,
such as bone & muscle
Leukemias & Lymphomas – cancers of blood forming tissues, such
as bone marrow, spleen & lymph nodes.
Cancer Cells Differ from
Normal Cells
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Cancer cells are largely
unrestrained by the system that
normally controls cell division.
They are not usually affected by
density-dependent inhibition.
They continue to divide even in
high densities.
Many cancer cells have cell
cycle control systems that
proceed past checkpoints even
in the absense of growth
factors.
Cancer cells have reduced need
for anchorage & often grow
without be attached to a
surface.
Treatment for Cancer
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Chemotherapy
Radiation
Both attempt to halt
the spread of cancer
by stopping them
from dividing.