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Cell Growth and Division
Growth vs. Division
• When an animal or plant grows, what
happens to its cells?
• Does an animal get larger because each
cell increases in size (grows) or because it
produces more of them?
Why can’t cells grow forever?
• REASON 1: Not enough DNA!...as the
cell increases in size, it keeps the same
amount of DNA. Eventually the cell will
grow too much for the DNA to control all
its activities
Memory Trick: Think of DNA like a library
of books. If a town (cell) is too big, people
may have to wait for books!
Why Can’t Cells Grow Forever?
• REASON #2: Surface area of membrane
doesn’t increase as quickly as cell volume
Too little membrane  not enough exchange of
materials (nutrient absorption and waste
removal) between cell and environment
Cells need a:
• (Large surface area) for more materials to
pass in and out of the cell…
• (Small volume) less distance within the cell
for materials to have to travel
Specialized Cells
• Villi (stomach lining)
– absorbs food & nutrients
• Alveoli (lungs)
– exchanges oxygen and carbon dioxide (oxygen in and
carbon dioxide out)
• Root hairs (plants)
– absorbs water and minerals
ALL of these specialized cells have:
- increased surface to volume ratios because
- their function is to absorb large quantities of material
(more surface area allows for more efficient absorption rates)
Surface Area & Volume
• Surface area of cube
6 sides x area of each side
6 x length x width = surface area
• Volume of a cube
length x width x height
Which of the cubes would have a
larger surface area to volume ratio?
• Calculate it…
1 cm3
2 cm3
3 cm3
6 cm2
24 cm2
54 cm2
1 cm3
8 cm3
27 cm3
Surface Area
Volume
Surface area :
Volume
6:1
(24 : 8)
(54 : 27)
3:1
2:1
The Solution?
• Before a cell becomes too large, it divides
to form 2 “daughter cells”.
• This process is called cell division
• Cell division can only happen once a cell
has made a copy of its DNA so that each
daughter cell can have a full genetic library
Cell Division in Prokaryotes
• DNA is copied and divided into
two cells
• Creates two identical daughter
cells
• Process is called Binary Fission
Cell Division in Eukaryotes
• DNA is found in
the nucleus
• Chromatin is DNA
& protein threads
that coil into
chromosomes
during cell
division.
Structure of Chromosomes
• Coiled threads of DNA and proteins.
• Contains genes (hereditary information)
• Must successfully divide for new cells to
be made
Structure of Chromosome
Chromatids: 2 identical
copies
Of DNA that are
connected to
Form a chromosome
Centromere: the area
where chromatids are
attached (usually at the
middle of the chromatids)
Chromosome #’s
• The cells of every organism have a
specific number of chromosomes
Cell Type
# of Chromosomes
Fruit Flies
8
Carrots
18
Human
46
Diploid vs. Haploid Cells
• Diploid Cells (2n) = 2 sets of
chromosomes…one from each
parent (Example: human body
cell has a diploid number of 46)
• Haploid Cells (n)= only have 1
set of chromosomes (Example:
Sperm or Egg Cell has a
haploid number of 23)
Types of Chromosomes
• Sex chromosomes =
determine the sex of
an organism; either X
or Y
• Autosomes = all the
other chromosomes in
an organism
Cell Cycle
•
•
•
The series of events that cells go through as
the grow and divide
During the cycle, a cell grows, prepares for
division, and divides to form 2 daughter cells,
each of which then begins the cycle again
2 Main Parts
1) Interphase
2) Cell Division
Interphase
• Time between
divisions where cell
grows and replicates
DNA
• 3 Parts
1) G1 = cell grows
and matures
2) S = DNA is
copied
3) G2 = cell
http://www.cellsalive.com/cell_cycle.htm
prepares for
division
Mitosis
• This involves division of the
nucleus
• 4 Phases
1) Prophase
2) Metaphase
3) Anaphase
4) Telophase
Part 1: Prophase
• Coiling of DNA into
chromosomes
• Nuclear membrane
breaks down
• Mitotic Spindle forms
using centrosomes /
centrioles
What is the mitotic spindle?
• 2 Structures located in the
cytoplasm called
centrioles move towards
opposite sides of the
nucleus
• Microtubules (protein
tubes) called spindle
fibers grow from the
centrioles
• Spindle fibers attach to
chromosomes at the
centromere and help pull
chromatids apart
NERD
Part 2: Metaphase
• Spindle fibers move
chromosomes to the
center of the dividing
cell
Part 3: Anaphase
• Chromatids of each
chromosome
separate at the
centromere and
move toward
opposite poles of the
dividing cell
Part 4: Telophase
• Chromosomes reach
poles of cell
• Spindle breaks down
• Chromosomes uncoil
into chromatin
• Nuclear envelopes
and 2 nucleoli form
Cytokinesis
• Division of the cytoplasm
• Animal Cell: Cleavage Furrow (cell
membrane pinches in )
• Plant Cell: Cell Plate (cell wall grows
between daughter cells)
Mitosis in Real Cells
http://www.cellsaliv
e.com/mitosis.htm
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Modern Cell Theory – With Cell
Division!
Original Cell Theory:
1) All living things are made of cells.
2) Cells are the basic unit of structure and function in living things
3) All cells come from pre-existing cells
Additions to the Cell Theory:
1) Energy flow (metabolism and biochemistry) occurs in cells
2) Cells contain hereditary information (DNA) that is passed from cell
to cell during cell division
3) All cells are basically the same in chemical composition in
organisms of similar species. (ex: all cell membranes are made of
phospholipids ; all plant cell walls are made of cellulose – a
polysaccharide)
Rate of Cell Division
• The rate of cell division is different for different types of
cells
Division Rate
Do not divide after tissue is
mature
Cell Type
Nerve Cells, Muscle Cells
Do not normally divide but
Liver Cells
can be stimulated to do so
(ex: if a portion of the liver is
removed surgically)
Divide constantly and
rapidly
Skin Cells, Epithelial Cells,
Sperm Cells, Bone Marrow
Cells
Fun Facts about Cell Division
Rate in Various Cell Types
• The average red blood cell lives
only about 120 days. There are
about 2.5 trillion of them in an
adult body. To maintain this
number, about 2.5 million new red
blood cells must be produced each
second by the division of stem
cells in the red bone marrow.
• All told, about 2 trillion cell
divisions occur in an adult human
every 24 hours; about 25 million a
second!
Cell Specialization
• A single-celled organism is called unicellular.
Examples: bacteria, algae (plant), yeasts (fungi)
• These organisms perform ALL of their functions in
ONLY ONE CELL using different organelles!!!
• Multicellular organisms are made up of MANY cells.
• These organisms have cell specialization— different
types of cells with different structures for different
jobs/purposes.
• As cells specialize, only DNA related to the functions of a
particular cell remains active.
Stem Cells
• All cells in your body came from a single fertilized cell
(sperm + egg).
• Most cells become specialized before birth. Once a cell
becomes specialized, it cannot be changed into another
type of cell.
• Stem cells are cells that have not become specialized.
• Stem cells can be found in developing embryos or adult
bone marrow
Cells – What do they really look
like?
• Cells have drastically different shapes and
sizes depending on their functions
Levels of Cell Organization
• Cells—the smallest unit of life
Example: Neuron
• Tissues—a group of similar cells
working together
Example: Nervous Tissue
• Organs—a group of similar tissues
working together
Example: Brain, Spinal Cord, Eye
• Organ Systems—a group of organs
working together
Example: nervous system
• Organism—the entire multicellular living
thing
Example: YOU!