Chapter 10 Cell Growth and Division
10-1 Cell Growth
Does an animal get larger because its
cells increase in size or because it
produces more of them?
Most living things grow by producing more cells, the cells of a
small animal are almost the same size as an adult animal.
They both have the same size cells!
2 reasons cell divides: (don’t write anything yet)
-The larger the cell becomes the more
demands the cell places on its DNA (DNA
overload)
-The more trouble the cell has moving
enough nutrients and wastes across the
cell membrane. (Membrane traffic jams)
1. DNA “Overload”
 DNA stores the info that controls cells fxn.
1000 books
1000 books
As the cell size increases it does not make
more DNA for the cell. If the cell were to grow
without limit, an information crisis would occur.
Compare this to a small town with a library…
2. Exchanging Materials
 Food, oxygen and water enter through a cell
membrane and wastes leave the cell in the
same way.
 The rate of exchange depends on the surface
area of the cell.
 Surface area = total area of its cell membrane
 The rate at which food and O2 are used up and
waste products are produced depends on the
cell’s volume.
Ratio of Surface Area to Volume
 To obtain the ratio: divide the surface area by
the volume
 Surface area/volume= ratio
 Volume = length x width x height
So a 1 cm cell would be: 1 x 1 x 1 = 1cm3
 S.A. = length x width x # of sides
So a 1 cm cell would be: 1 x 1 x 6 = 6 cm2
 In this case, the S.A. to vol. ratio would be 6:1
Ratio of Surface Area to Volume
 The volume increases much more rapidly than
the surface area, causing the ratio to decrease
 Think of the town:
The small town has one, two-lane main street. As the number
of people increase the size of the street remains the same.
The people encounter more traffic and traffic jams occur. The
cell experiences the same thing . As it grows it is harder to
move materials into and out of the cell. That is why cells do not
grow very big.
Why do we need more cells?
1. To grow/get bigger
2. To repair/heal a wound
3. To replace dead cells (G.I. tract, skin)
Cell Division
 Before it gets too large to cause “traffic”
problems the cell divides into 2 daughter
cells.
 Before division the cell replicates its DNA so
that each cell receives its own identical
“library”
Chromotin
Chromotin is the form that DNA is in most
of the time in a cell. It is loose and
unorganized.
Chromosomes
 A chromosome is the
form that DNA is in
before a cell divides.
It is X shaped and
organized.
 If the DNA is not
organized it cannot
be equally divided
between the two new
cells.
Chromatids
Chromatids are half of the chromosome X.
Each pair of chromatids contains identical
DNA.
They are known as “sister chromatids”
Chromatids
The sister chromatids are identical so that
each new “daughter cell” gets a copy of
the same DNA.
Centromere
 A centromere holds
the two sister
chromatids together
like a belt.
Histones
Histones
Histones are disk shaped proteins that
help to organize the DNA within the
chromosome.
Let’s review!
1. What are two reasons why a cell would
divide?
2. What is a chromosome? What does it
contain and where can it be found?
3. What is the difference between a
chromosome and chromotin?