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Why Do Cells Divide?
Why Do Cells Divide?
1. Surface area to volume ratio

Efficiency of moving materials into the cell
– The larger the cell becomes the less efficient
it is
– The volume of the cell increases faster then
its surface area
– Cells that grow too large no longer have
enough surface area to take in nutrients and
remove waste
Ratio of Surface Area to Volume in Cells
Cell Size
1 cm x 1 cm x 6
= 6 cm2
2 cm x 2 cm x 6
= 24 cm2
3 cm x 3 cm x 6 = 54 cm2
Volume
(length x width x height)
1 cm x 1 cm x 1 cm
= 1 cm3
2 cm x 2 cm x 2 cm
= 8 cm3
3 cm x 3 cm x 3 cm = 27 cm3
Ratio of Surface Area
to Volume
6/1 = 6:1
Surface Area
(length x width x 6)
© Pearson Education, Inc.
24 / 8 = 3 : 1
54 / 27 = 2 : 1
2. Cells get old and worn out
 Just
like anything else, cells get worn
out. They need repair, and, just like
an old car, eventually need
replacement.
 The best way to fix what ails a cell is
to replace that cell all together.
What does a cell need to do
first?
•In order to reproduce, a
cell must make copies of
its DNA and organelles.
•The new cell needs all
the stuff the old cell has.
DNA
 DNA
must be copied in a process
called DNA replication.
 After
replication, the DNA is all
stringy and tangled (called
chromatin), but must be separated
into a new cell. Before this can
happen, the DNA must coil up into
chromosomes.
Chromosomes

Threadlike structures made up of DNA that coils
around proteins. Contains genetic information
that is passed form parents to offspring.
chromosomes
Genes
(DNA)
Chromatids (Sister)

One of two identical
parts of a duplicated
chromosome
Centromere
Sister
chromatids
Number of Chromosomes






Fruit Flies: 8
Carrots: 20
Humans: 46
Fern: 480
White Ash: 138
Turkey: 82
Human Karyotype
HUMAN CHROMOSOMES
Genetic Abnormalities
The Cell Cycle
© Pearson Education, Inc.
Next…
we will learn about DNA.