The Cell cycle
CHAPTER 8 PART 2
Cell Growth
 As
organisms grow, do they grow because cells
get larger and larger or because more cells are
produced?
 Cells divide instead of growing larger for two
reasons:
As
a cell grows larger, there are more demands placed
on its DNA.
As
a cell grows larger, the cell has more trouble moving
enough nutrients and wastes across the cell
membrane.
 Deoxyribonucleic
Acid (DNA) is the genetic
information of a cell. In eukaryotes it is found in the
nucleus.
 A cell typically only has one copy of its’ DNA.
Imagine
going to the library to use the computers. As
more and more students arrive, there are not enough
computers for everyone.
 All
materials enter and/or leave the cell through
the cell membrane.
 The rate at which things can pass through the cell
(through active or passive transport) is dependent
on the surface area of that cell.
 The rate at which resources are used up and
waste leaves the cell is a dependent on the
volume of that cell.
 You
have to look at the ratio of surface area to volume.
The higher it is, the more efficient the cell is. Surface area is
calculated as length x width x # of sides.
 Let’s
look at three cells.
 If
a cube has height of 1 cm what is the surface area? ______.
What is the volume? ______ Therefore the ratio is ______.
 If
a cube has height of 2 cm what is the surface area? ______.
What is the volume? ______ Therefore the ratio is ______.
 If
a cube has height of 3 cm what is the surface area? ______.
What is the volume? ______ Therefore the ratio is ______.
 If
you think of a town it’s like cars on a main street. If the
number of cars is increased, but the road isn’t widened, it
becomes more and more crowded.
Cell Division
 Before
a cell grows too large, it will divide into two
“daughter cells”.
 The
process of division is called Cell Division.
 Before
a cell can divide, it must replicate all of its’ DNA. It
will begin with one copy and end with two copies.
 This
replication gives one copy of DNA to each daughter
cell.
 This
also raises the surface area to volume ratio of each
cell.
The Cell Cycle
 Prokaryotes
can just divide into two cells, but
eukaryotes must go through a lot of preparation.
Cell division is in three sections:
Interphase-The
Mitosis-The
cell grows and replicates DNA.
nucleus divides.
Cytokinesis-The
divide.
cytoplasm and the rest of the cell
 In
eukaryotes, DNA is carried in the form of
chromosomes. How many chromosomes are present
depends on the type of organism.
 Fruit
flies have 8 chromosomes.
 Humans
have 46 chromosomes.
 Chromosomes
mitosis.
can only be seen in most cells during
 Before
cell division each chromosome is replicated and
has two different “sister chromatids”. When the cell
divides, the chromosome splits and each daughter cell
gets a chromatid.
 The
chromatids are connected in an area called a
centromere.
Cells
are always in the cell cycle. It consists of
4 main phases:
Mitosis
and cytokinesis are in M phase.
Two “Gap” phases where the cell grows. They
are called G1 phase and G2 phase.
DNA replication occurs in S phase.
Events of the Cell Cycle
 First,
in G1 phase, cells grow and make new
organelles and proteins.
 Second, in S phase, the chromosomes are
replicated. Usually a cell proceeds to division if it
enters S phase.
 Third, in G2 phase, many of the organelles and
molecules needed for cell division are produced.
It
 All
is the shortest of the 4 phases.
three of these phases take place in Interphase.
Mitosis
The fourth stage is mitosis. It consists of 5
stages:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
After
these cytokinesis occurs.
 In
prophase, the DNA condenses into
chromosomes. The centrioles separate and a
spindle begins to form.
 In
prometaphase the nuclear envelope
breaks down and the centrioles begin to stretch
out their spindles to find the chromosomes.
 In
metaphase the chromosomes line up in the
middle of the cell and a spindle fiber connects to
each centromere.
 In
anaphase the sister chromatids are pulled
apart towards opposite ends of the cells.
 In
telophase the chromosomes gather at
opposite ends of the cell and revert to their usual
form (chromatin). Two new nuclear envelopes begin
to form.
Totally lame mitosis song!
Cytokinesis
 The
cytoplasm pinches in half and two daughter
cells are formed. Each daughter cell has a
duplicate set of chromosomes.
In
animal cells the cell membrane pinches inward until
two equal parts are formed.
In
plant cells a structure known as a cell plate forms
between the nuclei and develops into a separating
membrane. The cell wall then forms and spreads
outwards.
Controlling the Cell Cycle

Proteins called cyclins regulate the timing of the cell cycle in
eukaryotic cells.

They can be divided into two groups:
 Internal
Regulators are proteins that respond to events inside of the cell. For
example: making sure that the cell does not proceed to G2 phase or mitosis
unless all of the chromosomes have been replicated.
 External
Regulators are proteins that respond to events outside of the cell.
For example: if there is not enough space outside of a cell for others to
exist, the cell may not enter or proceed past G1 phase.
 This
is also why when a scab forms over a cut it does not rise much above
the surface of the skin. It limits new cells to within a certain distance of the
others.
G0 phase
 G0
phase is when a cell is in a dormant stage. This
occurs for some cells such as heart or nerve cells
which are not replicated and typically survive for a
long time.
 It can also occur for cells that do not have enough
resources to enter cell division.
 It typically occurs before cytokinesis or after G1
depending on the type of cell.
Uncontrolled Cell Growth
 In
uncontrolled cell growth, the checkpoints are bypassed
or do not work correctly and the cells grow and divide
whether or not they are ready.
 Cancer
is when some of a body’s cells lose the ability to
control growth.
 The
masses of cells formed by the uncontrolled growth are called
tumors. These can damage surrounding tissues.
 When
cancer metastasizes, cancer cells break loose from the
tumor and spread throughout the body affecting other areas of
cells.
 The
loss of control can be caused by many different
factors depending on the type of cancer such as:
 Tobacco
 Radiation
 Viral
Exposure
Infections
 Damage
to the p53 gene which halts the cell cycle until all the
chromosomes have replicated.
 Tumors
can be:
 Benign:
not usually caused by cancer. It does not spread to
surrounding healthy cells or tissue.
 Malignant:
cancerous cells that can invade and destroy
surrounding healthy cells or tissue.