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Unit #5 Cell Cycle
Chapter 9 Pages 242-257
Introduction
 Many events in nature occur in cycles: the
seasons, the tides, etc. Living organisms are
no exception. One cycle common to most
living things is the cell cycle. As a cell goes
through this cycle, it will pass through two
periods: a period of growth and a period of
division. This unit will be divided into these
corresponding sections—here we go!
Part #1 Interphase
I.
The Cell Cycle
A. A cell’s life follows the cell cycle. The
cell cycle can be depicted in Figure 1. The
cell cycle is the sequence of growth. DNA
replication, growth, and cell division are
events that all cells go through.
 Figure 1
1. Interphase
a. Interphase can be divided into 3 phases:
i. G1 phase At the beginning of interphase, the
cells are brand new. They are very small and do
not have very much chemical energy (ATP).
During this phase, the cell grows rapidly and
makes ATP. Many metabolic activities are
occuring. For example, proteins are being made
and enzymes are being produced. Of the 3
phases of interphase, the G1 phase lasts the
longest. G stands for growth.
1. Interphase
ii. S phase After acquiring sufficient energy,
the cell enters into the S phase of interphase.
During this phase, the cell copies or
replicates its chromosomes (DNA). This
process takes a lot of energy in the form of
ATP. This must be done so the cell can
divide. The S stands for synthesis.
1. Interphase
iii. G2 phase Because the cell used a lot of its
energy in the S phase as it duplicated its
chromosomes, the cell undergoes a second
growth and energy-producing phase called
the G2 phase. The energy made during this
phase is used in the upcoming mitosis.
During this time, mitochondria and other
organelles are made. Cell parts (such as the
centrioles that are needed for cell division)
are made at this time. Chromosomes
become tightly coiled.
1. Interphase
b. Some cells have a cell cycle that lasts for 20
minutes (bacteria), while other cells have a
cell cycle that lasts for 87 hours. Others,
such as nerve cells, lose their ability to divide
once they reach maturity. Liver cells maintain
their ability to divide, but do not normally do
so unless part of the liver is removed. Then
the liver cells will divide until the liver reaches
its former size.
II. Cell Size Limitations
A. A cell spends a good amount of time in
interphase getting the cell ready to undergo
mitosis/cell division.
B. Why does a cell need to divide? Why can’t it
just keep growing until it is 14 ft tall and 5 ft
wide?
1. Most cells are between 2 and 200
microns in diameter. A blood cell is pretty
small, with a diameter of just 8 microns.
Nerve cells can be much larger—up to 1
mm, but they have very small diameters.
The cell with the largest diameter is the yolk
of an ostrich egg—8 cm!
2. Diffusion of nutrients into the cell and
waste out of the cell limit cell size. The cell
membrane allows a steady supply of nutrients
such as oxygen and glucose to enter the cell.
Glucose has to enter the cell and travel to the
mitochondria, where it can then be converted
to energy. As long as the distance that
something (like glucose) has to travel is short,
diffusion is an effective way for things to get
from one place to another in and around the
cell.
Let’s say, however, that we had a cell with a
diameter of 20 cm (about 8 inches)—
basically, a really big cell! It would actually
take months for glucose and oxygen to reach
a mitochondria at the center of the cell—the
cell would die if it took nutrients that long to
reach the appropriate organelles!
This point can be illustrated in Figure 2. Notice
how the surface area (the size of the cell’s
surface) does not increase at as fast of a rate
as the volume (inside) of the cell.
 Figure 2:
3. DNA limits cell size. We have discussed
how the DNA contains the blueprints for
which proteins should be made in the cell.
There is a limit as to how quickly the
blueprints for these proteins can be copied in
the nucleus and made on the ribosomes.
The cell cannot survive unless there is
enough DNA to support the protein needs of
the cell. Some cells that are big have more
than one nucleus in order to help with protein
production.
Questions to answer:
 On the back of your notes paper, please
record and answer the following:
 In the U.S., 1 in ____ people develop cancer
at some point in their lives.
 The number of people that I have known with
cancer = _____
 Types of cancers that I’ve heard of:
III. Control of the Cell Cycle
A. Normal control of the cell cycle
1.
2.
Certain enzymes control the cell cycle.
Which enzymes get made in the cell is
controlled by the DNA (located on the
chromosomes in the nucleus). A certain
segment of DNA that codes for a certain
enzyme or any type of protein is called a
gene.
B. A mistake in the cell cycle may lead to cancer
1. Cancer can be the result of a change to one or more
genes that code for enzyme(s) that are involved in
controlling the cell cycle. When the cycle of cell
division is not controlled, cells divide more rapidly.
Cancer cells are rapidly dividing cells that can
eventually form masses of tissue called tumors.
These tumors (if not treated or if treatment does not
work) cause other vital organs and organ systems
not to work; this can lead to death.
2. Cancer is the second leading cause of
death in the U.S.—it is second only to heart
disease.
a. In the U.S., breast, colon, prostate,
and lung cancer are the most common
types of cancer.
3. What causes cancer?
a. Environmental factors:
i. Different people, different countries
= different types of cancer at
different rates
ii. Breast = high in U.S., low in Japan
Stomach = low in U.S., high in Japan
iii.Smoke, air & water pollution, UV radiation
cause genes to be damaged
or mutated, causing cancer
b. Genetic factors:
i. A small number are hereditary…People
who inherit specific gene mutations
have a higher risk of developing certain
forms of cancer as compared to the
general population.
4. How can cancer be prevented?
a. A healthy lifestyle!
i. A diet that is low in fat and high in
fiber (lots of fruits and veggies)
ii. No tobacco
iii.Vitamins (A, C, E, Ca, carotenes)
Part #2--Mitosis
I. Mitosis defined
A. Mitosis is another word for cell division.
When a cell divides, it is a continuous
process. During mitosis, things are
constantly moving to the two ends of the
cell. However, mitosis can be divided
into 5 distinct phases.
B. Mitosis is a type of cell division in which
the number of chromosomes in the 2
resulting daughter cells is the same as
the number and kind of chromosomes in
the original parent cell. For example, if
the parent cell started out with 46
chromosomes, then after mitosis, each
daughter cell would have 46 chromosomes. How
many chromosomes does every human cell
(except egg and sperm cells) have?
II. Phases of Mitosis
A. Refer to the cell cycle drawing. Does a cell
spend more of its time in interphase or
mitosis?
B. Why does a cell divide (undergo mitosis)?
 So, it is pretty important that a cell undergoes
mitosis or it will die. What if we could stop the
process of mitosis in cancer cells—would that
be good? YES! One cancer-fighting drug
used right now is taxol.
C. The 5 phases of mitosis
1. Phase #1 = Prophase
a. First phase
b. Longest phase
c. Prophase occurs right after the G2 phase
of interphase is complete, so the chromosomes
have duplicated themselves. Each chromosome
is attached to its duplicate with a structure called
a centromere. The original chromosome and its
duplicate are called sister chromatids.
d. What happens during prophase?
1. The chromosomes begin to coil up.
2. At the beginning of prophase, the nuclear
membrane begins to disintegrate; by the end
of prophase, it is totally gone.
3. The nucleolus disappears.
4. In animal cells, centrioles begin to move the
poles of the cell.
5. The spindle fibers begin to form. These are
structures that will help the chromosomes
move to the opposite ends of the cell.
2. Phase #2 = Metaphase
a. Second phase
b. What happens during metaphase?
1. The doubled chromosomes become attached
to the spindle fibers
2. The chromosomes are pulled by the spindle
fibers and line up on the equator
3. The chromosomes line up such that each new
cell will receive an identical set of
chromosomes that is identical to the parent
cells.
4. Asters (in animal cells) become more visible.
3. Phase #3 = Anaphase
a. Third phase
b. What happens during anaphase?
1. The chromosomes separate from each
other. They are pulled apart from each
other and move towards the poles by
the shortening of the spindle fibers.
4. Phase #4 = Telophase
a. Fourth phase
b. What happens during telophase?
1. As the chromosomes reach the opposite poles
of the cell, telophase begins.
2. The chromosomes begin to uncoil.
3. The spindle fibers begin to break down, as they
are no longer needed.
4. The nucleolus reappears.
5. A new nuclear membrane forms around each
new set of chromosomes as they sit at the poles.
5. Phase #5 = Cytokinesis
a. ‘Cyto’ stands for cytoplasm, and ‘kinesis’ means
movement.
b. What happens during cytokinesis?
1. Equal amounts of cytoplasm and the organelles
within it move to each of the two opposite halves
of the cell. In an animal cell, a furrow will form
as the cleavage furrow pinches across the cell.
In a plant cell, a cell plate is formed across the
equator. A new cell wall will form on each side
of the cell plate.
2. The two new cells are called daughter cells.
They are very small.