CHAPTER 11
THE REPRODUCTION
OF
CELLS
Mitosis

Cells divide to make more cells.
While all the other organelles can
be randomly separated into the
daughter cells, the chromosomes
must be precisely divided so that
each daughter cell gets exactly
the same DNA.
 Mitosis is normal cell division,
which goes on throughout life in all
parts of the body. Meiosis is the
special cell division that creates
the sperm and eggs, the gametes.
 Mitosis and meiosis occur in
eukaryotes. Prokaryotes use a
different method—”fission” to
divide.
 Humans have 46 chromosomes,
23 from each parent. Every cell
has the same 46 chromosomes
Each species has a characteristic
number of chromosomes: corn
ahs 20, house flies have 10,
chimpanzees have 48.
Chromosomes

The essential part of a
chromosome is a single very long
strand of DNA. This DNA
contains all the genetic
information for creating and
running the organism.
 The DNA is packaged by proteins
bound to it. At different times,
these proteins cause the DNA to
be spread out like spaghetti in a
bowl, or tightly condensed into the
X-shaped chromosomes we can
see in the microscope.
 Chromosomes have


Chromatids
Centromere
More Chromosomes
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Before replication,
chromosomes have one
chromatid.
After replication, chromosomes
have 2 sister chromatids, held
together at the centromere
In mitosis, the two chromatids
of each chromosome separate,
with each chromatid going into
a daughter cell.
Remember that diploid cells
have two copies of each
chromosome, one from each
parent. These pairs of
chromosomes are NOT
attached together.
Cell Cycle
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Some cells divide constantly: (cells in the embryo, skin cells, gut lining cells, etc.)
Other cells divide rarely or never: only to replace themselves.
Actively dividing cells go through a cycle of events that results in mitosis. Most of
the cycle was called “interphase” (the cell increases in size, but the chromosomes
are invisible.)
The 3 stages:
*G1 (“Gap”) is the period between mitosis and S, when each chromosome has 1
chromatid. It is the time when the cell grows and performs its normal function. The
*S phase (“Synthesis”) is the time when the DNA is replicated, when the
chromosome goes from having one chromatid to having 2 chromatids held
together at the centromere.
*G2 is the period between S and mitosis. The chromosome have 2 chromatids,
and the cell is getting ready to divide.
Machinery of Mitosis

The chromosomes are pulled
apart by the spindle, which is
made of microtubules. They
attach to each centromere, and
anchored on the other end to
centrioles.
 There are 2 centrioles, one at
each end of the spindle. The
chromosomes are lined up
between the poles of the spindle.
 When they contract, the
chromosomes are pulled to the
opposing poles.
Prophase
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In prophase, the cell begins
the process of division.
1. The chromosomes
condense (long & thin to
short & fat).
2. The nuclear envelope
disappears.
3. The centrioles move to
opposite poles. During
interphase, the pair of
centrioles were together just
outside the nucleus.
4. The spindle starts to
form, growing out of the
centrioles towards the
chromosomes.
Kinetochore vs Nonkinetochore

Kinetochore = protein
structure located at
centromere regions
 Kinetochore microtubules
= attach to the
kinetochores!! (PULL
CHROMOSOMES)
 Nonkinetochore
microtubules = overlap
with nonkinetochore
microtubules from the
opposite pole. (ELONGATE
CELL)
Metaphase


chromosomes line up on
the equator (metaphase
plate) of the cell, with the
centrioles at opposite
ends and the spindle
fibers attached to the
centromeres.
Entire structure –
nonkinetochore
microtubules +
kinetochore microtubules
= spindle
Anaphase

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In anaphase, the
centromeres divide.
At this point, each
chromosome goes from
having 2 chromatids to
being 2 chromosomes,
each with a single
chromatid.
Then the spindle fibers
contract, and the
chromosomes are pulled
to opposite poles.
Telophase
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Nonkinetochore
microtubules elongate the
cell
The spindle disintegrates
The nuclear envelope reforms around the two sets
of chromosomes.
Daughter nuclei begin to
reform
The cytoplasm divides
into 2 separate cells.
Cytoplasmic Division
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The organelles (other than the
chromosomes) get divided up into the
2 daughter cells passively: they go
with whichever cell they find
themselves in.
Plant and animal cells divide the
cytoplasm in different ways.
In plant cells, a new cell wall made of
cellulose forms between the 2 new
nuclei, about where the chromosomes
lined up in metaphase. Cell
membranes form along the surfaces of
this wall. When the new wall joins with
the existing side wall, the 2 cells have
become separate.
In animal cells, a ring of actin fibers
(microfilaments are composed of actin)
forms around the cell equator and
contacts, pinching the cell in half.
Summary of Mitosis
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Prophase:
•
•
•
•
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Chromosomes condense
Nuclear envelope disappears
Centrioles move to opposite sides of the cell
Spindle forms and attaches to centromeres on the chromosomes
Metaphase
• Chromosomes lined up on equator of spindle
• Centrioles at opposite ends of cell

Anaphase
• Centromeres divide: each 2-chromatid chromosome becomes
two 1-chromatid chromosomes
• Chromosomes pulled to opposite poles by the spindle

Telophase
• Chromosomes de-condense
• Nuclear envelope reappears
• Cytoplasm divided into 2 cells
PLANT vs ANIMAL

PLANT CELL
* Cell plate
ANIMAL CELL
* Cleavage furrow
REGULATION OF THE CELL
CYCLE

GROWTH FACTORS: occur in the presence of a
wound – cells respond and grow
 DENSITY-DEPENDENT INHIBITION: Cells grow
depending on the density of cells.
(nutrients/space/adhesion)
 RESTRICTION POINT: go/no go decision. If all
systems are “go” (all external and internal
conditions are favorable), the step proceeds.
 G0 phase = nondividing state.
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Cyclins – concentrations fluctuate
cyclically
Cdk complex called MPF (maturation
promotion factor (M-phase promoting
factor)
Near end of M-phase, MPF switches
off by activating an enzyme that
destroys cyclin.
1. cyclin is synthesized through the
cycle and accumulates during
interphase
2. cyclin attaches to Cdk and the
protein complex is activated at the
end of interphase
3. MPF coordinates mitosis by
phosphorylating varous proteins,
including other protein kinases
4. MPF is a cyclin-dependent kinase
enzyme that destroys MPF activity.
The Cdk component of MPF is
recycled, its kinase activity restored
by association with new cyclin that
accumulates during interphase.
Prokaryotic Cells: Binary Fission

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1.
2.
3.
“Division in half”
Most bacterial genes are carried on a
single, circular chromosome &
associated proteins.
Replicates
Attaches to different parts of the cell
membrane
Cell pulls apart and the replicate and
original chromosomes are separated.
Cancer
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Cancer is a disease of uncontrolled cell division. It starts with a
single cell that loses its control mechanisms due to a genetic
mutation. That cell starts dividing without limit, and eventually kills
the host.
Normal cells are controlled by several factors. They stay in the G1
stage of the cell cycle until they are given a specific signal to enter
the S phase, in which the DNA replicates and the cell prepares for
division. Cancer cells enter the S phase without waiting for a signal.
Another control: normal cells are mortal. This means that they can
divide about 50 times and then they lose the ability to die. This
“clock” gets re-set during the formation of the gametes. Cancer cells
escape this process of mortality: they are immortal and can divide
endlessly.
A third control: cells that suffer significant chromosome damage
destroy themselves due to the action of a gene called “p53”. Cancer
cells either lose the p53 gene or ignore its message and fail to kill
themselves.
Cancer Progression
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There are many different forms of cancer, affecting
different cell types and working in different ways. All
start out with mutations in specific genes called
“oncogenes”. The normal, unmutated versions of the
oncogenes provide the control mechanisms for the
cell. The mutations are caused by radiation, certain
chemicals (carcinogens), and various random events
during DNA replication.
Once a single cell starts growing uncontrollably, it
forms a tumor, a small mass of cells. No further
progress can occur unless the cancerous mass gets
its own blood supply. “Angiogenesis” is the process of
developing a system of small arteries and veins to
supply the tumor. Most tumors don’t reach this stage.
A tumor with a blood supply will grow into a large
mass. Eventually some of the cancer cells will break
loose and move through the blood supply to other
parts of the body, where they start to multiply. This
process is called metastasis. It occurs because the
tumor cells lose the proteins on their surface that hold
them to other cells.
Cancer Treatment
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Two basic treatments: surgery to remove the tumor, and radiation or
chemicals to kill actively dividing cells.
It is hard to remove all the tumor cells. Tumors often lack sharp
boundaries for easy removal, and metastatic tumors can be very
small and anywhere in the body.
Radiation and chemotherapy are aimed at killing actively dividing
cells, but killing all dividing cells is lethal: you must make new blood
cells, skin cells, etc. So treatment must be carefully balanced to
avoid killing the patient.
Chemotherapy also has the problem of natural selection within the
tumor. If any of the tumor cells are resistant to the chemical, they
will survive and multiply. The cancer seems to have disappeared,
but it comes back a few years later in a form that is resistant to
chemotherapy. Using multiple drugs can decrease the risk of
relapse.
MITOSIS LAB