Download 3. Mitosis

MITOSIS
Mitosis is normal cell division,
which goes on throughout life in
all parts of the body.
• 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 a process of cell division which
results in the production of two daughter
cells from a single parent cell.
• The daughter cells are identical to one
another and to the original parent cell.
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Machinery
of
Mitosis
The chromosomes are pulled
apart by the spindle, which is
made of microtubules.
The spindle fibers are attached
to each centromere(which is part
of the chromosome), and
anchored on the other end to a
centrosome( centriole).
There are 2 centrosomes, one at
each end of the spindle.
The Chromosomes are lined up
between the poles of the spindle
fibers.
When the spindle fibers contract,
the Chromosomes are pulled to
the opposing poles.
The cell then divides to separate
the two poles.
Interphase
• Interphase is the phase of the cell cycle in which the
cell spends the majority of its time and performs the
majority of its purposes including preparation for cell
division(like mitosis).
• Interphase is considered to be the 'living' phase of the
cell, in which the cell obtains nutrients, grows, reads
its DNA, and conducts other "normal" cell functions.
• Chromosomes are not clearly distinguished in the
nucleus, although a dark spot called the nucleolus
may be visible.
• The cell may contain a pair of centrioles .
• Stages of interphase includes G1, S, G2 phases
Interphase
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Interphase
stages
In a typical animal cell, mitosis can be
divided into four principals stages. They
are:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Prophase
• When mitosis begins, the chromatin of the
chromosome becomes gradually more and more
coiled so that the chromosome becomes
recognizable ( condensation) as a thread-like
structure that eventually acquires a rod-like
appearance.
• Towards the end of prophase, the two
chromatids constituting the chromosome
become distinct and the chromosome now has a
typical structure.
• While these changes are occurring in the
chromosomes, a number of other events
are also taking place.
• The nucleolus breaks down and
• The two centrioles separate and move to
opposite poles of the cell.
• They produce a number of microtubules
that pass from one centriole to the other
and form a spindle.
• At the end of prophase, the nuclear
envelope breaks down into vesicles.
• Centromeres captures spindle fibers
Prophase
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Prophase
Metaphase
• With the formation of the spindle,
chromosomes move to a position midway
between the two centrioles ( Equator)
Metaphase
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Metaphase
Anaphase
• The centromere of each chromosome
splits longitudinally into two so that the
chromatids now become independent
chromosomes.
• At this stage, the cell can be said to
contain 46 chromosomes.
• One chromosome of each such pair now
moves along the spindle to opposite poles
of the mitotic spindle.
Anaphase
• At this point, each individual chromosome goes from:
– 1 chromosome with 2 chromatids to:
– 2 chromosomes with one chromatid each.
Anaphase
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Anaphase
Telophase
• The two daughter nuclei are formed by
appearance of nuclear membranes.
• Chromosomes gradually elongate and
become indistinct.
• Nucleoli reappear.
• The centriole is duplicated at this stage or
in early interphase
Telophase
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Telophase
Cytokinesis
• Cytokinesis is the process when the
cytoplasm of a single eukaryotic cell is
divided to form two daughter cells.
• It is usually initiated during the late stages
of mitosis, and sometimes meiosis,
splitting a binucleate cell in two, to ensure
that chromosome number is maintained
from one generation to the next.
• The cytoplasm divides, the cell membrane
pinches inward ultimately producing two
daughter cells .
Cytokinesis
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Cytokinesis
Summary of Mitosis
• Prophase:
• Chromosomes condense
• Nuclear envelope disappears
• centrosomes move to opposite sides of the cell
• Spindle forms and attaches to centromeres on the
chromosomes
• Metaphase
• Chromosomes lined up on equator of spindle
• centrosomes 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
• Cytokinesis: the cytoplasm is divided into 2 cells
Cancer
• 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.
– Normal cells 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.
– Normal cells are mortal. This means that they can divide about 50
times and then they lose the ability to divide, and eventually 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.
– Normal 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
• 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
• 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: it’s hard for a cell to develop resistance to several
drugs at the same time.