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Mitosis
A little Vocabulary . . .
• Mitosis: division of the nucleus in autosomal
cells
– Autosomal Cells:
all cells except sex cells
More Vocabulary
• Meiosis: division of nucleus to
produce gametes in sex cells
– Sex Cells: cell that produce gametes
Ex: sperm/egg
• Chromotid: one of the 2 parts of
a duplicated chromosome.
”sister chromatids”
Vocab continued
• Chromatin: granular substance
in the
nucleus
*contains DNA and proteins called histones
• Chromosome: threadlike structure contains genetic
information
– Condense to from X-shaped structures
• Centromere: central part of chromosome where
chromatids attach
Genes and Proteins
• Proteins do the work of the cell:
– growth, maintenance, response to the environment,
reproduction, etc.
• Proteins are chains of amino acids.
– The sequence of amino acids in each protein is coded in the
DNA as a specific sequence of A, C, G and T bases: a gene.
• Each gene codes for a different protein.
Genes and Proteins
Continued…
• Key points:
– All cells in an organism have
the same genes.
– What makes cells different
from each other is that
different genes are turned on
and turned off in different
cells.
• The DNA must be copied
and then divided exactly so
that each cell gets an
identical copy.
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.
•
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.
Mitosis and Meiosis!
• 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. We
will discuss meiosis
separately.
• Mitosis and meiosis occur in
eukaryotes.
• Prokaryotes use a different
method—”nuclear fission” to
divide.
Chromosomes
•
Chromosomes are very long
strands of DNA.
•
contain genetic information
–
•
DNA takes 2 forms:
–
–
•
Needed to create and run an organism.
spread out like spaghetti in a bowl
proteins cause the DNA to become
tightly condensed into an X-shaped
chromosome
Each chromosome has a central
constricted region called a
centromere that serves as an
attachment point for the
machinery of mitosis.
More Chromosomes
• Chromosomes exist in 2
different states:
– Before DNA replication
•
chromosomes have one
chromatid.
– After DNA replication
• chromosomes have 2
sister chromatids, held
together at the
centromere.
• Each chromatid is one
piece of DNA with its
supporting proteins.
• In mitosis, the two
chromatids of each
chromosome separate,
with each chromatid
going into a daughter cell.
• Remember! diploid cells have
two copies of each
chromosome, one from each
parent. These pairs of
chromosomes are NOT
attached together.
Cell Cycle
• Some cells divide constantly:
– cells in the embryo, skin cells, gut lining
cells, etc.
• Other cells divide rarely or neveronly to replace themselves.
• Actively dividing cells go through a
cycle of events that results in mitosis.
•
Most of the cycle is called
“interphase”.
– During interphase the cell increases in
size, but the chromosomes are invisible.
• The 3 stages of interphase are called
G1, S, and G2.
Interphase
•
G1 (“Gap”) is the period between
mitosis and S, when each
chromosome has 1 chromatid.
–
–
•
•
The S phase (“Synthesis”)
DNA replication:
–
•
Cells spend mot of their time in G1.
Growth and normal funtion
the chromosome goes from having one
chromatid to having 2 chromatids held
together at the centromere.
G2 (“Gap”) is the period between S
and mitosis.
–
The chromosome have 2 chromatids, and
the cell is getting ready to divide.
Machinery of Mitosis
• Chromosomes are pulled apart by
spindle fibers, which are made of
microtubules.
1. Spindle fibers are attached to each
centromere (a part of the chromosome), and
anchored on the other end to a centrosome
2. There are 2 centrosomes, one at each end of
the cell.
3. The chromosomes are lined up in the middle
4. When the spindle fibers contract, the
chromosomes are pulled to the opposing
poles.
Stages of Mitosis!
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
• Just remember: PMAT!
Prophase
Division Begins!
1. The chromosomes condense.
The proteins attached to the DNA cause the
chromosomes to go from long thin structures to
short fat one, which makes them easier to pull
apart.
2. The nuclear envelope disappears.
3. The centrosomes move to opposite
poles.
During interphase, the pair of centrosomes were
together just outside the nucleus. In prophase
they separate and move to opposite ends of
the cell.
4. The spindle starts to form, growing out of
the centrosomes towards the
chromosomes.
Metaphase
• Chromosomes are lined
up on the equator
• The centrosomes are at
opposite ends and the
spindle fibers attached to
the centromeres.
•
Everything is aligned for
the rest of the division
process to occur.
Anaphase
• Centromeres divide.
• Each individual
chromosome goes from:
1 chromosome with 2
chromatids
to
2 chromosomes with one
chromatid each.
• Spindle fibers contract,
and the chromosomes
are pulled to opposite
poles.
Telophase
• In telophase the nucleus
actually divides.
• The chromosomes are at
the poles of the spindle.
• The spindle disintegrates
• The nuclear envelope reforms around the two sets
of chromosomes.
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
Cytokinesis
•
Organelles division
•
NOT a part of mitosis!!!!
•
Different in plants and animals!
•
In plant cells
– a new cell wall made of cellulose
forms between the 2 new nuclei,
– 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 forms around
the cell equator and contacts,
pinching the cell in half.
Cancer
• uncontrolled cell division.
• It starts when 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
– Mutations in genes called ONCOGENS
– The normal oncogenes provide control
mechanisms for the cell.
– mutations are caused by radiation, certain
chemicals 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
metastasize- they break loose and move through
the blood supply to other parts of the body, where
they start to multiply.
– It occurs because 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.