Cell Division by Mitosis
• all living things start
life as a single cell
• Growth results
when your body
forms new cells.
• New cells are
formed by cell
division
• There are two types of cell division
1. mitosis (division of body cells)
2. mieosis (division of sex cells)
• two cells are formed
from one cell
• new cells are
similar to old cell
• the new cells
replace old cells
• root cells, skin cells,
muscle cells are
examples
• - for many simple
organisms (like
bacteria and some
algae) mitosis =
reproduction
• mitosis is NOT a one step process
• as a cell divides some of the cell parts
change
• it forms two new cells. The number of
cells increases every time cells divide.
• the number of chromosomes in each
cell remains the same
Interphase (resting)
• “resting” describes
cells that have not
started to divide
• occurs between cell
divisions
• longest part of the
cell cycle
• chromatin is coiled
DNA inside the
nucleus,
– Later,chromatin
condenses to form
chromosomes
• during interphase,
the DNA is
replicated (copied)
and the cell
prepares for division
Interphase in an Onion Root Tip
Prophase (mitosis begins)
• during this phase the
chromatin condenses to
form chromosomes
• The original
chromosome and the
copied chromosome
form a pair, called a
chromatid.
• pair of sister chromatids
= chromosome
• joined together at a
central point called
the centromere
• the membrane
surrounding the
nucleus begins to
disappear
Prophase in an Onion Root Tip
Metaphase
• membrane
surrounding the
nucleus totally
disappears
• sister chromatids
line up in the middle
of the cell
(metaphase plate)
Metaphase in an Onion Root Tip
Anaphase
• chromatids separate
at the centromere
and move to
opposite ends of the
cell
– Separate using
spindle fibers
• once separated, the
chromatids are now
identical daughter
chromosomes
Anaphase in Onion Root Tip
Telophase
• nucleus forms
around both sets of
daughter
chromosomes
• cell splits and two
new cells are
formed (cytokinesis)
Telophase in an Onion Root Tip
• a cell that divides too fast can form a tumor
• cells divide too fast because of mutations
(changes) in the cells DNA
- mutations can occur because of
1. genetics
2. environmental factors
- UV radiation from sun
- cigarette smoke
The Cell Cycle and Cancer
The Stages of the Cell Cycle
1. Click on picture for cell cycle animation –
will go to www.cancerquest.org)
2. Use alt-tab keys to go between website
and power point presentation.
3. Click on blank space to proceed to next slide.)
There are several factors that regulate the cell
cycle and assure a cell divides correctly.
1.Before a cell divides,
the DNA is checked
to make sure it has
replicated correctly.
(If DNA does not copy
itself correctly, a gene
mutation occurs.
DNA replication animation:click on
DNA picture
2. Chemical Signals tell a cell when to start
and stop dividing.
(Target cells animation: click on go
sign)
Neighboring cells communicate with dividing
cells to regulate their growth also.
(Normal contact inhibition animation: click on petri dish)
Cancer is a disease of the cell cycle. Some of
the body’s cells divide uncontrollably and
tumors form.
Tumor in Colon
Tumors in Liver
DNA mutations disrupt the cell cycle.
Mutations may be caused
by:
1. radiation
2. smoking
3. Pollutants
4. chemicals
5. viruses
While normal cells will stop dividing if there is a mutation in
the DNA, cancer cells will continue to divide with mutation.
Due to DNA mutations, cancer cells ignore the
chemical signals that start and stop the cell cycle.
2 animations of cancer cells dividing: click on picture
Due to DNA mutations, cancer cells cannot
communicate with neighboring cells. Cells continue
to grow and form tumors.
Skin cancer
(cancer cells dividing: click on
picture.)
SUMMARY
Normal Cell Division
Cancer Cells
DNA is replicated
properly.
2. Chemical signals start and
stop the cell cycle.
3. Cells communicate with
each other so they don’t
become overcrowded.
1. Mutations occur in the
DNA when it is
replicated.
2. Chemical signals that start
and stop the cell cycle are
ignored.
3. Cells do not communicate
with each other and
tumors form.
1.
Treating Cancers
Cancer treatments include drugs that can stop cancer cells
from dividing.
• 1. In mitosis, how many cells are formed from
each dividing cell?
• 2. What is copied in the nucleus before the
cells divide?
• 3. If a cell has 46 chromosomes and divides
by mitosis, how many chromosomes will each
new cell have?