CELL DIVISION
According to the cell theory: all cells come from
preexisting cells. Why, when and how cells divide is the
subject of this unit.
Reproduction
Cells reproduce in order to make new cells.
A bacterium divides to
become two bacteria.
Growth
An organism gets larger because its cells reach a certain size
and then divide.
Each one of us started out as a single cell no bigger than the
period at the end of this sentence.
Repair and Replacement
Cells divide in order to repair or replace worn out or
damaged cells.
Young cells (human)
Old cells (human)
Here’s Kim Kardashian after an unfortunate sun tanning
incident. poor Kim fell asleep with her sunglasses on after
tanning and woke up looking like this. how many skin cells
did Kim damage….?
Lucky for Kim, cells divide when damaged or need to be replaced.
The damaged cells of a skinned
knee will divide quickly
Patrick Robinson’s Injured hamstring
will heal as a result of the muscle
and tendon cells diving.
What causes a Cell to Divide?
Cells grow to a certain size and then divide.
Bozeman Science Video
The efficiency of moving materials into the cell
decreases as the cell becomes larger.
The volume of the cell increases faster then its
surface area.
Cells that grow too large no longer have enough
surface area to take in nutrients and remove waste
Chromosomes - Video
A chromosome is DNA in a coiled, rod-shape form that
occurs during cell division.
The term chromosome originates from the Greek word “chroma” which means
color and “soma” meaning body. It is organized into a rodlike body of a tightly
coiled chromatin and visible only during cell division. It is a single twisted DNA
structure containing thousands of genes that makes up the blueprint of living
organisms.
Every species has a characteristic number of chromosomes in each cell.
Humans have 46 chromosomes or 23 pairs. The number of chromosomes
varies among species but is not an indication of the complexity of the
species.
FRUIT FLIES: 8
CARROTS: 20
HUMANS: 46
FERN: 480
GOLDFISH: 94
TURKEY: 82
POTATO: 48
PLUM: 48
CHIMPANZEE: 48
MITOSIS
Mitosis is the division of the cell nucleus in which the
chromosomes in the parent cell divide into sets. two identical
In mitosis, the number of cells increases without changing
the genetic material. Thus, mitosis is like making a
photocopy.
When injury occurs such as a scrape or burn, new cells grow
to replace the damaged ones-this happens as a result of
mitosis.
Mitosis occurs for the purpose of growth and replacement of
worn out or damaged cells.
Mitosis is the part of cell division that divides the
nuclear material of the cell.
Each resulting cell will have the same complete
set of chromosomes.
Mitosis takes place in non-sex cells also known as
body cells or somatic cells. Khan Academy Video
The Stages of mitosis in a plant cell
Controls on Cell Division
• Effects of controlled cell growth can be seen by
placing some cells in a petri dish containing nutrient
broth
• Cells grow until they form a thin layer covering the
bottom of the dish
• Cells stop growing when they come into contact with
other cells
• If cells are removed, the remaining cells will begin
dividing again
• Something can turn cell division on or off
Regulating Cell Growth
• Cyclins- proteins that regulate the timing of
the cell cycle in eukaryotic cells
– Internal regulators: proteins that respond to
events inside the cell
• i.e. make sure all chromosomes have been replicated;
make sure all chromosomes are attached to the spindle
before entering anaphase
– External regulators: proteins that respond to
events outside the cell
• i.e. embryonic development; wound healing
Uncontrolled Cell Growth
• Cancer- disorder in which some of the body’s own
cells lose the ability to control growth
• Cancer cells do not respond to the signals that
regulate the growth of most cells
• P53 gene halts the cell cycle until all chromosomes
have been properly replicated
– A damaged or defective p53 gene causes the cells to lose
the information needed to respond to signals that would
normally control their growth
Why this is important
• About 1 in 4 of us get cancer, 1 in 5 die of it- at
least.
• It’s very much a disease that can be
understood at the molecular level.
• Our understanding has not resulted in the
cures we’d like
No particular
relevance- painted by
Van Gogh in the
1880’s!
Cancers vary in terms of
getting a type of cancer and
dying from that type
Data 2000-2003
Basic Properties of cancer cells
Video
• Uncontrolled- grow at the usual rate, but then
keep growing when other cells would
normally stop- ignore stopping signals, or
grow w/o added signals.
• Invasive
• Immortal- normal cells undergo senescencetelomerase is 1 factor.
• Chromosomal abnormalities- aneuploid
Well-behavedstop growing
when they
cover the dish.
Not wellbehaved- keep
growing after
they cover the
dish.
INTERPHASE
Interphase looks like the resting stage, but inside
the nucleus a lot is happening! The nucleus makes
a copy of its genetic material (DNA) and the cell
gets bigger so it can divide.
PROPHASE
The DNA strands coil up to form thick rope-like chromosomes that you can see with
a light microscope. The nuclear membrane begins to disintegrate (break down).
In animal cells, spindle fibers form and are stretched across the cell by centrioles.
Centrioles are the parts of the cell that produce spindle fibers that the chromosomes
attach to when they move to opposite ends of the cell. Plant cells do not have
centrioles.
METAPHASE
Chromosomes line up in the middle of the cell.
Can you see the spindle fibers? What do they do?
Spindle fibers are what chromosomes attach to in order
to move to opposite ends of the cell.
ANAPHASE
The spindle fibers shorten and pull the centromeres apart. This causes the
chromosomes to move to opposite sides of the cell.
Anaphase ends when the chromosomes reach opposite end of the cell.
Centromeres are what hold the chromosomes together.
TELOPHASE
The spindle fibers disappear and the nuclear membrane reforms
around each new set of chromosomes. Chromosomes start to uncoil
and form strands of DNA. Cytoplasmic division takes place to form
two identical cells. This is called cytokinesis.
Cell Division can be summarized in the following:
•Each chromosome makes a copy of itself. Interphase
•The membrane around the nucleus begins to fade away, the
chromosomes begin to shorten and thicken. Prophase
•Chromosomes line up along the center of the cell. Metaphase
•The doubled chromosomes begin to separate and move to opposite
ends of t cell. Anaphase
•A nuclear membrane begins to form around both sets of
chromosomes. Telophase
•The cytoplasm separates as a new cell membrane forms. Cytokinesis