Unit #6
Cell Division:
Mitosis
Do we get larger because our cells
increase in size or because we
produce more of them?
The cells of an adult are no larger than the
cells of a child – there are just more of
them!
Why do cells divide?
A cell cannot grow forever. Eventually
it must divide. There are three
reasons why cell growth is limited:
• The cell’s DNA would not be able to meet
the needs of large cells
• Nutrients would not enter fast enough
• Wastes would not exit fast enough
Therefore, a cell divides into two
identical cells to prevent it from
becoming too large.
Before cell division, a
cell copies all of its
DNA so both new cells
have a complete set of
genetic information.
Chromosomes
Chromosomes
In eukaryotic cells, the genetic information that is
passed on from one generation of cells to the next
is carried by chromosomes.
Chromosomes- are made up of DNA and holds the
cell’s genetic information (genes).
The cells of every organism have a specific
number of chromosomes.
• Fruit fly cells have 8 chromosomes
• Carrot cells have 18 chromosomes.
• Human cells have 46 chromosomes
• Cows have 60 chromosomes
Well before cell division, each chromosome is
replicated
Because of this, when they become visible at the
beginning of cell division, each chromosome
consists of two identical “sister” chromatids as
shown below.
Checkpoint
The sister chromatids are identical
because
1. one is a copy of the other.
2. they are both copies of a third
chromatid that came before them.
3. they are both made of DNA and
protein.
Biologists divide the events of
mitosis into Four phases:
telophase
cytokinesis
G2: cell
growth
1. Prophase
G1: cell growth and
differentiation
mitotic cell
division
2. Metaphase
3. Anaphase
4. Telophase
S: synthesis
of DNA;
chromosomes
are duplicated
InterphaseResting
Depending on the type of cell, the four
phases of mitosis may last anywhere from a
few minutes to several days.
Remember
• Before Mitosis a cell copies all of its DNA
(Chromosomes ) so both new cells have
a complete set of genetic information.
Prophase
• chromosomes become visible (condense)
• nuclear envelope disappears
• centrioles move to opposite sides of the cell. Plant
cells do not have centrioles
• spindle begins to form and attaches to centromere
Checkpoint
The function of the spindle is to
a. break down the nuclear envelope.
b. help form a new nuclear envelope
c. help separate the chromosomes.
Metaphase
• chromosomes line up at the middle of the cell
Anaphase
• Sister chromatids separate at centromere into
individual chromosomes
• Chromosomes move toward poles of cell
• Anaphase ends when chromosomes stop moving
Check Point
• What happens during anaphase?
A. The chromosomes become visible.
B. The chromosomes line up across the center of the
cell.
C. The sister chromatids separate.
Telophase
• Chromosomes become less condensed
• Nuclear envelope forms around each cluster of
chromosomes
• Spindle disappears
• Start of cytokinesis
Video
Mitosis is complete.
However, cell division is not complete.
Cytokinesis
• occurs during telophase
• cytoplasm divides
Video
In most animal cells, the cell membrane is
drawn inward until the cytoplasm is pinched
into two nearly equal parts
In plants, a structure known as the cell plate forms
between the two cells.
Mitosis Animation
Mitosis Review
1. Name the main events of the cell cycle.
2. Describe what happens during each of
the four phases of mitosis.
3. What are chromosomes made of?
4. How is cytokinesis in plant cells similar
to cytokinesis in animal cells? How is it
different?
Cleavage
Cleavage is the name for the process of mitosis
that occurs in cells that forms a developing embryo.
Cleavage involves all the steps of mitosis, but after
cell division, it quickly goes through mitosis again.
This results in a mass of many small
undifferentiated cells. These cells will lead to the
development of an embryo from a fertilized egg
gastrula
Once a gastrula forms, cells start to arrange
themselves in different layers. The different layers
of cells will differentiate into different tissues, which
will form organs.
When the different tissues form, the mass of cells
is called an embryo.
Differentiation
• In most multicellular organisms, not all cells are
alike. For example, cells that make up the
human skin are different from cells that make up
the inner organs. Yet, all of the different cell
types in the human body are all derived from a
single fertilized egg cell through differentiation.
Cellular differentiation is a concept from
developmental biology describing the process
by which cells acquire a "type".
1. The outer layer of cells is called the ectoderm.
It will differentiate into nerve cells and skin cells
2. The middle layer of cells is called the mesoderm. It will
differentiate into cells that form the muscular system,
circulatory system, excretory system and endocrine
system
3. The inner layer of cells is called the endoderm.
It will differentiate into cells that form the digestive system,
pancreas, liver and lungs
What is cancer?
• Uncontrollable cell division resulting in a malignant and
invasive growth or tumor.
• If untreated -Metastasis is the spread of cancer from its
primary site to other places in the body (e.g., brain, liver).