Cell Growth
Limits to Cell Growth
Cells divide for two main reasons:
•DNA overload
•Exchanging materials gets too difficult
– DNA overload – as a cell increases in size, it
usually does not make extra copies of DNA and
this places more demands on the DNA. If a cell
increases with unlimited growth, an information
crisis would occur.
• The cell would no longer be able to serve the needs of
the growing cell.
– Exchanging Materials
• The rate at which food and oxygen are used up and
waste products are produced depends on the cell’s
volume. The larger the cell, the more quickly it uses
food and oxygen.
• The rate at which the exchange of materials takes place
depends on the surface area of the cell membrane.
The larger the cell grows, the more difficulty it has
moving materials across its cell membrane.
– Ratio of Surface Area to Volume
• Volume increases more rapidly than does the surface
area.
• If a cell got too large, it would be more difficult to get
sufficient amounts of oxygen and nutrients in and
wastes out.
• Large surface area to volume ratio is most beneficial
• Division of the Cell
– Before a cell gets too large, it divides forming
two “daughter” cells. This is called cell
division.
– Before a cell divides, the cell replicates, or
copies, it’s entire DNA.
– This replication of DNA solves the problem of
information storage because each daughter
cell gets one complete set of genetic
information.
Cell Division
Key Concepts:
• What are the main events of the cell cycle?
• What are the four phases of mitosis?
There are two main stages of cell division in
eukaryotic cells.
• Mitosis
• Cytokinesis
• Mitosis is classified as asexual, since the cells
produced (two daughter cells) are genetically
identical to the parent cell (identical number
of chromosomes).
• Mitosis is also a source of new cells when a
multicellular organism grows and develops.
Cell Division
In eukaryotic cell, the
genetic information of the
cell is carried on
chromosomes.
Chromosomes are made
of DNA.
Just before cell division, the
chromosomes replicate,
or copy themselves,
creating identical sister
chromatids, joined by a
centromere.
Chromosomes
– The genetic information carried on from one
generation of cells to the next is by chromosomes.
– Chromosomes are made up of DNA and protein.
– Human cells have 46 chromosomes.
– Most chromosomes are not visible in most cells
except during cell division.
Chromosomes (cont.)
– At the beginning of cell division, the chromosomes
condense into visible structures.
– Before cell division occurs, each chromosome
replicates.
– Each chromosome consists of two identical
“sister” chromatids separate from each other.
– Each pair of chromatids is attached to an area
called the centromere.
The Cell Cycle
• During the cell cycle, a cell
grows, prepares for
division, and divides to
form two daughter cells,
each of which then begins
the cycle again.
– The cell cycle consists of four phases of which the
first three are grouped together and called
INTERPHASE (G1, S, G2) followed by M phase
(mitosis).
•
•
•
•
G1 phase (gap 1)
S phase (synthesis)
G2 phase (gap 2)
M phase
Interphase
Events of Interphase
– G1 phase: period of activity in which cells do most
of their growing. Cell increases in size and
synthesize new proteins and organelles.
– S phase: chromosomes are replicated: the
synthesis of DNA molecules takes place. Key
proteins made.
– G2: shortest phase of three phases of Interphase.
Many of the required organelles and molecules
required for cell division are produced.
• When the events of the G2 phases are completed, the
cell is ready to enter the M phase and begin the
process of cell division.
Mitosis
Video Review
Mitosis
– There are four phases: prophase, metaphase,
anaphase, and telophase.
– Depending on the type of cell, the four phases
may last from a few minutes to several days.
Prophase
– Prophase
• The first and longest phase of mitosis.
• The chromosomes become visible.
• The centrioles, two tiny structures located in the
cytoplasm near the nuclear envelope, separate
and take up positions on opposite sides of the
nucleus.
• Spindle fibers begin to form.
• The nuclear envelope breaks down and the
nucleolus disappears.
Metaphase
• Often lasts only a few minutes.
• The chromosomes line up across the center of the cell.
• Microtubules connect the centromere of each
chromosome to the two poles of the spindle.
Anaphase
• The third phase of mitosis.
• The centromeres that join the sister chromatids split,
allowing the sister chromatids to separate and become
individual chromosomes. The chromosomes move until
they have separated into two groups near the poles of
the spindle. Anaphase ends when the chromosomes
stop moving
Telophase
• The fourth and final phase of
mitosis. The chromosomes
begin to disperse into a
tangle of dense material.
• A nuclear envelope re-forms
around each cluster of
chromosomes.
• The spindle begins to break
apart, and a nucleolus
becomes visible in each
daughter nucleus.
• Mitosis is complete.
Mitosis
• The 4 phases of mitosis are, from beginning to
end: prophase, metaphase, anaphase, telophase.
• Remember…
PMAT!
or…
The entire cell cycle is abbreviated
IPMAT!
(but Interphase isn’t part of mitosis)
Cytokinesis
– The division of the cytoplasm occurs.
– Cytokinesis usually occurs at the same time as
telophase.
– In animal cells, the cell membrane is drawn inward
until the cytoplasm is pinched into two nearly
equal parts.
– In plant cells, a cell plate forms midway between
the divided nuclei. The cell wall then begins to
appear in the cell plate.
Video
Mitosis
• Mitosis produces 2
identical daughter cells
from each parent cell.
• If a parent cell had 4
chromosomes, how
many chromosomes
would each of the 2
new daughter cells
have?
• Human cells have 46
chromosomes. When a
muscle cell undergoes
mitosis, how many
chromosomes will each
daughter cell it
produces have?
Regulating the Cell Cycle
• How is the cell cycle regulated?
• How are cancer cells different from other
cells?
Controls On Cell Division
– Cells in a Petri dish will continue to grow until they
come in contact with other cells.
– If the cells are removed from the center of the
Petri dish, the cells bordering the empty space will
begin dividing. The controls on cell growth can be
turned off and on.
– When you have an injury such as a cut in the skin
or a break in a bone, cells will begin to divide
rapidly. This produces new cells that will help in
the healing process.
Cell Cycle Regulators
– It was discovered that cells in mitosis contained a protein that when
injected into a non-dividing cell, would cause mitotic spindle to form.
– Cyclin and kinase are proteins that seemed to regulate the cell cycle.
– Cyclins regulate the timing of the cell cycle in eukaryotic cells.
– Internal Regulators
• Proteins that respond to events inside the cell.
• Allow the cell cycle to proceed only if certain processes have happened inside
the cell.
• For example, several regulatory proteins make sure that a cell does not enter
mitosis until all chromosomes have been replicated.
– External Regulators
•
•
•
•
Proteins that respond to events outside the cell.
Direct cells to speed up or slow down the cell cycle.
Growth factors are important external regulators.
They stimulate the growth and division of cells.
Uncontrolled Cell Growth
– Cancer – 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.
– They divide uncontrollably and produce masses called
tumors.
– The control over the cell cycle has broken down.
– Some causes of cancer include smoking tobacco, radiation
exposure, and even viral infections.
– A large number of cancer cells have a defect in a gene
called p53, which normally halts the cell cycle until all
chromosomes have been properly replicated.
– Cancer is a disease of the cell cycle.