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Cell Growth and Division
Chapter 8
Introduction:
• When an organism grows up, does it grow because
its cells get larger, or is it because the organism gets
more and more cells?
• In most cases, a living thing grows because it
produces more and more cells
– Your cells are the same size as a baby’s cells, you just
have a lot more of them.
Limits of Cell Growth
Recall that things (gas, food, water, and waste) are
constantly moving across the cell membrane
1. How quickly the exchange takes place depends on the
surface area of the cell.
2. How quickly materials are used or produced depends on
cell volume.
• If you take a typical cell and double its diameter,
what happens to the surface area of the cell
compared to the volume of the cell?
• You’ll notice that as the cell increases in size, the
volume increases faster than the surface area.
1 cm
Problems with getting too big
As a cell grows, its surface area increases at a
much slower rate than its volume.
Q: What’s wrong with that?
• The cell has problems moving needed materials in, and
getting wastes out.
• DNA cannot produce enough proteins to keep up with cell
needs  Information Crisis!
– The cell would have to slow down, making it less efficient.
• As the cell grows, the cell’s DNA may no longer be
able to make enough RNA/protein to support the
cell.
– The cell then needs to slow down its growth, making it
less efficient.
– To solve these problems, the cell undergoes a process
called cell division.
• Cell Division: The process whereby the cell divides into two
daughter cells
Rates of Cell Growth
• Cells can grow and divide at astonishing rates.
– E. coli can divide to form 2 new cells every 20 minutes
under ideal conditions.
• Ideal conditions for this growth cannot be
maintained for very long.
– If they did remain:
• A single cell would grow to a 14 kg mass of bacteria in 1 day.
• In 3 days, the mass of cells would equal the mass of the Earth.
Controls on Cell Growth
• An organism’s body controls the growth and
division of cells.
– Cells in certain places of the body, like in the heart and
nervous system, rarely divide, if at all.
– Cells of the skin and digestive tract grow and divide
rapidly through life to replace those that are worn out or
broken.
• We can observe controlled cell growth in the lab by
allowing cells to accumulate on petri dishes.
– Most cells will grow until they form a thin layer covering
the entire dish.
• When this happens the cells stop growing to prevent
overcrowding.
• Controls on cell growth and cell division can be turned on and
off.
– Ex: Cuts on the skin
Uncontrolled Cell Growth
• The consequences of uncontrolled cell growth in a
multicellular organism are severe.
– Cancer is a disorder where cells have lost the ability to
control their own rate of growth.
• Cancer will continue to grow and divide until the supply of
nutrients is exhausted (Death of the organism).
Cell Division: Mitosis and Cytokinesis
8-2
Introduction
• The division of eukaryotic cells occurs in 2 main
stages: mitosis and cytokinesis
– Mitosis: When the nucleus divides into 2 nuclei, each
with the same number and kinds of chromosomes as the
parent cell.
– Cytokinesis: The process by which the cytoplasm
divides, forming 2 distinct cells.
• The process is meticulous because large amounts of
DNA need to be separated accurately and
efficiently.
– With as many as 100 chromosomes to care for, the cell
must make certain that each new daughter cell gets only
one copy of each chromosome.
• Mistakes usually result in death of the cell.
Chromosomes
• Remember, chromosome are structures made from
DNA.
– They are only visible during mitosis
– All organisms have a specific number of chromosomes.
• Humans have 46 chromosomes. Goldfish have 94
• Chromosomes are made of chromatin.
– During cell division, the chromatin condenses to make
chromosomes
– It is composed of DNA and protein
– A chromosome’s DNA is wrapped around special
proteins called histones, which together form
nucleosomes. Nucleosomes pack together to form
chromosomes.
Chromosome Structure
• A chromosome is made of 2 identical parts called
chromatids. (These are often called sister
chromatids)
– A pair of chromatids are attached at the centromere,
which is usually located near the center of the
chromosome.
The Cell Cycle
• Cell Cycle: The period from the end of one mitosis
to the end of the next.
– The cell grows, divides into 2 daughter cells, and then
begin to grow again.
• The cell cycle is divided into 2 main parts, mitosis
and interphase, which is the time of cell growth
between divisions.
– Also during this time, a third process known as
cytokinesis occurs.
Interphase
• Interphase is the time of the cell cycle that falls
between cell divisions.
– Here, the cell grows and carries out normal functions.
– Interphase is the longest stage of the cell cycle and can
be divided into 3 smaller phases
• G1 (Gap 1): Cell growth and development take place.
• S (Synthesis): DNA replication occurs.
• G2 (Gap 2): Synthesis of organelles and other materials.
(Shortest of interphase).
Mitosis
• Mitosis is divided into 4 phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
– Prophase
• This is the longest stage of mitosis.
• Chromosomes appear
• Centrioles appear and separate, moving to opposite sides of
the cell (Animal cells only)
– Centriole: Tiny structures in the cytoplasm
– Spindle fibers attach to the centromeres
– The nucleus disappears/ breaks down
– Metaphase
• This is the shortest phase of mitosis
• Chromosomes line up across the center of the cell (Equator)
• Centromeres are connected to the poles of the spindle by
microtubules called asters.
– Anaphase
• Begins when centromeres split
– This causes sister chromatids to separate and move to opposite sides
of the cell.
– Ends when the movement of chromosomes stops
– Telophase
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Final phase of mitosis
The chromosomes uncoil to become chromatin
A nuclear envelope develops around each set of chromatin.
Centrioles and spindles disappear.
Cytokinesis
• Now that there are 2 identical nuclei, the rest of the cell can divide.
• Cytokinesis is the division of the cytoplasm
– In animal cells, the cell membrane pinches inward until 2 new
cells form (Called a furrow)
– In plant cells, a cell plate develops between the 2 nuclei and
works toward each side of the cell wall, forming a new part of
the cell membrane and cell wall.
– Cytokinesis usually begins during anaphase/telophase