Download Cell Growth and Division Chapter 10

Cell Growth and Division
Chapter 10
1
Limits to Cell Growth
• When a living
organism grows, it
produces more cells
instead of increasing
cell size.
• This is because of
two main reasons:
– DNA “overload”
– Exchanging
materials
DNA “Overload”
• The information that
controls a cell’s function
is stored in DNA
• When a cell is small, the
information stored in DNA
can meet all of the cell’s
needs
• If a cell were to grow too
large, there would not be
enough DNA
– “genetic library”
Exchanging Materials
• Cell growth is also
limited by the ability to
exchange materials
across the cell
membrane.
• This ability to
exchange materials is
based on the
relationship between
volume and surface
area.
Cell Size
Volume
(Length x Width
x Height)
Surface
Area (Length
x Width x # of
Surfaces)
Ratio of
Surface
Area to
Volume
1 cm x
1 1 cm x
1
cm x 1cm = 1
cm x 6
cm3
= 6 cm2
6:1
2 cm x 2cm x 2 cm x
2
2 cm
cm x 6
= 8 cm3
= 24 cm2
3:1
Limits to Cell Growth
• When cells grow, the ratio of
surface area to volume
decreases, making it harder
for cells to do a good job of
transferring things across the
membrane.
• Therefore, instead of growing,
a cell will split into two
daughter cells.
– This process is called cell
division
Cell Division
• Because a cell has only
one copy of its genetic
information (DNA), it
needs to copy it before it
can divide.
– Prokaryotic cells
simply copy the
genetic information
and split in two
– Eukaryotic cells have
a much more complex
way of dividing
Chromosomes
• In eukaryotic cells, genetic
information is passed from one
generation to another in
chromosomes
• Chromosomes are made up of
DNA and protein.
• Chromosomes are not visible
in most cells except during cell
division.
– At the start of cell division,
the DNA that is spread
throughout the nucleus
condenses into
chromosomes.
Chromosomes
• Before cell division, each
chromosome is replicated, or
copied.
• As a result, at the start of cell
division each chromosome
consists of two identical
chromatids.
• The two “sister chromatids” are
attached near the center by a
centromere.
• Human cells at the start of cell
division have 46 chromosomes,
each of which has a pair of
chromatids.
The Cell Cycle
• The life cycle of a cell is
known as the cell cycle.
• During the cell cycle, the
cell grows, prepares for
division, and divides into
two daughter cells.
• The cell cycle is divided
into two parts:
– Cell division
– Interphase
Interphase
• Interphase refers to the
periods in-between cell
division.
• Interphase is usually the
longest part of the cell
cycle.
• Interphase consists of
three individual phases:
– G1 Phase
– S Phase
– G2 Phase
Interphase
• G1 Phase: During this
phase, cells do most of their
growing, and make new
proteins and organelles.
• S Phase: During this phase,
chromosomes are
replicated.
• G2 Phase: During this
phase, the cell prepares for
division by preparing the
organelles needed for
division.
Mitosis
• The process of cell
division in eukaryotes
is called mitosis.
• Mitosis consists of 4
parts:
– Prophase
– Metaphase
– Anaphase
– Telophase
Prophase
• Prophase is the first and
longest phase of mitosis.
• During prophase,
chromosomes become
visible.
• Centrioles, two tiny
structures located in the
cytoplasm separate and
move to opposite sides of
the nucleus.
Prophase
• During prophase, the
spindle, or the fanlike
microtubule structure that
helps separate the
chromosomes is formed.
• The spindle connects the
centrioles to the
chromosomes.
• During prophase, the
nuclear membrane
breaks down.
Metaphase
• Metaphase is the
second, and shortest
phase of mitosis.
• During metaphase, the
chromosomes finish
attaching to the
spindle and line up
across the center of
the cell.
Anaphase
• During anaphase, the
sister chromatids
separate into individual
chromosomes and are
pulled apart.
• Anaphase ends when
the chromosomes stop
moving.
Telophase
• During telophase,
the previously
condensed
chromosomes will
begin to unwind.
• Additionally, a
nuclear envelope
re-forms around
each cluster of
chromosomes.
Cytokinesis
• After mitosis is
complete, the cell
contains two nuclei.
• As a result, the
cytoplasm has to be
split, a process that is
called cytokinesis.
• This creates two new
daughter cells.
Regulating the Cell Cycle
• It is fascinating to see the
different ways cells in a
multicellular organism move
through the cell cycle.
• For example, in humans,
most muscle and nerve cells
don’t divide while mature,
while skin and blood cells
move continually through the
cycle.
Controls on Cell Division
• If we look at cells dividing on a petri dish, they will divide
until they cover the whole bottom of the dish.
• If we take out some cells from the middle of the dish, they
will divide until the plate is covered again, at which point
cell division will stop.
Controls on Cell Division
• This is similar to how the body heals.
• When the body is cut, broken or bruised cells at the
edge of the injury are stimulated to divide rapidly
until the injury is healed.
– At this point, growth stops
Cell Cycle Regulators
• For many years,
biologists searched for
the substance that
regulates the cell cycle.
• In the 1980s, scientist
discovered that cells in
mitosis contain a
specific protein that
stimulates mitosis to
occur.
Cell Cycle Regulators
• This protein is called
cyclin because it controls
the cell cycle
• They did an experiment
where they injected cyclin
from cells undergoing
mitosis into non-dividing
cells, which caused the
cells to begin mitosis.
Cyclin
• There are many
different types of cyclin
proteins that are found
in mitotic cells
• Some are known as
internal regulators,
while others are known
as external regulators.
Internal Regulators
• Cyclin proteins that are
internal regulators
respond to events that
are inside the cell
• Examples would be cyclin
that prevents a cell from
entering anaphase until
all of its chromosomes
are attached to spindles.
External Regulators
• External regulators are
cyclin proteins that
respond to events that
are outside the cell.
• They direct cells to
speed up or slow down
the cell cycle, and make
sure that neighboring
tissues don’t disrupt
each other.
Cancer
• Cell growth and division must be carefully regulated.
• If uncontrolled, cancer will form.
– Cancer cells do not respond to the signals that
regulate most cells
• When a cancer replicates uncontrollably, it forms a
tumor that can harm surrounding tissue.