Download cell reproduction/division

Unit 4B
Asexual Reproduction
and Mitotic Cell Division
The Cell Theory states:
1) cells are the basic unit of life
2) all living things are made up of at least 1
cell, and
3) all cells come from pre-existing cells
(cell reproduction/division)
What is the purpose of cell
reproduction?
• In Unicellular Organisms: to form a new
organism-asexual reproduction
• In Multicellular Organisms:
a. Growth- addition of new cells
b. Repair- replacement of lost or injured cells
c. Reproduction- formation of reproductive cells
(egg + sperm) in sexual reproduction
Cell Reproduction
• As you grow from a baby to a teenager, do
your cells get bigger or do you gain more
cells?
Why do cells divide into more cells
rather than continue to grow?
2 Reasons:
1. Larger size puts larger demands on the
DNA (information shortage)
2. Larger cells are less efficient at moving
materials in and out of the cell
(exchanging materials)
1. Information Shortage
• DNA contains instructions for making proteins
for the cell
• As the cell gets bigger, the DNA stays the same
• If a cell continues to get larger, eventually the
DNA would not be able to make enough proteins
quickly enough for the cell
2. Exchanging Materials
• Cells pass material in and out through the
cell membrane
– Surface area (SA) of the membrane
determines the rate of exchange of materials
• Cells use and break down materials within
the cell
– Volume (V) of the cell determines how much
material is needed (larger cells need more
nutrients, O2, etc.)
Ratio of Surface Area to Volume
• A ratio is a comparison
– Ex. Surface Area: Volume
(Surface Area/Volume)
– The amount of surface area
for each unit of volume
• Volume = l x w x h
• Surface Area = l x w (x 6
sides)
Ratio of Surface Area to Volume
The greater the SA / V ratio, the easier it is for the cell to
exchange materials.
1.
2.
3.
Cube 1
SA (mm2)
V (mm3)
SA/V
Cube 2
Cube 3
Determine the SA/V Ratio for
cubes 2 & 3 and fill in the table
Cube 1
Cube 2
Cube 3
SA (mm2)
V (mm3)
SA/V
• Which cell has the greatest SA /
V ratio?
• So, which cell has the easiest
time moving material in and out
of the membrane and getting
nutrients, O2, etc. to all places in
the cell?
1
.
2
.
3
.
Determine the SA/V Ratio for
cubes 2 & 3 and fill in the table
Cube 1
Cube 2
Cube 3
SA (mm2)
V (mm3)
SA/V
• From one cube to the next, the SA
increased by a factor of ______.
• The V increased by a factor of _______.
• Which is increasing faster as the cell gets
bigger? (circle) SA or V
Ratio of Surface Area to Volume
• As a cell grows, its volume increases much
faster than its surface area
• If a cell becomes too large, not enough nutrients
can come through the membrane to feed the
whole cell.
• Similarly, wastes from the huge cell would not be
released fast enough.
• Therefore, smaller size allows for efficient
movement of materials in, out, and throughout
the cell.
Thus, instead of growing too large,
cells divide in half!
• This increases the SA / V ratio
• 1 cell divides into 2 “daughter” cells.
• This process is called cell division
How a high SA/V ratio aids in diffusion into a cell
Before a Cell Divides…
1. The cell gets the signal to divide
2. It replicates (copies) all of its DNA
3. Then, the cell divides and each daughter cell
gets a complete copy of the DNA
1 chromosome
Copy of that
chromosome
Replicated Chromosomes (DNA and associated proteins)
Cell Division Solves the Growth
Problems
1. Each daughter cell gets its own DNA so
there is no information shortage
2. Volume is reduced in division, so efficient
exchange of material can occur through
the cell membrane
Practice
Two factors important in the transport of
materials into or out of cells:
• The surface area of the cell membrane
should be large / small
• The volume of cell matter that materials
have to travel through to get to all parts of
the cell should be as small / large as
possible.
Practice
• The best combination of surface area
and volume factors is one in which
• the surface area is
small / large
• and the volume is
small / large
• the surface area to volume ratio: SA /V is
small / large
• Cells that conduct transport most
efficiently will be (larger / smaller) cells.
Structure Leads to Function
• Different cell types in the body
have different functions
– Ex: muscles cells and nerve
cells have different functions
Muscle Cells
• All cell types are structured to
maximize their surface area to
volume ratio!
Nerve Cell
• This allows for efficient
How do the structures of
movement of materials.
these cells maximize
SA/V ratio?
The Process of Cell Division
Chromosome Recap
• Packages of DNA and histones highly
condensed.
Prokaryotic Chromosomes
• Prokaryotes have no nucleus
• Usually prokaryotes have 1 circular
chromosome in the cytoplasm.
• Not all prokaryotes have histones
Eukaryotic Chromosomes
• DNA binds to proteins called histones
• DNA and histones condense to form chromatin
Eukaryotic Chromosomes (cont.)
• Chromatin is condensed completely to form
chromosomes after the DNA has replicated and
the cell is ready for division.
• Chromosomes make it possible to separate DNA
precisely during cell division.
Chromosomes can be seen in
two forms:
1. Single-Arm: are composed of a
single chromatid
2. Double- Arm (Replicated Form,
Duplicated form): are made up of paired,
genetically identical chromatids,
called sister chromatids.
• Sister chromatids are joined at the
centromere.
• Because the sister chromatids are formed
during replication of DNA, they are identical
right down to the nucleotide sequences!
The Cell Cycle
• The cell cycle is the series of events that
cells go through as they grow and divide
• The events in the cell cycle are:
– Growth
– Preparation for division
– Division into two daughter cells
Prokaryotic Cell Cycle
• Also called binary fission
(a form of asexual
reproduction)
• Cell grows, doubles its
DNA, and splits in two,
dividing the DNA and
cytoplasm between the
daughters.
Eukaryotic Cell Cycle- Cell
division in body (somatic) cells
• Consists of 4 phases:
–
–
–
–
G1 (Growth)
S (DNA Replication)
G2 (Preparation for Cell Division)
M (Cell Division)
Eukaryotic Cell Cycle
• Only during M
phase does
division actually
occur.
• The other 3
phases are part of
Interphase- the
“in-between”
period of growth
G1 Phase: Cell Growth
• Cells increase in size
• Synthesize new proteins
• Make new organelles
G1
S Phase: DNA Replication
• New DNA is
synthesized
(chromatin replicated)
• At the end of S Phase
the cell will have twice
the amount of DNA
S
G2 Phase: Preparing for Cell
Division
• Shortest of the phases
in Interphase
• Cell produces
organelles and
molecules necessary
for division
• Now the cell is finally
ready to divide
G2
M Phase: Cell Division
• The making of 2
daughter cells
• Very quick compared to
the lengthy interphase.
• Consists of Mitosis
(division of the nucleus)
and Cytokinesis
(division of the
cytoplasm).
M
Mitosis
• Consists of 4 phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
– Remember PMAT
Prophase
• Longest phase of mitosis
• Duplicated chromatin
condense to form the
double armed
chromosomes seen in the
picture.
• Both copies attach to each
other at the centromere
Prophase (cont.)
• The spindle begins to
form
– System of microtubules
that will help separate the
duplicated chromosomes.
– Spindle fibers extend from
the centrosome regions
where centrioles are
located (no centrioles in
plant cells)
– Centrioles move to
opposite ends (poles) of
the cell
Prophase (cont.)
• At the end of prophase…
– Chromosomes coil more
tightly
– The nucleolus disappears
– The nuclear envelope
breaks down
Metaphase
• Shortest part of Mitosis
• Centromeres (and thus
chromosomes) line up
across the center/middle
of the cell
• Spindle fibers connect
each centromere to both
poles of the spindle
Anaphase
• Sister chromatids
separate into single armed
chromosomes and begin
to move apart
• Separated chromosomes
move along spindle fibers
to either pole of the cell
• When the movement
stops, anaphase is over
Telophase
• The condensed
chromosomes begin to
spread out (back into
chromatin)
• A nuclear envelope
reforms around both
clusters of chromosomes
• Spindle breaks apart
• Nucleolus reforms in each
daughter nucleus
• Mitosis is now complete
Cytokinesis
• The second portion of
M phase
• Splits the cytoplasm in
half, and splits the cell
in two
• Usually occurs at the
same time as
telophase
Cytokinesis Differs in Animal and
Plant Cells
• Animal Cells- the cell membrane is drawn
inward by microfilaments creating a
cleavage furrow
– The cytoplasm is pinched into 2 parts until the
daughter cells separate
• Plant Cells- a cell plate forms in between
the daughter cells.
– Cell membrane and cell wall form at the cell
plate from vesicles containing membrane and
cell wall materials
Cytokinesis Differs in Animal and
Plant Cells
Stages of
Mitosis
Animal Cell
Plant Cell
After cytokinesis is complete…
• The cell may enter G1 again and
continue the cycle
• OR the cell may enter a phase
called G0 or resting phase.
– During this time a cell is not
preparing for division but rather is
making protein and doing normal cell
functions.
– Different cells stay in G0 for different
amounts of time. (Nerve? ________
Skin? _______)
Compare plant and animal cell
division:
• Differences:
– Centrioles in animal cells only
– Cleavage furrow for animal cell cytokinesis
and cell plate for plant cell cytokinesis
• Similarities:
– The rest!
Regulating the
Cell Cycle
Cell Growth and Cell Division are
Controlled Carefully in Multicellular
Organisms
• These controls can be turned on and off
• Example: if you get a cut, new cells fill in
the space only until the cut is healed
• How do cells know
when to divide?
Cyclins and Regulatory Proteins
• Cyclins- A family of
proteins that regulate
the timing of the cell
cycle in eukaryotes.
– Act like policemen at
different points in the cycle
to prevent it from going
forward if mistakes have
occurred.
– Work with a family of
proteins called CDK
Other regulatory proteins:
• Proteins inside and outside the cell that help
regulate the cell cycle
Internal Regulatory Proteins
• Proteins that respond to events inside the
cell, letting the cell know whether or not to
proceed with the cell cycle.
• Examples:
1. An internal regularity protein keeps the cell from
entering mitosis until the chromosomes have
replicated.
2. A different protein prevents the cell from entering
anaphase until the spindle fibers have attached to all
chromosomes.
External Regulatory Proteins
•
•
•
Proteins that respond to events outside
the cell
Direct the cell to speed up or slow down
the cell cycle
Examples:
1. Growth Factors- tell the cell to grow and
divide
2. Inhibitory signals- proteins on the surface of
other cells; tell the cell to slow down or stop
the cell cycle
- This prevents excessive replication
Cells die and are replaced
all the time
• Two ways that cells end their life cycle
1. Accidental death by damage or injury
2. Programmed cell death called apoptosis.
- helps shape structures
during development
- kills infected cells
- kills cells with DNA
damage or cancer
- etc.
Cancer:
Uncontrolled Cell Division
• Cancer- a disorder in
which body cells lose
the ability to control
division
- Cancer cells do not
respond to signals that
regulate cell growth
and thus, divide
uncontrollably.
Cancer is a disease of the cell cycle!
Tumors
• A mass of cells
• Cancerous tumors are
called malignant- invade
and destroy surrounding
tissue
• Not all tumors are
cancerous
– Some are benign – do not
spread to surrounding
healthy tissue or around
the body
What Causes Cancer?
• Defects/mutations in
genes that regulate
the cell cycle
– Example: the protein
from the gene p53
normally halts the cell
cycle until all
chromosomes have
replicated (an internal
regulatory protein)
– If this gene is defective,
apoptosis will not occur,
and the cell cycle will
proceed continuously
Normal
Problem with
replication occurs
Cell division proceeds normally
What Causes Cancer? (cont.)
• The defective/mutated gene is not the
same for all cancers
– Some cancer cells do not respond to internal
regulation, others do not respond to external
regulation, etc.
Possible Reasons for Defective
Cell Cycle Genes
•
•
•
•
•
•
Innate mutation (born with it)
Smoking or chewing tobacco
Radiation exposure
Other defective genes
Viral infections
Etc.
Will a mutation in a somatic cell mean the
organism’s offspring will also have the
mutation?
Or if you get skin cancer during your lifetime,
will your children be born with the mutations
for skin cancer?
Treatments for Cancer
• Some tumors can be removed by surgery
• Radiation is used to kill cancerous tumors
• Chemotherapy- using chemicals (drugs) to
kill cancer cells
– These drugs target rapidly dividing cells
– Will also kill normal, healthy
cells that are dividing (Like?)
– Scientists seek to find a
drug that kills cancer cells
but not healthy cells