Transcript Cell Growth and Division

Cell Growth and Division
• Limits to growth
Cell Growth
– Stress on DNA
– Difficulty moving nutrients/wastes
across membrane
• Ratio of surface area to volume
• Volume increases at a faster rate than
surface area
Cell Division
• Before a cell becomes too large, it divides,
producing 2 daughter cells
– Each daughter cell is an exact replica of the
parent cell
• Before the cell divides, the DNA is
replicated, so each new cell will have the
same genetic information as the parent
cell
Cell Division
• 2 stages (eukaryotes)
– Mitosis- division of the cell nucleus
– Cytokinesis- division of the cytoplasm
• Asexual reproduction (esp for unicellular
organisms)
• Also used for new cells as an organism
grows and develops
Chromosomes
• Threadlike structure
within the nucleus
containing the genetic
information (DNA) that
is passed from one
generation of cells to
the next
• Cells of every organism have a specific number
of chromosomes
– Fruit flies = 8; Humans = 46; carrots = 18
Chromosomes
• Chromatin- granular material visible within
the nucleus; consists of DNA tightly coiled
around proteins
• Chromatid- one of two identical “sister”
parts of a duplicated chromosome
• Centromere- an area where the
chromatids are attached to one another
The Cell
Cycle
• 2 major
phases
– Interphase
– Mitosis
Interphase
• 3 phases
– G1 phase= cells do most
of their growing
• Increase in size and
synthesize new proteins
and organelles
– S phase= chromosomes are replicated and the synthesis
and DNA molecules takes place
• Usually if a cell enters S phase and begins replication, it completes
the rest of the cycle
– G2 phase= many of the organelles and molecules required
for cell division are produced
• Shortest of the 3 phases of interphase
Mitosis
• Divided into 4 phases
– Prophase
– Metaphase
– Anaphase
– Telophase
• Followed with Cytokinesis
• Depending on cell- may last a few minutes
to several days
Prophase
• 1st and longest phase
of mitosis
• Events
– Chromosomes become visible
– Centrioles separate and move to opposite sides
of the cell
– Chromosomes become attached to fibers in the
spindle at the centromere
– Chromosomes coil more tightly
– Nucleolus disappears
– Nuclear envelope breaks down
Metaphase
• Often lasts only a
few minutes
• Events
– Chromosomes line
up across the
center of the cell
– Microtubules
connect the
centromere of each
chromosome to the
two poles of the
spindle
Anaphase
• Centromeres split
• Sister chromatids
separate and move to
opposite poles
• Anaphase ends when
chromosomes stop
moving
Telophase
• Chromosomes begin to
disperse into a
chromatin
• Nuclear envelope reforms around each
cluster of chromosomes
• Spindle begins to break
apart
• Nucleolus becomes
visible
Cytokinesis
• Occurs at the same time as
telophase
• Animal cells:
– Cell membrane is drawn inward
until the cytoplasm is pinched into
2 nearly equal parts
• Plant cells:
– Cell plate forms midway between
the divided nuclei
– Cell wall begins to appear in the
cell plate
• Result? 2 new identical cells
Controls on Cell Division
• Effects of controlled cell growth can be seen by
placing some cells in a petri dish containing
nutrient broth
• Cells grow until they form a thin layer covering
the bottom of the dish
• Cells stop growing when they come into contact
with other cells
• If cells are removed, the remaining cells will
begin dividing again
• Something can turn cell division on or off
Regulating Cell Growth
• Cyclins- proteins that regulate the timing of
the cell cycle in eukaryotic cells
– Internal regulators: proteins that respond to
events inside the cell
• i.e. make sure all chromosomes have been
replicated; make sure all chromosomes are
attached to the spindle before entering anaphase
– External regulators: proteins that respond to
events outside the cell
• i.e. embryonic development; wound healing
Uncontrolled Cell Growth
• Cancer- disorder in which 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
• P53 gene halts the cell cycle until all
chromosomes have been properly replicated
– A damaged or defective p53 gene causes the cells to
lose the information needed to respond to signals that
would normally control their growth