Transcript Cell Growth and Division

Chapter 10
 What
are some reasons/examples where you
can think of smaller being better?
 As
living organisms get older, they naturally
tend to grow in size
 The
growth in size is due to the accumulation
of more cells, not the growth of each
individual cell
 On
average, the cells of adult organisms are
the same size as young organisms
 There
are 2 main reasons why cells divide
rather than grow indefinitely
1.
Larger cells place more demands on cells DNA
2.
Larger cells have trouble exchanging nutrients
and waste across the membrane
DNA Overload

The DNA controls all of the cells functions


Information stored in DNA can meet the needs of
small cells, but wouldn’t be enough to meet
those of a larger cell


ie – what the cell does and what type of cell it is
No extra DNA copies are produced after a cell is made
(library in a small town example)
If there are any DNA “issues” there would be
more trouble for the organism
Exchanging & Using Materials
 The
rate of exchange depends on a cells
Surface Area
 The rate of usage depends on a cells Volume
 As
a cell grows in size, its volume increases
much faster than its surface area


As a result, the cell would be using materials at a
faster rate than what it can get rid of waste and
take in new nutrients
(small town and 2 lane road example)
 Before
a cell has the chance to become too
large, it will divide into 2 “daughter” cells
 Cell
Division – the process by which a cell
divides into 2 daughter cells
 Cell
division solves any problems associated
with cell growth


Each daughter cell gets a copy of the DNA
Each daughter cell will have an increase in
surface area to volume
A
cell can not just simply divide without any
type of preparation
 The
1.
2.
3.
process of cell division includes
Preparation
interphase
Nucleus division
M Phase
Cytoplasm division

The genetic information that is passed from one
generation to the next is carried by
chromosomes


Chromosomes are made of DNA (includes genetic info
and proteins)
Chromosomes are mostly only visible during cell
division
Prior to cell division each chromosome is copied into
2 sister chromatids
 The chromatid pair is attached at a centromere
located in the center
 During cell replication, one chromatid is sent to each
daughter cell

 The
cell cycle is the series of events cells go
through as they grow and divide
 Cell
1.
2.
3.
4.
cycle consists of 4 phases
G-1 phase
S phase
G-2 phase
M Phase
Interphase
 the “in-between” period in the cell cycle
when the cell is preparing for division
 Is a period of intense cell growth and
activity
 Consists of 3 phases
1) G-1 Phase
 Phase where the cell does most of its
growing
 Proteins and organelles can also be produced
2) S Phase
 Chromosomes are replicated and DNA is
synthesized
 Remember ~ S for SYNTHESIS
3) G-2 Phase
 Last and shortest phase in interphase
 Many of the organelles and molecules
required for cell division are produced
 When G-2 is complete the cell is ready to
enter M-phase
M Phase
 The process of cell division
 Comprised of 2 steps
1.
2.

Mitosis – division of the cell nucleus
Cytokinesis – division of the cell cytoplasm
Remember ~ M for MITOSIS!
 There
are 4 main
phases of mitosis
Prophase
2. Metaphase
3. Anaphase
4. Telophase
1.
 Depending
on the
organism, each
phase can last
anywhere from a
few minutes to
several days!
The cell prior
to Prophase
(interphase)
 The

1st and longest phase of mitosis
Takes 50% - 60% of mitosis’ total time
 The
chromatin condenses into visible
chromosomes
 The
nuclear envelope breaks down

The centrioles move to opposite sides of the
nucleus


Tiny structures in the cytoplasm which produce
spindle fibers
Spindle fibers form from the centrioles

Fanlike microtubule structure that acts to separate
the chromosomes
The end of
prophase
moving into
metaphase
 The

2nd phase of mitosis
Often lasts only a few minutes
 The
chromosomes line up
across the middle of the cell
 The
spindle fibers attach to
the centromere
 Remember
~ M for MIDDLE

The 3rd phase of mitosis

The spindle fibers pull at the
centromere, separating the sister
chromatids

Each sister chromatid moves away/apart toward
separate poles of the cell

Anaphase is complete when the individual
chromosomes (chromatids) stop moving

Remember ~ A for AWAY
 The
4th and final phase of
mitosis
 The
condensed
chromosomes begin to
reform back to chromatin
 The
nuclear envelope
begins to reform around
the chromosomes
 At
the end of telophase, mitosis is complete
and we are left with 2 new nuclei
 Cytokinesis
cytoplasm

is the division of the cell’s
Is the completion of the M phase
 Cytokinesis
usually takes place at the
same time as telophase
 As
a result we have 2 new “daughter” cells
which are identical to the “parent” cell

Cell growth and cell division must be
carefully controlled

Different cells of the body divide at
different rates
Nervous and muscles cells rarely divide once
formed
 Cells of skin, digestive tract and marrow divide
often


Newly divided cells make it possible to
replace old cells that are worn out or break
down
 Within
the laboratory we can look at cell
growth


Cells tend to stop growing when they come into
contact with other cells
When cells are removed, growth will start again
 In



our bodies we can see the same event
When there is an injury such as a cut, cells at the
edge are stimulated to divide
New cells are produced and healing begins
By the end of the healing process, cells stop
diving
 Scientists
wanted to know what tells
cells to divide


Performed experiments on dividing cells
and noticed level of protein cyclin rose
and fell with progression of cell cycle
Cyclin – regulates the cell cycle

Caused spindle formation when placed in nondividing cells
 Cyclins
– family of proteins that
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 when certain
processes have happened inside the cell

Ex) makes sure the cell doesn’t enter mitosis
until all chromosomes have been replicated

External Regulators – proteins that respond to
events outside the cell
Direct the cell to speed up or slow down the cell
cycle
 Ex) Growth Factors – tell the cell when to grow
and/or divide



Especially important in embryonic development and
wound healing
Ex) Molecules on Neighboring Cells – cause cells to
slow down growth or stop their cycle

Prevents excessive cell growth or tissues interrupting
others
 The
consequences of uncontrolled cell
growth can be severe on an organism
 Cancer
– disorder where cells do not respond
to the bodies signal to regulate cell growth


As a result, cells divide uncontrollably
and form masses called tumors
Cancer cells can break loose from
tumors and spread throughout the
body
 Control

has broken down for some reason…
Cells no longer respond to internal or external
regulators
 Exposure



Tobacco
Radiation
Viral infections
 Genetic

Defect
P53 gene – supposed to halt cell cycle no longer
functions