Download cell cycle - Humble ISD

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UNIT VI – CELL CYCLE
The third statement of the Cell Theory states that all cells come from _ preexisting cells ___. The continuity of life is based on the reproduction of cells or
__ cell division ___. In most organisms, cells increase to a certain size and then
divide into two cells. This cycle of growth and cell division is known as the _ cell
cycle _. The cell cycle is defined as the period of time from the beginning of cell
division_ to the _ beginning of the next . This type of cell division is _ asexual
_ reproduction. In asexual reproduction, the genome or _DNA_ of the cell is
exactly replicated resulting in _ two identical cells __.
I. IMPORTANCE OF ASEXUAL REPRODUCTION (p. 241-243)
A. Single-celled Organisms
Single-celled organisms belonging to kingdoms __Archaebacteria__,
_Eubacteria__, _Protista___, and _Fungi__, use asexual reproduction to
_reproduce – make new organisms___.
B. Multicellular Organisms
1. Growth & Development – Organisms grow through _ cell division _,
rather than unlimited growth of a cell.
 Surface Area to Volume Ratio – As the size of an object increases,
the _volume________ increases at a much faster rate than the
_surface area________; therefore, the surface area to volume ratio
becomes a _smaller___ number.
 In a cell, the _cell membrane___________ represents the surface
area and the _cytoplasm_____________ represents the volume. At a
certain point, a cell can no longer meets its needs and maintain
_homeostasis________; therefore, cells only grow to a certain size.
2. Renewal & Repair – Cell division is used to replace cells that die from
normal wear & tear; for example, _skin cells_____ and _blood
cells______. Other types of cells maintain the ability to divide, but keep
it in reserve unless severely damaged; for example, _liver
cells____. Other cells do not appear to divide at all in a mature human;
for example, _muscle cells_______ and _nerve cells_____.
II. CELL CYCLE IN PROKARYOTES (p. 475)
All _bacteria_ undergo a type of cell division known as _binary fission_.
Binary fission is a less complex, faster process than eukaryotic cell
division because bacteria lack a _nucleus_, _membrane bound
organelles__ and have only _one_ chromosome.
III. CELL CYCLE IN EUKARYOTES - AN OVERVIEW
There are two main parts to the eukaryotic cell cycle:
A. Interphase - Accounts for about _90%__ of cell cycle. This is the period
of time in which a cell is carrying out _normal metabolic activities,
_doing its cell job_______. _Protein_____ synthesis is occurring at a
high rate. The cell’s DNA is in the form of _chromatin_____, long, fine
strands of DNA wrapped in _protein____.
B. M-Phase – Period of time in which cell division occurs. Consists of two
main events:
 Mitosis – Division of the _nucleus___
 Cytokinesis – Division of the _cytoplasm____
IV. A CLOSER LOOK AT INTERPHASE
There are three stages to interphase:
1. G1 - Cell grows, carries out normal cell activities such as _protein
synthesis, cell respiration_. Organelles are replaced if needed.
_Protein_ production is very high.
2. S - _Synthesis_. Replication of _DNA_ takes place. Known as the
“Point of No Return”. Replication takes place in the _nucleus_ of
the cell.
3. G2 - Preparation for _mitosis_. __Centrioles_ and __all organelles_
are replicated.
Cell at the END of interphase
Protein
microtubules
that will form
spindle
fibers
a. _DNA_ has been replicated – still
in chromatin form.
b. _Centrioles_ have been replicated
in _animal_ cells only. This
replication results in 2 pairs of
centrioles, composed of
_microtubules__.
c. The _nuclear envelope_ and
_nucleolus are still present.
B. Mitosis, also known as the _M - Phase_, is described in four stages, but it is a
continuous process.
1. PROPHASE – This is the _longest_ phase of __mitosis_.
Microtubules form Spindle
fibers – some are kinetochore
and some are non-kinetochore
microtubules
Centrioles
Sister chromatids attach at
the centromere
Nuclear envelope
disintegrates
1. _Nucleolus_ and _nuclear envelope__
disassemble.
2. Replicated DNA thickens & condenses 
Results in _chromosomes_ made up of 2
sister chromatids held together by the
_centromere_.
3. _Centrioles_ begin moving to opposite sides
of
the cell.
4. _Microtubules_extend to form spindle fibers
from centriole to centriole. Some of these
fibers, known as kinetochore microtubules,
begin to attach at the kinetochore of each
chromatid. (located at the centromere).
Other fibers, known as non-kinetochore
microtubules span the cell to aid in support
and structure.
Sister
Chromatids
Late Prophase
2. METAPHASE – This is the _shortest_ phase of mitosis.
Centrioles
Kinetochore microtubules
Spindle fiber network
Sister chromatids
1. _Spindle fiber network_ network is
fully formed with _centrioles_ at
opposite ends.
2. Each _sister chromatid_ is attached
by their kinetochore (at the
centromere) to a spindle fiber from
each pole.
3. Sister chromatids align in _equator
(middle)__ of cell.
3. ANAPHASE
Centromeres
Daughter
chromosomes
Kinetochore microtubules
Non-kinetochore microtubules
1. _Centromeres__ split.
2. Sister _chromatids_ are pulled
apart by _kinetochore
microtubules_ to opposite ends of
the cell.
3. Genetic material is now known as
daughter _chromosomes__.
4. Cell is elongating; preparing for two
new nuclei.
4. TELOPHASE – By the end of telophase . . .
Chromosomes
uncoil to
Chromatin
Nuclear envelope
reforms
1. Complete set of _chromosomes_ at each pole
of the cell.
2. _Spindle fibers _ disassemble.
3. New _nuclear envelope_ forms around
nucleus.
4. _chromosomes_ uncoil  _chromatin_.
5. _nucleolus_ reforms  _ribosomes_ are
produced _protein_ synthesis resumes 
cell _metabolic activity_ resumes
5. CYTOKINESIS – This is the final step, which begins during _telophase_.
Cytokinesis differs in plant and animal cells because plant cells have a
_cell wall_.
a. Animal Cell – In the final stages of telophase, the _cell membrane_
pinches in. This is referred to as a _cleavage_furrow_. This cleavage
furrow deepens until the parent cell is pinched in two, producing two
Cleavage
separate, identical _daughter cells_.
furrow
forming
New daughter cells
b. Plant Cell – There is no cleavage furrow in plant cells. Instead, during late
telophase, _vesicles_ from the Golgi apparatus move to the center of the
cell where cellulose and other materials they contain come together to
form a cell plate ->cell wall
http://www.youtube.com/watch?v=pOsAbTi9tHw
VI. CONTROL OF THE CELL CYCLE
A. Timing of the Cell Cycle
The timing of the cell cycle is regulated by proteins known as _cyclins___ that bind
with enzymes known as ______kinases_________to form a complex called
_cyclin-dependent kinases____ or _CDKs____. These _enzymes________ are
responsible for monitoring checkpoints in the cell cycle.
 If the progression of the cell cycle is too slow, _dying cells are not
replaced___________________.
 If the cell cycle progresses too quickly, the result is uncontrolled cell growth or
_cancer_________. This is harmful for many reasons
o If a cell spends all of its time dividing, _it is not doing its cell job_____
o The rapidly- dividing, nonfunctional cells steal _nutrients_______ from
healthy, functioning cells causing _them to die_______.
o The rapid replication of _DNA____ results in a greater risk of
_mutations____.
B. Stem Cells
Stem cells are _undifferentiated_____ cells that have the ability to _reproduce
forever_______. Stem cells that can give rise to many types of cells are said to be
pluripotent___________. Although stem cells are found in certain locations in
adults, most research involves the use of _embryonic_____________ stem cells,
due to the fact they are considered to be “_immortal_______” and capable of
unlimited specialization. Scientists hope that stem cells may be used as implants to
replace _damaged tissues & organs_____, but there are many
_legal____________ and _ethical_______ concerns.
C. Number of Cell Divisions per Cell
Each cell is programmed to divide a certain number of times, and then it will not
divide again. This is monitored by _telomeres_________, a series of DNA
_nucleotides_______ found at the tips of each _chromosome______. As DNA is
_replicated____ in __S____ of the cell cycle, the telomeres shorten. Once they
reach a critical length, the DNA does not replicate again, and the
cell does not _divide______. _Cancer cells___ and _stem cells____ produce an
enzyme known as _telomerase____, which restores the chromosomes back to
their original length, thereby theoretically making the cell capable of _unlimited____
cell division.