Name _________________________________________________________
Test Date _Fri, 12/10_______
UNIT 8 – THE CELL CYCLE
The third statement of the Cell Theory states that all cells come from __pre-existing cells______. 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 _one 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 like _bacteria____ and _yeast____ use asexual reproduction to _reproduce_______________.
B. Multicellular Organisms
1. Growth & Development – Organisms grow through _making more cells_______, rather than unlimited growth of
a cell because if a cell gets too large, it cannot meet its own needs and maintain _homeostasis______. A cell
grows to a certain size, and then it divides.
2. Renewal & Repair – Cell division is used to replace cells that die from normal wear & tear; for example,
_skin 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, _nerve cells, muscle cells_________. However, recent experimental findings appear to
dispute this long-standing _theory_______.
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
(p. 245 - 248)
A. Interphase - Accounts for about _90%___ of cell cycle. This is the period of time that a cell is carrying out
_normal cell activities________________. During interphase, a cell’s DNA is in the form of _chromatin______, long,
thin strands of DNA wrapped in _protein____.
There are three stages to interphase:
1. G1 - Cell _grows____, carries out _cell job______. _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. Replicated DNA produces two identical
_sister chromatids____. Each pair of chromatids is held together at the middle by a _centromere_____.
3. G2 - Preparation for _cell division__. __Centrioles_________ in animal cells and other organelles are
replicated.
B. Mitosis - Also known as the _M phase________, is described in four stages, but it is a continuous
process
1. Prophase
a. _Nuclear envelope_____ and __nucleolus____ disassemble.
b. The “angel hair pasta” strands of DNA _thicken___, coil and condense to form _chromosomes_____, which are
visible with a compound light microscope. Each chromsome is made up of two identical
_sister chromatids_______.
c. _Centrioles_______ begin moving to opposite sides of the cell.
d. _Microtubules_______ extend to form __spindle fibers______ from centriole to centriole.
2. Metaphase
a. _Spindle fiber____ network is fully formed with _centrioles_____ at opposite ends.
b. Each _sister chromatid___ is attached to spindle fiber at the _centromere______.
c. Sister chromatids align in _middle (equator)____ of cell.
3. Anaphase
a. Spindle fibers shorten, splitting _centromeres______.
b. Sister _chromatids______ are pulled apart to opposite ends of the cell.
4. Telophase – By the end of telophase . . .
a. Complete set of _chromosomes________ at each pole of the cell. Chromosomes uncoil 
_chromatin_________.
b. New _nuclear envelope______ forms around nucleus.
c. _Spindle fibers______ disassemble.
d. _Nucleolus__ reforms  _ribosomes__ are produced  _protein__synthesis resumes  cell
_metabolism________ resumes
5. Cytokinesis – This is the final step in _mitosis_____. 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 separate, identical _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 the _cellulose_______ and
other materials they contain come together to form a _cell plate → cell wall______________________.

______________G1 of Interphase________________
IV. A CLOSER LOOK AT THE CELL CYCLE
V. THE CELL CYCLE & MEDICAL RESEARCH
(pp. 250-253)
A. Timing of The Cell Cycle
The timing of the cell cycle is regulated by _enzymes____ attached to proteins called _cyclins_______.
These cyclin compounds form checkpoints for each stage of the cell cycle. In the case of _cancer____ or a
_tumor______, the cells malfunction and do not respond to this control mechanism resulting in uncontrolled
growth and cell division of these abnormal, non-functional cells.
B. Stem Cells
Stem cells are unspecialized cells that have the ability to _reproduce forever________ and, under appropriate
conditions, _differentiate_______ into specific types of cells. Stem cells that can give rise to many types of cells are
said to be _pluripotent_________. Although stem cells are found 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 or diseased
tissues/organs, but there are many _ethical__ concerns.