Download Cell Cycle Notes

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Test Date _Fri, 12/9
UNIT 6 - DNA & THE CELL CYCLE
The third statement of the Cell Theory states that all cells come from _pre-existing 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 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 _2_______
_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 (p. 245)
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 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:
 G1 - Cell _grows______, carries out normal cell activities such as _cellular respiration, metabolism________.
Period in which _protein_______ production is highest
 S - _Synthesis____________. Replication of _DNA__; known as the “_Point of No Return___”. Replication takes place in
the _nucleus___ of the cell.
 G2 - Preparation for _cell division___. _Centrioles______ are replicated in _animal___ cells.
At the end of interphase . . .
________________
replicated
________________
_____
Protein microtubules that
will form spindle fibers

_DNA___ has been replicated – still in
_chromatin______ form.
_Centrioles_________ have been replicated in
_animal_______ cells only. Results in 2 pairs of
centrioles, composed of _microtubules_______.
The _nuclear envelope____ and _nucleolus__ are
still present.


_____________
_________
_
____________________
V. A CLOSER LOOK AT MITOSIS (pp. 246-248)
Mitosis, also known as the _M - Phase_, is described in four stages, but it is a continuous process.
A. PROPHASE – In prophase, the nucleus is preparing to divide. This includes . . .
_________________________

____________________
____________________
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______________________
________________________
_Nucleolus_____ and _nuclear__________
_envelope____ disassemble.
Replicated _DNA___ thickens & condenses 
results in _chromosomes_____ made up of 2
_sister chromatids__ held together at the
_centromeres________
_Centrioles________ begin moving to opposite
poles of the cell.
_Microtubules______ form _spindle fiber
network______. Specific spindle fibers attach
to the sister chromatids at the
_kinetochore___.
centromere). Other fibers, known as non-kinetochore
B. METAPHASE – In metaphase, the sister chromatids are organized in the microtubules
_middle____span
of the
thecell.
cell to aid in support and
structure.

_____________________

______________________
__________________

______________________
_Spindle fiber______ network is fully formed with
_centrioles______ at opposite ends.
Each _sister chromatid_________ is attached to a
spindle fiber at the _kinetochore________ located
at the _centromere____________
Sister chromatids align at the _equator_______ of
the cell.
C. ANAPHASE – In anaphase, the sister chromatids are pulled _apart_______.
______________________
______________________
______________________
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_________________
________________________
________________________
D. TELOPHASE – By the end of telophase . . .
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____________
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______________
_Centromeres__________ split.
Sister _chromatids_________ are pulled
apart by _spindle fibers___________; move
to opposite ends of the cell.
Genetic material is now known as daughter
__chromosomes_______.
Cell elongates; prepares for two new nuclei.
Complete and _identical_______ set of
_chromosomes_______ at each pole of the cell.
_Spindle fibers _____________ disassemble.
New _nuclear envelope_____________ forms around each
group of chromosomes.
DNA uncoils  _chromatin__________
_Nucleolus___________ reforms  _ribosomes________
are produced _protein synthesis_ resumes  cell
_metabolism__________ resumes
__________________
E. CYTOKINESIS – This refers to the actual division of the _cytoplasm_____________. Cytokinesis begins during
_telophase______ and differs in plant versus animal cells because plant cells have a _cell wall________.
1. Animal Cell – In the final stages of telophase, the _cell membrane_________ pinches in. This is referred to as a
_cleavage furrow_________ and is due to the action of _microfilaments (actin)____. This cleavage furrow
deepens until the parent cell is pinched in two, producing two separate, _identical__________ daughter cells.
2. 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______________, which eventually
develops into a _cell wall____________.
_____________________________
_
____________
_____________________

___________________ Cell
_____________
_______
_______
_
______________________ Cell
_____________________
_____________________
VI. CONTROL OF THE CELL CYCLE
(pp. 250-253)
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.
A CLOSER LOOK AT DNA & REPLICATION
I. DNA – THE DISCOVERY OF THE MOLECULE OF INHERITANCE (pp. 287-294)
DNA contains the _genetic code_______ and the _working instructions______________ for a cell. Scientists in the early
1900s realized that inheritance was dependent on _genes_____ found on _chromosomes_____ and that chromosomes were
composed of _DNA__ and _protein_____. It wasn’t until the 1940s that scientists established _DNA______ as the
molecule of inheritance, and still another 15 years passed before the structure of DNA was discovered.
A. Griffith (1928)
 Discovered that bacteria can take up genetic material from environment in a process known as _transformation______.
 Performed experiments with _mice_____ and bacteria known as _Pneumococcus______.
o Mixed harmless living bacteria with dead _pathogenic_______ bacteria and injected them into mice.
o Mice died.
B. Avery, et.al. (1944)
 Set up experiments to identify “transforming substance” discovered by Griffith
 Used enzymes that destroyed carbohydrates, lipids, proteins, and RNA . . . transformation still occurred. Used an enzyme
that destroyed DNA, _transformation did not occur; mice survived_________________.
 Concluded that _DNA_____ stores and transmits the genetic information from one generation of an organism to the next.
 Scientific community did not completely accept their conclusion
C. Hershey & Chase (1952)
 Used a virus known as a _bacteriophage_____ which infects _bacteria_____.
 Viruses are composed of _DNA___and _protein____; require a _host___ cell in order to replicate.
 Hershey & Chase recognized that virus must inject its genetic material into the bacterial cell for replication to take place.
 Used labeled _isotopes_______ to determine whether DNA or protein was injected into bacterium.
 Hershey & Chase concluded that the genetic material of the bacteriophage was _DNA____.
D. Watson & Crick (1953)
 Used molecular models to solve the 3-dimensional structure of DNA
 “Borrowed” an x-ray of DNA taken by _Rosalind Franklin_________ to solve the puzzle
 Proposed a _double helix_______ structure with a backbone of alternating _deoxyribose______ and _phosphate
groups______ with _nitrogen bases____ paired in the middle
II. STRUCTURE OF DNA
 _Deoxyribonucleic acid_______ is a _nucleic acid____ made up of _nucleotide___ monomers. Each DNA
_nucleotide_________ is composed of :
o _Deoxyribose
(5-carbon sugar)___
o _Phosphate group_________________
o _Nitrogen base________________________
 Strong, stable _covalent_____ bonds hold the components of each nucleotide together.
 In addition, nucleotides form strong _covalent_____ bonds with other nucleotides, resulting in a long strand of nucleotides.
 Double helix created by _hydrogen_____ bonding between _nitrogen bases_____
 The diameter of DNA is uniform due to specific pairing of nitrogen bases
o Purines – double-ringed nitrogen bases
 _guanine______________
 _adenine______________
o Pyrimidines - single-ringed nitrogen bases
 _thymine______________
 _cytosine______________
o _Purine____ always base pairs with a _pyrimidine_____, specifically …
 _Adenine____ base pairs with _thymine_____
 _Guanine____ base pairs with _cytosine_____
o _Watson____ and _Crick_____ determined this structurally because the _diameter____ of DNA is uniform.
o _Chargaff____ determined this chemically. His chemical analysis of DNA from many different organisms always
showed that the % of adenine = % _thymine___ and the % of guanine = the % _cytosine___. These findings
are known as _Chargaff’s rules_____.
 The two strands of nucleotides are _anti-parallel_________; that is, they run opposite to each other.
 It is the _pattern_____ and _number____ of nucleotides that makes each gene unique.
DEOXYRIBONUCLEIC ACID
adenine
covalent bond
cytosine
deoxyribose
guanine
hydrogen bond
nitrogen bases
nucleotide
phosphate group
purines
pyrimidines
thymine
5’ end
3’ end
III.
DNA REPLICATION (pp. 295-299)
Prior to _cell division____, the DNA must _replicate_____. This occurs in _S____ of _interphase____.
During this part of the cell cycle, DNA is in the form of _chromatin____. DNA can replicate itself exactly due to
_Chargaff’s (base pairing) rules____. Replication occurs in the _nucleus_____ of the cell and requires the participation of
several _enzymes______.
Replication may be summarized in the following steps:
 DNA is “unzipped” by the enzyme, _helicase__. Helicase breaks the _hydrogen______ bonds between the _nitrogen
bases______. The point where the unzipping begins is known as the _origin of replication_____. The continuation of the
unzipping is called the _replication fork_____. This occurs in several places in each _chromosome_, much like a
_broken zipper_____.
 _Single-strand binding proteins___________ hold the separated DNA strands apart.
 Another enzyme, _topoisomerase_____ moves ahead of the _helicase______ to relieve tension in the double helix as it is
being unzipped.
 The enzyme, _DNA polymerase_______ adds nucleotides to both sides of the DNA molecule according to _base
pairing______ rules; however, DNA polymerase has two restrictions:
1) It can only add nucleotides in a _5’ – 3’____ direction.
2) It can only add nucleotides to an existing strand.
 Before DNA polymerase begins moving in nucleotides, an _RNA primer_____ must be put in place. Eventually this primer
is removed and replaced with DNA nucleotides.
 The two sides of the DNA molecule are replicated differently because nucleotides can only be added in a _5’ – 3’____
direction:
o One side of the DNA molecule is being replicated in the same direction as the _replication fork____. This is
known as the _leading____ strand. Nucleotides are moved in by _DNA polymerase____ in a _5’ – 3’_____
direction as the DNA is unzipped.
o On the other side of the DNA molecule, nucleotides are added _away_______ from the replication fork. This is
known as the _lagging_________ strand. Short segments of nucleotides are synthesized, each with a new _RNA
primer_______, as the DNA is unzipped. These short segments are known as _Okazaki fragments_______.
Eventually, another enzyme known as _ligase____ “glues” together the _Okazaki_____ fragments to create a
continuous strand of nucleotides.
 Another type of _DNA polymerase______ proofreads the replicated DNA to minimize the chance of _mutation_____.
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
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The result is two _identical________ DNA molecules, each new DNA consists of one _original___ strand and one
_new___ strand. This is known as the _semi-conservative______ model of replication.
These two identical DNA molecules are the _sister chromatids_____ that proceed through _mitosis________.
Once replication has been completed, the cell moves into _G2___ and then, _prophase___ of _mitosis___.
IV. A DETAILED VIEW OF DNA REPLICATION