CHAPTER 1: OVERVIEW OF THE CELL CYCLE
THE THREE STAGES OF INTERPHASE:
INTERPHASE
BEFORE A CELL CAN ENTER CELL DIVISION, IT NEEDS TO PREPARE ITSELF BY
REPLICATING ITS GENETIC INFORMATION AND ALL OF THE ORGANELLES. ALL OF THE
PREPARATIONS ARE DONE DURING THE INTERPHASE. INTERPHASE PROCEEDS IN
THREE STAGES, G1, S, AND G2. CELL DIVISION OPERATES IN A CYCLE. THEREFORE,
INTERPHASE IS PRECEDED BY THE PREVIOUS CYCLE OF MITOSIS AND CYTOKENESIS.
G1 PHASE: AFTER MITOSIS IS COMPLETE THE NEW DAUGHTER CELL BEGINS TO
ACCELERATE ITS BIOCHEMICAL PROCESSES WHICH WERE SLOWED DOWN BY MITOSIS.
THE LENGTH OF THE G1 PHASE CREATES THE DIFFERENCE BETWEEN FAST DIVIDING
CELLS AND SLOWLY DIVIDING CELLS. THE G1 PHASE CAN BE SLOWED BY REDUCING
THE NUTRIENTS AVAILABLE IN A SYSTEM - THUS THE CELL WILL TAKE LONGER TO
BUILD UP THE RESOURCES NECESSARY FOR CELL DIVISION. IF THERE IS A SEVERE
DEPLETION IN NUTRIENTS THE CELLS CAN VIRTUALLY STOP GROWING. IT IS
INTERESTING TO NOTE THAT CELLS THAT AREN'T GROWING ARE ALWAYS STOPPED IN
THE G1 PHASE, BEING MITOTICALLY ARRESTED. THIS SUGGESTS THAT ONCE THE
CELL ENTERS THE S PHASE, IT IS COMMITTED TO CELL DIVISION, REGARDLESS OF THE
EXTERNAL CELL CONDITIONS.
S PHASE: THE S PHASE BEGINS WITH THE REPLICATION OF THE CELLULAR DNA.
THIS IS DESCRIBED IN FURTHER DETAIL IN DNA REPLICATION. WHEN THE CELLULAR
DNA HAS BEEN DUPLICATED, LEAVING THE CELL WITH TWICE AS MANY
CHROMOSOMES (EACH CHROMOSOME IS MADE UP OF TWO IDENTICAL CHROMATIDS),
THE CELL MOVES ONTO THE G2 PHASE.
G2 PHASE: DURING THIS PHASE PROTEINS, SUCH AS KINASE (WHICH CATALYZES
PROTEIN PHOSPHORYLATION), WHICH ARE NECESSARY FOR CELL DIVISION ARE
SYNTHESIZED AT THIS TIME. THE CHROMOSOME BEGINS TO CONDENSE AND THE
PROTEINS NECESSARY FOR CONSTRUCTION OF THE MITOTIC SPINDLE ALSO ARE
SYNTHESIZED. WHEN THE CHROMOSOMES BECOME VISIBLE THE CELL ENTERS THE
FIRST STAGE OF MITOSIS, PROPHASE.
G0- GAP 0 (G0): THERE ARE TIMES WHEN A CELL WILL LEAVE THE CYCLE AND QUIT
DIVIDING. THIS MAY BE A TEMPORARY RESTING PERIOD OR MORE PERMANENT. AN
EXAMPLE OF THE LATTER IS A CELL THAT HAS REACHED AN END STAGE OF
DEVELOPMENT AND WILL NO LONGER DIVIDE (E.G. NEURON)
THE APPROXIMATE TIME A SKIN CELL STAYS IN EACH PHASE OF THE CYCLE IS 48
HOURS.
CHAPTER 2: CANCER-UNCONTROLLABLE CELL DIVISION
FOR ANY NUMBER OF REASONS, CERTAIN CELLS IN THE BODY CAN DEVELOP
ALTERATIONS IN THEIR DNA UPON REPLICATION. OFTEN, THESE CELLS JUST DIE OFF
BECAUSE THEY ARE INFERIOR AND DON'T HAVE THE ABILITY TO SURVIVE AS NORMAL
CELLS WOULD. HOWEVER, SOME OF THOSE DNA ALTERATIONS CAUSE A CELL TO "GO
CRAZY" FOR LACK OF A BETTER EXPRESSION.
THE ALTERATIONS IN THE CELL CAN RESULT IN SHORTER CELL CYCLES, MEANING
THAT THEY WILL REPLICATE AND DIVIDE MORE QUICKLY. SOMETIMES THE
ALTERATIONS WILL AFFECT NORMAL APOPTOSIS OR PROGRAMMED CELLULAR DEATH.
THIS MEANS THAT OUR CELLS ARE ONLY DESIGNED TO LIVE FOR A CERTAIN AMOUNT
OF TIME BEFORE THEY ARE REPLACED BY NEWLY FORMED CELLS. IF APOPTOSIS IS
INTERRUPTED, THE OLD CELLS DON'T DIE OFF THE WAY THEY SHOULD. THEY HANG
AROUND WITH THE NEW CELLS AND EVERYONE JUST KEEPS REPLICATING.
ULTIMATELY, CANCER IS UNCONTROLLED CELL DIVISION GONE TO THE EXTREME.
CHAPTER 3: MITOSIS
MITOSIS IS THE PROCESS BY WHICH A EUKARYOTIC CELL SEPARATES THE
CHROMOSOMES IN ITS CELL NUCLEUS INTO TWO IDENTICAL SETS, TWO SEPARATE
NUCLEI. IT IS GENERALLY FOLLOWED IMMEDIATELY BY CYTOKINESIS, WHICH
DIVIDES THE NUCLEI, CYTOPLASM, ORGANELLES AND CELL MEMBRANE INTO TWO
CELLS CONTAINING ROUGHLY EQUAL SHARES OF THESE CELLULAR COMPONENTS.
WHEN A PARENT CELL GOES INTO THE PROCESS OF MITOSIS THE ENDING RESULT IS
TWO DAUGHTER CELLS EXACTLY THE SAME AS THE PARENT CELL. MITOSIS HAS 5
MAIN PROCESSES.
INTERPHASE
THE CELL IS ENGAGED IN
METABOLIC ACTIVITY AND
PERFORMING ITS PREPARE FOR
MITOSIS (THE NEXT FOUR
PHASES THAT LEAD UP TO AND
INCLUDE NUCLEAR DIVISION).
CHROMOSOMES ARE NOT
CLEARLY DISCERNED IN THE
NUCLEUS, ALTHOUGH A DARK
SPOT CALLED THE NUCLEOLUS
MAY BE VISIBLE. THE CELL MAY CONTAIN A PAIR OF CENTRIOLES (OR MICROTUBULE
ORGANIZING CENTERS IN PLANTS) BOTH OF WHICH ARE ORGANIZATIONAL SITES FOR
MICROTUBULES.
PROPHASE
CHROMATIN IN THE
NUCLEUS BEGINS TO CONDENSE
AND BECOMES VISIBLE IN THE
LIGHT MICROSCOPE AS
CHROMOSOMES. THE
NUCLEOLUS DISAPPEARS.
CENTRIOLES BEGIN MOVING TO
OPPOSITE ENDS OF THE CELL
AND FIBERS EXTEND FROM THE
CENTROMERES. SOME FIBERS
CROSS THE CELL TO FORM THE MITOTIC SPINDLE.
METAPHASE
SPINDLE FIBERS ALIGN
THE CHROMOSOMES ALONG THE
MIDDLE OF THE CELL NUCLEUS.
THIS LINE IS REFERRED TO AS
THE METAPHASE PLATE. THIS
ORGANIZATION HELPS TO
ENSURE THAT IN THE NEXT
PHASE, WHEN THE
CHROMOSOMES ARE SEPARATED,
EACH NEW NUCLEUS WILL
RECEIVE ONE COPY OF EACH CHROMOSOME.
ANAPHASE
THE PAIRED
CHROMOSOMES SEPARATE AT
THE KINETOCHORES AND MOVE
TO OPPOSITE SIDES OF THE
CELL. MOTION RESULTS FROM A
COMBINATION OF KINETOCHORE
MOVEMENT ALONG THE SPINDLE
MICROTUBULES AND THROUGH
THE PHYSICAL INTERACTION OF
POLAR MICROTUBULES.
TELOPHASE
CHROMATIDS ARRIVE AT
OPPOSITE POLES OF CELL, AND
NEW MEMBRANES FORM
AROUND THE DAUGHTER
NUCLEI. THE CHROMOSOMES
DISPERSE AND ARE NO LONGER
VISIBLE UNDER THE LIGHT
MICROSCOPE. THE SPINDLE
FIBERS DISPERSE, AND
CYTOKINESIS OR THE
PARTITIONING OF THE CELL MAY ALSO BEGIN DURING THIS STAGE.
CYTOKINESIS
IN ANIMAL CELLS,
CYTOKINESIS RESULTS WHEN A
FIBER RING COMPOSED OF A
PROTEIN CALLED ACTIN
AROUND THE CENTER OF THE
CELL CONTRACTS PINCHING THE
CELL INTO TWO DAUGHTER
CELLS, EACH WITH ONE
NUCLEUS. IN PLANT CELLS, THE
RIGID WALL REQUIRES THAT A
CELL PLATE BE SYNTHESIZED BETWEEN THE TWO DAUGHTER CELLS.
Chapter 4: Meiosis Overview
Meiosis is a specialized process of cell division that produces gametes (eggs and
sperm). It is a differentiation pathway, distinct from the mitotic cycle of normally
dividing cells. In humans, it is estimated that at least 10% of conceptions have
defects that probably occurred during meiosis of either paternal or maternal
gametes. Most of these will result in miscarriage. Understanding the process of
meiosis is therefore fundamental to understanding human health and development.
This page uses simple schematics to compare mitosis to meiosis in a generalized
cell.
First, consider mitosis (typical cell division) in a normal diploid cell, with two
chromosomes. The chromosomes are duplicated during DNA replication, and the
duplicates, called sister chromatids, remain attached to one another via sister
chromatid cohesion, until chromosome segregation when the spindle pulls the pairs
apart. Each daughter cell then receives one of the sisters from each homologue
pair. Importantly, this means that the daughter cells are exact copies of the mother
cell, with two copies of each chromosome, so they can go through the same
process again. Thus, we call this process the cell cycle. This is the usual process of
division by which the cells of our bodies renew themselves.
Human somatic cells, with their full set of 46 chromosomes, have what geneticists
refer to as a diploid number of chromosomes. Gametes have a haploid number
(23). When conception occurs, a human sperm and ovum combine their
chromosomes to make a zygote (fertilized egg) with 46 chromosomes. This is the
same number that the parents each had in their somatic cells. In doing this, nature
is acting conservatively. Each generation inherits the same number of
chromosomes. Without reducing their number by half in meiosis first, each new
generation would have double the number of chromosomes in their cells as the
previous one. Within only 15 generations, humans would have over 1½ million
chromosomes per cell and would be a radically different kind of animal. In fact,
when a zygote has an extra set of chromosomes, it usually is spontaneously aborted
by the mother's reproductive system--it is a lethal condition.
Chapter 5: Meiosis I
This is a specialized process of cell division that produces gametes (eggs and
sperm). It is a differentiation pathway, distinct from the mitotic cycle of normally
dividing cells. In humans, it is estimated that at least 10% of conceptions have
defects that probably occurred during meiosis of either paternal or maternal
gametes. Most of these will result in miscarriage. Understanding the process of
meiosis is therefore fundamental to understanding human health and development.
This page uses simple schematics to compare mitosis to meiosis in a generalized
cell.
First, consider mitosis (typical cell division) in a normal diploid cell, with two
chromosomes. The chromosomes are duplicated during DNA replication, and the
duplicates, called sister chromatids, remain attached to one another via sister
chromatid cohesion, until chromosome segregation when the spindle pulls the pairs
apart. Each daughter cell then receives one of the sisters from each homologue
pair. Importantly, this means that the daughter cells are exact copies of the mother
cell, with two copies of each chromosome, so they can go through the same
process again. Thus, we call this process the cell cycle. This is the usual process of
division by which the cells of our bodies renew themselves.
Chapter 7: comparison of mitosis & meiosis
Prior to meiosis or mitosis, DNA replication occurs. In both meiosis and
mitosis, the chromosomes condense and become visible under the microscope. In
meiosis, homologous pair up or synapse, and crossing over takes place.
Homologous chromosomes then align themselves along the metaphase plate and
each homologue is drawn to the opposite end of the cell.
Meiosis
Occurrence of crossing over: yes
Occurs in: humans, animals, plants, fungi
Number of daughter cells: 4
Creates: sex cells only: female egg cells or male sperm eggs
Type of reproduction: sexual
Mitosis
Occurrence of crossing over: no
Occurs in: all organisms
Number of daughter cells: 2
Creates: makes everything but sex cells
Type of reproduction: asexual
References
http://www.google.com/imgres?imgurl=http://www.carolguze.com/images/cell
%2520division/comp2.jpg&imgrefurl=http://www.carolguze.com/text/442-3cell_cycle_mitosis_meiosis.shtml&h=360&w=480&sz=50&tbnid=nLDUXPjejGk
9WM:&tbnh=83&tbnw=111&prev=/search%3Fq%3Dcomparison%2Bof
%2Bmitosis%2Band%2Bmeiosis%26tbm%3Disch%26tbo
%3Du&zoom=1&q=comparison+of+mitosis+and
+meiosis&docid=ilxAnJnMv0ftSM&hl=en&sa=X&ei=JfPkTuqeEKe2AW8t93WBA&sqi=2&ved=0CFoQ9QEwBg&dur=2516
http://highered.mcgraw-hill.com
http://www.diffen.com/difference/Meiosis_vs_Mitosis
http://www-bcf.usc.edu/~forsburg/meiosis.html
http://anthro.palomar.edu/biobasis/bio_2.htm
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html
http://www.cellsalive.com/mitosis.htm
http://library.thinkquest.org/C004535/text/interphase.html
http://www.cellsalive.com/cell_cycle.htm