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General Biology Final Study Guide
MITOSIS
Mitosis is the asexual process of cell reproduction (cell division) in which one “parent”
cell divides to produce two new “daughter” cells. The daughter cells are clones of the
parent cell. In other words, they are genetically identical to the parent.
Mitosis has 4 phases
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
1) Prophase
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o
o
DNA condenses to form chromosomes.
Centrioles and spindles form
Nuclear envelope (membrane) begins to break up.
2) Metaphase
o
The Spindle attaches to the kinetochores of the chromosomes and lines the
chromosomes up along the midline.
3) Anaphase
o
The Spindle pulls sister chromatids to opposite sides of the cell.
4) Telophase
o
o
o
The chromosomes decondense back into chromatin form.
Spindle disappears.
Nuclear envelope (membrane ) reforms.
Cytokinesis
Cytokinesis occurs after telophase and is the process of proteins “pinching” the single cell into 2
newly formed daughter cells.
Development
You are made up of trillions of cells. All of your cells have the same DNA. So... how come we get so
many different types of cells, if all our cells have the same set of “instructions”? Each cell will end up
“reading” only part of the genetic code “instruction manual”. During development and throughout each
cell’s life, each cell will come across different chemical signals (like growth factors) that activate certain
parts of the genome.
Zygote to Blastocyst
A fertilized egg is called a zygote. The zygote then becomes 2 cells, then 4 cells,
then 8 cells, then 16 cells and then 32 cell followed by 64 cells. Between the 32
to 64 cell stage, the mass of cells is called a blastocyst. All cells of the blastocyst
are STEM CELLS. That means I can take any of those cells and make your twin…
or any other cell type I want.
Sonic hedgehog (SHH)
At the blastocyst stage the cells begin secreting growth factors like Sonic
hedgehog (SHH). The strongest cell will continue to secrete SHH as the other
cells stop making SHH, causing a concentration gradient.
Different concentration gradients "turn on" different genes.
The cells secreting SHH will become the head. Each of the cells in the
blastocyst will be getting a different concentration of SHH depending on where
it is in relation to the head. It is at this point that the cells will begin to
differentiate from stem cells to into different types of cells. The different
concentrations of SHH “turn on” only certain genes in certain cells.
Mitosis for Wound Repair
When you are an adult, your body will still undergo mitosis for cell repair and
cell replacement. Cells react and respond to changing environments. How
does your body know when it need to grown more cells to heal a wound? Cells
communicate to each other using chemical signals. When you cut yourself, the
cells along the edge notice that their neighbor is gone. The cell responds by will
entering the cell cycle and mitosis to make more cells to fill in the ‘gap’.
We need mitosis for…
1. Growth
2. Repair
3. Replacement
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How did YOU get from Baby to Grown-Up? The answer is MITOSIS!
All the Cells
of your Body (except for your oocytes or spermatozoa) Reproduce by MITOSIS! You
see, cells undergo what we like to call "The Cell Cycle". 90% of this cycle involves the
cells preparations for division in mitosis.
IN YOUR BODY Millions of cells divide every second. Your body has ~10 trillion
cells. Some cells divide a lot and others almost never divide! For example, neurons,
muscle cells and red blood cells don't divide once they are fully differentiated
(mature). This explains why brain injuries and heart ailments are so detrimental to the
sufferer. Recovery from injury to these areas is difficult, and, depending on the extent of
damage, can be lethal
Neurons that are fully differentiated (fully mature) do not divide. Neural stem
cells can divide just fine. So, why don't most neurons divide? There are 3 likely reasons.
First, neurons that are fully differentiated are highly specialized and can be very large.
The brain responds to brain injury, by “rewiring” itself, and even reallocating lost
functionality to new brain regions. Therefore, the brain has found an alternative to
replacing damaged neurons. Nowhere is the “rewiring” ability of the brain more apparent
than it is in the story of Cameron, a girl with half her brain. See the news clip of this
amazing story at the link below. Cameron underwent a hemispherectomy procedure at
Johns Hopkins Medical Center to treat extreme epilepsy. She had to teach her LEFT side
of her brain, to take over the functions that the missing RIGHT hemisphere would have
performed. Most notably, the ability to control the LEFT side of the body.
Muscle cells that are fully differentiated (fully mature) do not divide. Why Don’t
Muscle Cells Divide? The short answer is, “Nobody really knows.” These cells are large
and complex and have very specific functions and a limited amount of “space”, so these
may be key factors.
Red blood cells do not undergo mitosis once they are fully differentiated. Fully
differentiated (mature) red blood cells are specialized for one task; getting oxygen to your
cells and taking carbon dioxide away from cells. Red blood cells (RBCs) are rather odd
cells, because 1) They have NO nucleus; 2)They DO NOT have DNA; 3)They have NO
organelles; 4)They cannot make RNA; and 5)They cannot divide!
red blood cells are made from hemocytoblasts. Your bone marrow makes hemocytoblasts
which are stem cells that have the ability to differentiate into any of the cellular
components of the blood. They can become red blood cells, white blood cells or platelets
(clotting factors) . The hemocytoblasts DO have a nucleus and do
produce enzymes and proteins. When the RBCs mature and pour into blood they lose
their nucleii. So, a mature RBC functions only till the enzyme and protein stock in them
are exhausted. That's the reason for a short life span of RBC of about 120 days.
Some Cells Divide A LOT! The cells that line the inside and outside of our bodies,
divide a lot since they . The cells in the living layer at the base of the skin divide often to
replenish the cells of the skin that are lost all the time. The cells that line the digestive
system, particularly the intestines are also amongst the rapid dividers.
The Stages of the Cell Cycle
The two main stages of the cell cycle are
1. interphase
2. mitosis
Interphase
In interphase, the cell is undergoing preparations for mitosis.
Interphase is separated into 3 divisions or phases;
1. G1 (Gap1)
a. the cell increases in size
b. doubles its cytoplasmic contents and organelles
2. S-Phase (DNA Synthesis)
a. At the beginning of the S phase, each chromosome is single. Each chromosome
doubles it’s contents to form two sister chromatids joined at the centromere.
3. G2 (Gap 2)
a. a period of rapid cell growth
b. protein synthesis
c. cell readies itself for mitosis
What is G0? G0 is a resting phase where the cell has left the cycle and has stopped
dividing. In G0 the cell can be either…
o
o
Quiescent – lay dormant and may start dividing again
Senescent – get old and eventually undergo apoptosis (cell death)
Cell Cycle Checkpoints
There is tight regulation of Cell Growth. Cells don’t just grow when they feel like it! You want to make
enough cells, but not too many cells. When cells start reproducing when they aren’t supposed to, a
TUMOR is formed. The Cell Cycle is the process by which the body regulates cell division (aka cellular
reproduction)
The cell cycle is controlled at three checkpoints (G1, G2 and M). Check Points for is
essential for accurate, healthy Cell Division (Reproduction). It is essential that the daughter cells
are exact duplicates of the parent cell. Mistakes in the duplication or distribution of the
chromosomes lead to mutations that may be passed forward to every new cell produced from an
abnormal cell. Uncontrolled cell growth leads to tumors and ultimately, cancer.
The G1 Checkpoint - The G1 checkpoint determines whether all conditions are favorable
for cell division to proceed. The cell will only pass the checkpoint if it is an appropriate size and
has adequate energy reserves. At this point, the cell also checks for DNA damage. A cell that
does not meet all the requirements will not progress to the S phase. If this happens, the cell has 2
options. It can HALT the cycle and attempt to remedy the problematic condition, or the cell can
advance into G0 (inactive) phase and await further signals when conditions improve..
The G2 Checkpoint If a cell meets the requirements for the G1 checkpoint, the cell will enter
S phase and begin DNA replication. The G2 checkpoint is the most important, because it is the
last check point before the cell enters MITOSIS! The G2 checkpoint checks for cell size,
protein reserves and check to be sure that all of the chromosomes have been accurately
replicated without mistakes or damage. If the checkpoint mechanisms detect problems with
the DNA, the cell cycle is halted and the cell attempts to either complete DNA replication or
repair the damaged DNA. If the DNA has been correctly replicated, cyclin dependent kinases
(CDKs) signal the beginning of MITOSIS (mitotic cell division).
The M Checkpoint The M Checkpoint occurs near the end of the metaphase stage of mitosis.
The M checkpoint is also known as the spindle checkpoint because it determines whether all the
sister chromatids are correctly attached to the spindle microtubules. Because the separation of
the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until the
kinetochores of each pair of sister chromatids are firmly anchored to at least two spindle fibers
arising from opposite poles of the cell.
Genetic variance is brought about in Meiosis by 1) crossing over and 2) independent assortment.
Genetic Inheritance
Even though we all have the same genes, the type of genes are different. You can think of it as
having different flavors of ice cream. Ice cream has a lot of different flavors, but they are still ice
cream. In genetics, we call the different types of genes, Alleles. The form of the gene can be
either DOMINANT or RECESSIVE. A dominant gene is the one that can mask the expression
of the a recessive gene.
Humans have 2 full sets of chromosomes. One set from your mom that came from the egg that
made you, and one set came from the sperm that fertilized the egg. So you have 2 sets of 23
chromosomes, for a total of 46 chromosomes. SInce you have 2 full sets of chromosomes, this
means that you have 2 genes for each of your traits (one gene from mom and one gene from
dad). Some genetic traits are dominant and will be expressed (outwardly displayed), even if the
other equivalent gene is a recessive form.
An individual would have to be homozygous recessive in order to have the recessive trait.
This means that they have inherited a recessive gene from mom and a recessive gene from dad.
Genotype- the actual allele combination in an individual
Phenotype- outward manifestation or expression of an individuals genotype
Alleles- alternative forms of the same gene
Dominant is the one that can mask the expression of the other
Recessive the gene whose expression is able to be masked
Homozygous recessive-the individual has inherited a dominant allele (form of the gene) from
both the mother and the father.
Homozygous dominant the individual has inherited a recessive allele (form of the gene) from
both the mother and the father.
Heterozygous the individual has inherited a dominant allele (form of the gene) from one parent
and recessive allele (form of the gene) from one parent
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Benefits of Genetic Variation
Meiosis increases genetic variation due to the random recombination of genetic material (from mom and
dad). It is for this reason that we consider meiosis sexual reproduction. Asexual reproduction, like we see
in mitosis, yields genetically identical offspring (clones of the parent cell). Sexual reproduction is
important for survival of the species. For example, when a potentially detrimental change occurs in the
environment, a species having a wide variety of genetic variation will have a much better chance that at
least some of the individuals of the species will survive.
A very significant example of this in human history is the Black Plague (a.k.a. The black Death) that
took the lives of approximately one-half of Europe's population in the 1300's.
When viruses invade the body, they do not have the ability to reproduce on their own. They must invade
the hosts's cells and then hijack the reproductive machinery of the cell to produce more copies of the
virus. Then the copies of the virus cause the host cell to burst open, freeing thousand of copies of the virus
that can then continue the cycle of cell invasion and virus replication. Eventually, as the amount of the
virus increases in the body, the victim will become ill and display the symptoms of the illness.
In order for viruses to invade the host cells in the first place, the host cell must carry specific receptor that
has the complementary shape to the virus. You can think if the receptor as a LOCK and the virus as the
KEY. If the virus doesn't fit, it cannot "open the door" and gain entrance to the host cell. In the case of the
Black Plague, some of the European population carried a specific type of genetic mutation that caused the
receptor that the virus uses to gain access to the cell, to be misshapen! The virus was unable to invade the
cells of these individuals, so the virus could not reproduce and make these people sick. The mutation gave
them immunity to the virus! A similar story can be found more recently in the discovery of the Delta 32
mutation that allowed some people to be immune to the HIV virus! More interestingly, this mutation
(Delta 32) prevents both HIV and plague bacteria from entering human cells and causing
infection. Genetic diversity allows offspring to have a better chance of surviving environmental changes
(natural selection).
Meiosis is separated into Meiosis I and Meiosis II. Meiosis I and II each have phases that are
named after the phases of mitosis, but there are some very important differences.
Meiosis I
Prophase I
- During prophase I, differences from mitosis begin to appear. As in mitosis, the
chromosomes begin to condense, but in meiosis I, they also pair up.
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Each chromosome carefully aligns with its homologue partner so that the two match up at
corresponding positions along their full length. For example, chromosome 1 of dad will
line up and overlap with chromosome 1 of mom; chromosome 2 of dad will line up and
overlap with chromosome 2 of mom, and so on.
In Prophase I the cell has 46 chromosomes, 23 from mom and 23 from dad. The genetic material
has already been doubled in the S-Phase of Interphase, but the material is connected at the
centromere.
In Prophase I, these chromosomes condense forming the familiar 'X' shape. In this form, the
chromosomes are made up of 2 genetically identical sister chromatids.
In late Prophase I, homologous chromosomes will "pair up" and "overlap" and they swap genetic
material in a process called "crossing-over".
Metaphase I
After Prophase I, Metaphase I begins. Here, the spindle begins to "capture" the homologous
chromosomes and move them towards the middle of the cell (metaphase plate). The
chromosomes are lined up side by side with their homolog. The spindle will attach to the
kinetichore of the ONLY ONE of the homologous chromosomes. In this way, one homologous
chromosome goes to one pole of the cell and the other homolog goes to the other side of the cell.
Remember that the homologs are no longer identical due to cros-over!
HOW IS THIS DIFFERENT FROM MITOSIS? During metaphase I, it is homologue pairs of
chromosomes that line up, not individual chromosomes as in mitosis.
The spindle attaches to the kinetochore of hmologous chromosomes and pulls each of
them to opposite sides of the cell, towards the centriole.