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INTRODUCTION
Biotechnology refers to technologies utilizing or based on biology – a formal
definition from the United Nations Convention on Biological Diversity states that
biotechnology is “any technological application that uses biological systems, living
organisms, or derivatives thereof, to make or modify products or processes for
specific use.” Biotechnology is used in a variety of fields, including agriculture,
food science and medicine and is often associated with genetic manipulations.
This chapter will provide some background information on genetics, will apply that
information to the techniques of cloning and will end with a discussion of a hot
topic in the world of biotechnology – stem cells.
GENETICS
You have probably heard the expression, “it runs in the family”, but have you ever
wondered what exactly that means? Or why you have the
same bright red hair as your dad? Or why all of your uncles
are bald? A lot of questions like these can be answered by
genetics. Genetics refers to the study of genes, or DNA.
DNA stands for deoxyribonucleic acid and it acts as a sort
of instruction manual for every living thing. DNA is found in
almost all cells and is very, very small! Despite the fact that
DNA is so small, it is really important!
The structure of DNA
DNA looks like a ladder that has been twisted, a spiral
staircase of sorts – this twisted shape is called a helix. (You
will often hear of DNA referred to as the double helix
because it has two interwoven helical strands). Even though
DNA looks like a ladder, it behaves more like a zipper.
Picture the two sides of a ladder or a zipper.
Figure 1: Deoxyribonucleic
acid (DNA) contains the
“recipe” for making
organisms. Source:
http://en.wikipedia.org/wi
ki/Image:DNA-structureand-bases.png
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5’
3’
AT
CG
TA
Sugar
deoxyribose
& phosphate
phosphate
backbone
backbone
GC
GC
AT
3’
5’
nucleic acid
Figure 2: When DNA is
unfolded from its helical
structure, it looks like a
ladder, with the base pairs
as the rungs and the
deoxyribose and phosphate
as the sides
These two parts are the backbone of DNA and are made
up of a type of sugar molecule called deoxyribose that is
joined together with phosphate bonds.
These
“backbones” have different ends, called the 3’
(pronounced “3 prime”) and 5’ (“5 prime”) ends. Since one
side runs 3’5’ and the other runs 5’3’, we call the
strands antiparallel.
The teeth of the zipper (or the rungs of the ladder) are
made of nucleic acids. There are four types of nucleic
acids in DNA: A (for adenine), G (for guanine), C (for
cytosine) and T (for tyrosine). A can only pair with T and
G can only pair with C. An A-T link is held together by
two bonds (bonding is the linking of chemical structures
to each other) while a G-C link is held together by three
bonds. This makes G-C bonds stronger.
This model for DNA is called the Watson and Crick
model, named after James D. Watson and Francis Crick
who received the Nobel Prize for their model in 1962. Another researcher,
Rosalyn Franklin was also very important in figuring out the double helix shape of
DNA.
What does DNA do?
Okay, I bet you’re thinking, how do these nucleic acids turn into a blueprint for
making people? There are around 3 billion pairs of Gs, Cs, Ts and As in humans that
are arranged in a pattern, which is called the DNA sequence.
Different
combinations of these nucleic acids mean different things. Think of how many
combinations can be made with 6 billion G, C, T and As! It’s just like a code; in fact
it’s actually called the genetic code. Important parts of the DNA sequence are
called genes. They code for specific things like hair colour, for example.
If you think of how many ways that every person is the same, it makes sense that
most of your DNA is exactly like your mom’s, or your best friend’s or the Queen of
England’s. In fact, your DNA is even 99% the same as a mouse’s DNA! Even though
each person has very similar DNA sequences, there are some small differences
that make each person’s DNA unique.
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How do those CSI guys tell people apart by their DNA?
Since every person’s DNA is a little bit different, we can use DNA to distinguish
one person from another. Solving crimes using DNA evidence is part of the field
of forensic biology. Basically, after DNA evidence like hair, blood or skin has been
collected from a crime scene, DNA is extracted, or removed, from the cells.
Then, a few steps are followed using special machines. What you end up with is a
way to compare different people.
Here’s how it works:
All of the
The DNA gets cut up by special scissors!!!
STEP
1. Special proteins, called
restriction enzymes, act like scissors and
cut up the DNA. These enzymes are
specific, so that a certain restriction
enzyme can only cut a particular sequence
cutof
up pieces
sizes.
A, C, ofGDNA
andareT.different
For example,
one
Figure 3: Restriction enzymes (represented by the
scissors) cut DNA at specific sequences.
enzyme
called
HpaII will only cut
when it sees the sequence CCGG. Another restriction
enzyme is the BamHI, which recognizes this sequence: 5’GGATTC-3’ and cut between the two guanines (G).
Figure 4: After being
cut up by restriction
enzymes, you end up
with different sized
pieces of DNA.
In Figure 3, imagine that the green scissors are HpaII.
They can only cut CCGG, represented here by the green bar.
When the enzymes cut up the DNA, it ends up in many
pieces, of many different sizes (Figure 4).
A special machine sorts the DNA by size.
Step 2. All of the cut up pieces are placed in a
machine that will sort the pieces by size. That
machine is called a gel electrophoresis machine.
How it works is that when the pieces of DNA get
put in the gel, they move from the top to the
bottom. The thing is, different sized pieces move
at different speeds. Little pieces are speedy and
move quickly through the gel, while large pieces are
slow and stay close to the top of the gel.
(Little pieces are fast, so they move faster to the bottom.)
BIG
TOP
LITTLE
BOTTOM
Figure 5: Gel electrophoresis lets you
separate pieces of DNA based on their
size
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WANT TO KNOW MORE?
If you noticed, the word electrophoresis sounds a lot like electricity. That is
because the machine uses electricity to get the pieces to move. Have you ever
played with magnets? If you had a bar magnet then one side would be negative and
one side would be positive. If you tried to bring the two negative ends together,
they would repel each other. DNA has a negative charge. So if you make the gel
have a negative charge at the top and a positive charge at the bottom, the negative
pieces of DNA will try to get away from the negative charge in the gel. That’s why
the pieces of DNA move!
So, now that you understand how these gels work, but I bet you want to know how
exactlyWe
you
two people
apart. Well, remember how everyone’s DNA is a
arecan
ALLtell
a little
bit different!
little bit different? Each person’s
DNA will be a little bit different at
those cut sites too. In Figure 6, can
Uncle
Norm of
Õs DNA
Queen
England
BIG
you see how the green cut site is
LITTLE
large for the Queen of England but
small for Uncle Norm? Well, when
PrimeUncle
MinisterNorm
Harper Õs DNA
these two people’s DNA is all cut up,
it will make different sized pieces.
Figure 6: Pieces of DNA from the Queen of
When those pieces are run on the gel,
England and from Uncle Norm. Note how there
they will each make a different
are some differences.
pattern.
Our DNA has different sizes of pieces so it
makes a different pattern when it ’s all cut up.
Queen
of England
Uncle NormÕ
s DNA
Here is an example of what two different
people’s gels could look like (Figure 7).
(Remember, they wouldn’t actually be
different colours!) You can see that you can
tell the difference between the Queen of
England and Uncle Norm by the pattern the
pieces of DNA make.
Uncle Norm
Prime Minister HarperÕ
s DNA
Figure 7: Gel electrophoresis of DNA from the
Queen of England and from Uncle Norm.
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Here is an example of what it can look like for real:
1 piece of DNA
another piece of
DNA
Six different people
Figure 8: A real picture of a gel electrophoresis of DNA from six
different people. Picture courtesy of Karla Bretherick from UBC.
Why do people say that I have my mum’s nose and my dad’s chin?
In Figure 9, you see a very simple
version of what is known as a
pedigree. In this pedigree Mr. &
Mrs. Smith have three children.
Mr. Smith
Mrs. Smith
Jane has her dad’s eyes and nose,
but her mum’s mouth. Peter has his
mum’s nose and eyes but his dad’s
mouth. Sue has her mum’s eyes and
her dad’s nose and mouth. It
seems like all three children are a
Sue
Jane
Peter
mixture of mum and dad. This is
ene
pretty much what happens in
Figure 9: A pedigree of the Smith family.
humans!
(There isn’t a single
“nose” gene or “mouth” gene – instead there are many, many genes that, together,
result in a given trait). The DNA that carries all this information gets handed
down from parents in packages called chromosomes. This is known as inheritance.
Humans have a total of 46 chromosomes, half from each parent.
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So Jane, Peter and Sue got 23 chromosomes from their dad and 23 from their
mum. The chromosomes from Mr. Smith pair up with the same chromosomes that
came from Mrs. Smith making 23 pairs. A picture of the set of chromosomes that a
person has is called a karyotype. While paired together some parts of the
chromosomes swap places making the
chromosomes that the children have
a chromosome
slightly different from Mr. & Mrs.
Smith’s chromosomes (a process called
recombination). So what’s the biggest
difference between Peter and his two
sisters Jane and Sue? Well he’s a boy,
which means he has a Y-chromosome. Ychromosomes are special because only
boys have them and getting a Ychromosome is what makes boys, well
boys. But… don’t get too excited boys,
the Y chromosome is WAY tinier and has
less genetic material than the X
Figure 10: A karyotype is a picture of the set
chromosome So Peter’s last pair of
of chromosomes belonging to a person.
Source:
chromosomes is made of an Xhttp://www.genome.gov/Pages/Hyperion/DIR/VIP/Glossa
chromosome, which he got from his mum,
ry/Illustration/karyotype.cfm?key=karyotype
and a Y-chromosome, which he got from
his dad. Both his sisters got 1 X-chromosome from each parent for a total of 2.
Why is it important to know about pedigrees and karyotypes?
So, in the pedigree Jane has the genes from Mr. Smith that say “brown eyes” and
“round nose”, she may also have genes that say “high blood pressure” inherited
from her dad. Her dad may have received this gene from his dad, and Jane may
pass this gene to her children when she’s older. So just by looking at the pedigree
you can follow the gene. In medicine today special doctors, called medical
geneticists, do this all the time so that people know much more about the chances
of getting a “bad” gene.
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Want to know more?
Here is a karyotype that is
different from normal. Can you
spot what is different in this
karyotype?
Figure 11: Can you spot the abnormality in this
karyotype? Karyotype courtesy of Snezana Arsovska
In this karyotype there is an extra chromosome 21. When a person has an extra
copy of any chromosome this is called trisomy. Trisomy 21 causes a specific
disease that you may have heard of called Down Syndrome.
CLONING
The word “clone” indicates two things that are
genetically identical. This means that you can
have cloned molecules of DNA, cloned cells
that contain the same DNA, and cloned
organisms (e.g., animals).
DNA cloning is used all the time to study
genes of interest.
Cell cloning is very
important to study cellular function and
development.
Animal cloning is more
controversial, but could offer many benefits
to scientific research. In this section, cellbased cloning of a DNA molecule, cell-free
cloning of a DNA molecule, and organism
cloning will be discussed.
Figure 12: “Cloning” means to make
two things that are genetically
identical. This can be cloned DNA,
cloned cells or even cloned organisms!
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Cell-based cloning of a DNA molecule
Often researchers want to clone a specific “gene of interest” to study gene
function (i.e., what it does), structure (i.e., what it looks like), or sequence (i.e., the
number and order of A, C, G and T’s). The techniques used might differ depending
on what organism the gene came from, where it will end up, and the type of gene
(e.g., large or small, or complex structures). There are four major components of
cell-based DNA cloning: 1. Making a recombinant DNA molecule by ligation
reactions, 2. Transformation, 3. Selection and propagation of transformed cell
clones, and 4. Isolation of recombinant DNA. Let’s look at each step in more detail!
1. Making recombinant
ligation reactions
DNA
molecules
by
A recombinant DNA molecule is a piece of DNA
that has been linked to another piece of foreign
DNA.
If we want to clone a gene, we need to link the
gene of interest to another piece of DNA such as a
vector. A vector is also made of DNA and can be
transferred between different organisms. So, if
you link your gene of interest to a vector, it acts
like a “vehicle” to transfer your gene to a simpler
organism where it can be copied rapidly.
Figure 13: Plasmid molecules. An
illustration of what plasmids look
like in nature. Plasmids are DNA
molecules that can contain genes
of antibiotic resistance. Source:
http://universe-review.ca/I1071-plasmid.jpg.
A plasmid is a type of vector (Figure 13). A plasmid is a circular piece of DNA.
Bacteria often have plasmids present in their cells and use them in the case of
“emergencies”.
For example, plasmids commonly carry genes for antibiotic
resistance. These are genes that wouldn’t be required under normal growing
conditions but are very helpful in the presence of an antibiotic, such as penicillin,
ampicillin, erythromycin, etc., which normally will kill the cell.
Scientists can use antibiotic resistance to figure out which bacteria contain the
plasmid and which don’t. More about this later!
In order to insert the gene of interest (to be cloned) into the plasmid, the plasmid
must be opened. Remember the restriction enzymes we learned about in the
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Genetics section? Well, in addition to being used to cut up DNA for gel
electrophoresis, they are also used to open up plasmids!
Cutting with a restriction enzyme leaves what is called an overhang. This is a
sequence of nucleotides that remain “dangling” beyond the edge of the
complementary strand. If the gene of interest is cut on either end (at the 5’ and
3’ end) with the same enzyme, it will have complementary overhangs (i.e., the
pieces will match and so they can be joined together), also known as “sticky ends”.
The overhangs of the cut plasmid can then be ligated (i.e., joined) to the overhangs
of the cut gene because they are complementary. Another enzyme, DNA ligase, is
needed to ligate the pieces of DNA together. This is how we can insert a gene of
interest into a plasmid (Figure 14). Once the gene is in the plasmid, it’s ready to be
cloned!
Figure 14. Ligation of a gene of
interest into a plasmid. Both the
plasmid and the gene are digested
with the same restriction enzyme.
The overhanging regions created by
the digestion are complementary,
therefore the gene will bind to the cut
plasmid (with the help of ligase).
2. Transformation
Plasmids are a vector to aid in the cloning of a gene. However, a plasmid can’t clone
a gene on its own – it needs a host system to make copies of the plasmid (and
therefore, make copies of your gene of interest). The most efficient copying host
is bacteria, specifically E. coli (Escherichia coli). This is because E. coli divide and
grow very rapidly. A culture consisting of one cell can grow and divide into billions
of cells in just 24 hours! If E. coli are carrying the plasmid with your gene of
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interest, every time the bacteria duplicates it will also duplicate the plasmid.
Therefore, with an overnight growth period you could potentially have billions of
copies (clones) of your gene!
How can we alter E. coli so they carry a plasmid? Many types of bacteria will
naturally take up DNA molecules from their environment. In addition, E. coli can be
treated with chemicals to make them more competent (make it easier) to take up
DNA. You can then incubate the treated cells with a mixture of your plasmid and
salt. The osmotic pressure (i.e., high salt concentration on the outside of the cells,
lower salt concentration inside the cells) created and the presence of the plasmid
DNA causes a flow into the cells (Figure 15).
Figure 15. Transformation and selection of transformed cells for
cloning. A. Bacterial cells are transformed with plasmid DNA. B. Transformed
bacteria are selected based on antibiotic resistance. Resistant lines can then be
grown up to harvest large concentrations of the cloned plasmid.
3. Selection and propagation of transformed cell clones
So how do we know which bacteria picked up the plasmids (with the gene we are
trying to clone) and which didn’t?
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Well, when we grow the bacteria, we do so in the –presence of an antibiotic.
Remember how the plasmids carry cool genes that give antibiotic resistance to
bacteria? Bacteria WITH plasmids will be able to live and grow on the Petri dish
with antibiotics. Bacteria WITHOUT plasmids will die. So we know that all the
bacteria that are living, growing and duplicating contain the plasmid -- and
therefore the gene we are trying to clone! This is called antibiotic selection. Once
you have selected a colony of bacteria that are carrying the plasmid you can grow
the bacteria to clone billions of copies of your gene. (Figure 3B).
4. Isolation of recombinant DNA
The final step is to harvest the high concentration of your plasmid (and therefore,
the gene) from the bacteria. To do this, the bacteria will be lysed (the cell
membrane is broken open) and the plasmid DNA will be separated from the
bacterial DNA and RNA. This is done using a high salt and low pH solution (because
plasmid DNA can withstand these conditions but regular DNA and RNA cannot).
Once the cells are lysed and the solutions have been added, the plasmid can be
separated using centrifugation (spinning at high speed). Presto! You now have a
whole bunch of copies of that specific gene you are cloning!
Organism cloning
Organism cloning is the production of two individuals (the same organism) that have
identical DNA. Identical twins are a natural example of human clones. Their DNA,
therefore, are exact copies.
Techniques have been developed that allow researchers to take the nucleus (which
contains DNA) from one cell and put it into another cell from which the nucleus has
been removed. This is called nuclear transfer technology because you are
transferring the nucleus from one cell to another. This technique can be used to
create cells that are clones of each other, because they carry the same DNA. In
the case of animal cloning, the cloned cell can be an oocyte (an egg cell from a
female), which can then be implanted into a female and could grow into an embryo,
and eventually a cloned individual organism (Figure 4). This is how the first cloned
sheep (Dolly) was created.
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Figure 16. Nuclear transfer technology to create a clone. The DNA in the cloned
cell will be identical to that of the donor, therefore a nuclear DNA clone.
INTRODUCTION TO STEM CELLS
What is a “stem cell?”
Most of us have heard the term “stem cell” from newspapers and TV news
reports. Stem cells were not this popular until 10 years ago and have become a
popular as well as an important field of scientific research. So…exactly what is a
stem cell and why is it important?
In general, a stem cell is defined as an unspecialized (with no specific
function) cell type that can self-renew (make copies of itself) and differentiate
(make cells that do have a specific function). Cells in our body usually get old, die,
and then are replaced by new cells. Some cells die fast and are replaced fast,
which means they have a high turnover (like skin cells). Other cells are slower in
this process and hence have a low turnover (like muscle cells or T cells). But in the
end, most of the cells die and are replaced. As a whole population, stem cells have
a ridiculously low turnover because they possess the ability to “self-renew” – this
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means they can make daughter cells that are also stem cells. This allows them to
go on making specialized/differentiated cells with some of their progeny and while
also saving some cells to make keep the numbers of stem cells up. Taken together,
the special properties of stem cells make it possible to generate different types of
cells such as blood cells, neural cells, muscle cells, and many more using just one
single stem cell.
There are two types of stem cells: embryonic stem cells (also called ES
cells) and adult stem cells. ES cells are the most most powerful type of stem cell
and can make all the other types of adult stem cells, while adult stem cells can
typically only make the cells in a given tissue type (i.e.: blood stem cells make T
cells, B cells, red blood cells, etc)
Embryonic Stem Cells
Before knowing what an embryonic stem cell is, we have to start from where
WE came from: a fertilized egg. During the process of fertilization, a sperm from
male and an egg from female come together to generate a fertilized egg, or
zygote. A zygote is a single cell that will continue to develop, dividing into a cluster
of cells, at which stage it is called a blastomere. A blastomere divides into 2, 4, 8
blastomeres (the cluster of blastomeres is called morula), and eventually form a
blastocyst with a hollow centre (Figure 17). A blastocyst has the ability to implant
itself in the uterus of a female (the process is called implantation), and then grow
into fetus, and eventually become an individual. Embryonic stem cells come from
the inner cell mass of blastocyst. These cells can generate any cell type in our
body (except the cell types that grow outside of the embryo (extra-embryonic) –
which are still required for the embryo to grow!) , which is very amazing if you
think about it!! This characteristic is called pluripotency (pluri: several, just think
plural; potency: potential).
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Inner Mass
(Embryonic
Stem Cell)
First
Division
Sperm
and egg
Second
Division
2
Fertilized Blastomere
blastomeres
egg
Third
Division
4
blastomeres
After many
Divisions
8
blastomeres
Blastocyst
Figure 17: Development of embryo from fertilized egg
Adult Stem Cells
Adult stem cells are quite a bit different from embryonic stem cells. The
best-known example of an adult stem cell is the blood stem cell (called
hematopoietic which comes from the Latin “hemato” (iron/blood) and “poiesy” (the
verb “to make” – also the root of poetry), although much is known about adult stem
cells in other organ systems too. It has been shown that adult stem cells can be
found in the liver, pancreas, skin, muscle, and possibly more! A hematopoietic stem
cell can generate red blood cells, platelets, and various types of white blood cells
(Figure 18). In this case, the adult stem cells can only generate a specific set of
cells (i.e., blood cells) but not every cell in a body, and therefore they are called
multipotent instead of pluripotent. Compared to embryonic stem cells, adult stem
cells are even more limited in the cell types they can make (Table 1).
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B-lymphocyte
Lymphoid
Progenitor Cells
Hematopoietic
Stem Cells
T-lymphocyte
Natural Killer Cells
Red blood cells
Platelet
Myeloid Progenitor
Cells
Basophil
Neutrophil
Eosinophil
Macrophage
Figure 18 Various types of hematopoietic cells generated from hematopoietic stem
cells
Table 1 Comparing embryonic stem cells with adult stem cells
Embryonic Stem Cells
The kind of cells they can All cells in a body except
generate
extra-embryonic tissue
Plasticity
Pluripotent
Source
Embryo (inner cell mass of
blastocyst)
Adult Stem Cells
A limited set of cells,
usually in the same system
Multipotent
Adult tissue
Stem Cells And Our Society
Why Are Stem Cells Important?
The study of embryonic stem cells has become very popular because of two
major reasons. First of all, because stem cells can produce an unlimited number of
cells and can become many different cell types. They could possibly be used to
replace cells, following damage or those lost when we are sick with certain medical
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conditions. For example, type 1 diabetes is an illness when the insulin making cells
inside the pancreas are dead and lost. People who have type 1 diabetes have no
insulin, which is supposed to help those people to take glucose from the blood and
store glucose in cells. Because of the lack of insulin, these patients have high
blood glucose and suffer from its medical complications. If we can generate
insulin-secreting cells using stem cells, then we can inject these cells into the
patients to reverse their condition. Or, if we can find stem cells in the pancreas of
these patients, we can stimulate those stem cells to increase the number of insulin
secreting cells in the patient. A similar scenario can apply to many other diseases
related to cell injury or cell death, including Alzheimer’s, Parkinson’s, Muscular
Dystrophy, spinal cord injury, heart disease, and the list goes on. That’s why many
scientists are trying to find ways to make more stem cells, and also make stem
cells differentiate into the cell types they want.
Another reason why stem cell research is so popular is because stem cells
might be associated with certain types of cancer. Essentially, cancers result from
the uncontrolled growth of cells which can cause many problems (for instance,
leukemia makes too many white blood cells in the bone marrow and takes up all the
space, which prohibits the production of other blood cell types used to carry
nutrients around the body.
Often, cancer can be controlled, either by
chemotherapy (using chemicals to destroy the body’s cells, hoping that the cancer
cells will die in the process) or surgery (to remove “cancerous growths” – this is
limited to only some cancer types). But sometimes even though we think that all the
cancerous cells have been removed, the condition relapses (comes back), more
serious than before. Many scientists believe that it is because the cancerous cells
removed are the downstream cells generated from stem cells. And if we just
remove the cancerous cells but don’t do anything to the stem cells that they come
from, the chance of relapse increases. By learning more about stem cells, we might
be able to find better ways to stop cancer from spreading and to treat cancer
more efficiently.
Stem Cell Research: Ethical aspects
So, even though it seems like stem cells might be used for treating many
diseases, using them (especially using embryonic stem cells) has great concerns.
This is mostly because embryonic stem cells are taken from the blastocyst, which
(like it does in the real world) can implant in the uterus and develop into a being
like you and me. Many groups, therefore, object to the use of embryonic stem cells
for research despite the possible clinical use. Many ethical questions need to be
addressed in this field – should embryos be considered as individuals? When does
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life begin? Should scientists be able to generate embryos for research? What
about if they are going to go to waste anyhow? Clearly, all scientists need to think
about the implications of their work.
Stem Cell Research: What the Future Holds
Stem cells are an important key to future medical treatments. However,
more research is needed before the stem cells are used for clinical treatment. We
still need to know more about how stem cells differentiate and self-renew, how to
target or isolate stem cells in adult tissue, and how to control stem cell
differentiation. We also need to keep in mind that ethical issues need to be dealt
with before we learn how practical stem cell research is for humans. There are
many more questions to answer and hopefully one day we will see the
implementation of stem cell therapy in medicine!
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References
Karp. G. Cell and molecular biology: Concepts and experiments. (3rd ed.) John
Wiley & Sons, inc. 2002.
Kimball's Biology Pages (Stem Cells):
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Stem_Cells.html#Using
_Stem_Cells_for_Human_Therapy_-_The_Problems
Griffiths, Anthony J.F., Miller, Jeffrey H. Suzuki, David T, Lewontin, Richard C.,
and
Gelbart,
William
M.
An
Introduction
to
Genetic
Analysis
Seventh Edition. 2000. W.H. Freeman & Company. New York, NY.
Report on Stem cells: http://stemcells.nih.gov/info/scireport/
Stem Cell Information, the official National Institute of Health resource for stem
cell research (USA): http://stemcells.nih.gov/index.asp
Strachan, Tom, and Read, Andrew P. Human Molecular Genetics 2nd Edition.
John Wiley & Sons Inc. Rexdale, ON.
1999.
United Nations Convention on Biological Diversity
http://www.biodiv.org/convention/articles.asp?lg=0&a=cbd-02
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