Module Background: Genetic Engineering
Cell Structure and Function
At a microscopic level, humans, animals and plants are all composed of cells.
Everything from reproduction to infections to repairing a broken bone happens at the
cellular level. In order to understand new frontiers like biotechnology and genetic
engineering, first you need to understand cells and cellular processes.
All living organisms are composed of one or more cells. Organisms grow by
increasing their size and their number of cells. Every cell must be able to enclose
itself from the external environment. The barrier used to surround the cell is called the
plasma membrane. Most cells have internal structures called organelles, which carry
out specific functions for the cell. There are two types of cells - prokaryotic cells and
eukaryotic cells.
Prokaryotic cells:
Prokaryotic cells are found in bacteria. Bacteria are about the simplest cells that
exist today. Bacteria are single, self-contained, living cells. Escherichia coli bacteria
(or E. coli bacteria) are about one-hundredth the size of a human cell, so it is invisible
without a microscope. Bacteria are also a lot simpler than human cells. A bacterium
consists of an outer layer called the cell membrane, and inside the membrane is a
watery fluid called the cytoplasm. Cytoplasm might be 70-percent water. The other
30 percent is filled with proteins called enzymes that the cell has manufactured, along
with smaller molecules like amino acids, glucose molecules and ATP.
Though simple, prokaryotic cells have genetic information called
Deoxyribonucleic acid (DNA) in a region of the cell known as the nuclear area. The
plasma membrane of these cells may be surrounded by a cell wall, which offers added
protection from the external environment. Many of the prokaryotic cells also have
flagella - long fibres that extend from the surface of the cell and help with movement.
The figure below depicts an E. coli bacterium.
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Eukaryotic cells:
Eukaryotic cells (like human cells) are much more complex than bacteria. They
contain a special nuclear membrane to protect the DNA, additional membranes and
structures like mitochondria and Golgi bodies, and a variety of other advanced
features. However, the fundamental processes are the same in both bacteria and
human cells.
Eukaryotic cells include both animal and plant cells and are characterized by their
highly organized membrane bound organelles found within the plasma membrane.
The largest and most important of the organelles is the nucleus, containing the
heredity information (DNA) in the form of chromatin and the nucleolus that
synthesizes ribosomes important for protein synthesis. The area outside of the
nucleus is called the cytoplasm - a clear fluid that usually constitutes a little more
than half of the volume of the cell. All of the other organelles are suspended in the
cytoplasm.
Endoplasmic Reticulum (ER) is attached to the outer membrane of the nuclear
envelope. The ER is a complex system of membranes that fold to make
interconnected compartments inside the cell. There are two types of ER – (1) the
rough where ribosomes are attached to the ER and protein synthesis occurs and, (2)
the smooth where lipid (fat) synthesis occurs. Ribosomes are also found floating free
within the cytoplasm. As stated earlier, ribosomes are the sites where the cell
assembles enzymes and other proteins according to the directions of the DNA.
Although they are considered organelles, ribosomes are not bound by a membrane.
The golgi apparatus is a series of closely stacked flattened membrane sacs that
receive newly synthesized proteins and lipids from the ER to the golgi apparatus in
small, membrane-bound transport packages. These packages are called vesicles. The
golgi apparatus modifies the proteins or lipids before repackaging them in new
vesicles for their final destination. Some of the vesicles remain in the cell while others
are expelled to carry out a function outside the cell.
The energy for the cell is produced in the mitochondria. Molecules of food are
broken down to release energy in the mitochondria. This process is called cell
respiration. The mitochondrion has an outer membrane and a highly folded inner
membrane called the cristae. The area within the cristae is called the matrix. On the
folds of the cristae is the region for the production of energy molecules (cell
respiration). The following figure shows the intricate nature of a typical eukaryotic
cell.
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Eukaryotic plant cells can be differentiated from animal cells by the presence of
the three distinctive structures found in plant cells:
1. Cell Wall
2. Vacuole
3. Chloroplast
Plant cells have an external boundary outside their plasma membrane called the cell
wall. It is a relatively inflexible structure that surrounds the plasma membrane. Plant
cells also contain a single large vacuole, which is a sac of fluid surrounded by a
membrane. They often store food, enzymes, and other materials needed by the cell.
Some vacuoles even store waste products. The vacuole in plants produces turgor
pressure against the cell wall for support. Most waste is sent to the lysosomes. These
are organelles that contain a digestive enzyme to break down worn out cell parts, food
particles, and invading viruses or bacteria. The membrane surrounding the lysosome
prevents the digestive enzyme from leaking out in to the cell and destroying important
parts of the cell.
Plant cells also contain an organelle called the chloroplast. This is the site
where light energy is converted into chemical energy. The energy is then stored in
food molecules including sugar and starch. The chloroplast contains a green pigment
called chlorophyll that traps the energy from the sunlight and gives plants their green
colour. The chloroplast like the mitochondria has a double outer membrane and a
folded inner membrane where the light energy is captured and photosynthesis takes
place.
The Cell Cycle:
The cell cycle is the sequence of growth and division of a cell. The cell goes
through different phases representing the important phases in the life of a cell. As a
cell proceeds through its cycle there are two general periods - one of growth and the
other of division. The growth period of the cell is known as interphase. Most of the
cell’s life is spent in interphase. During the G1
phase the cell is making energy (ATP –
adenosine triphosphate) repairing itself, and
excreting waste. The cell undergoes normal
functions while growing. During the S phase,
the cell copies its chromosomes (see DNA
replication). After the cells have been
duplicated the cell enters G2 phase during
which the cell manufactures cell parts
involved in cell division. The cell division part
of the cell cycle is called mitosis. At the end
of mitosis, the result is two identical cells (two
daughter cells).
Mitosis:
The process by which one cell divides its nucleus and then its cytoplasm to
from two daughter cells containing a complete set of the chromosomes is known as
mitosis. There are four phases of mitosis, which include prophase, metaphase,
anaphase, and telophase. Interphase is often included in discussions of mitosis, but
interphase is technically not part of mitosis, but rather encompasses stages G1, S, and
G2 of the cell cycle.
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The phases of Mitosis:
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.
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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.
Mitosis results in the formation of two identical cells carrying the same
genetic information. The new cells can carry out all of the same functions as their
parent cells. The cells can grow until the limitations of the cell causes cell division to
occur again. Mitosis can occur in all somatic cells found in animals.
Meiosis:
Mitosis describes the process by which the nucleus of a cell divides to create
two new nuclei, each containing an identical copy of DNA. Almost all of the DNA
duplication in your body is carried out through mitosis. Meiosis is the process by
which certain sex cells are created. If you're male, your body uses meiosis to create
sperm cells; if you're female, it uses meiosis to create egg cells. Others cells in your
body contain 46 chromosomes: 23 from your father and 23 from your mother. Your
egg (or sperm) cells contain only half that number -- a total of 23 chromosomes.
When an egg and sperm unite to make a fertilized egg, the chromosomes add up to
equal 46.
Meiosis takes place in germ line cells. Cells contain two copies of information
within the nucleus (diploid – 2n). If two cells come together for reproduction and
pass on their copies into the daughter cells. The daughter cells will have too many
copies of the genetic information (4 copies). Meiosis allows the cell to divide into
four cells each with one set of the genetic information (haploid – 1n). When that cell
comes into contact with another cell like it (containing only one set of information)
the cells can join to make one cell with two copies of differing information. This
allows for cells to differ from their parent cells.
Meiosis consists of two separate divisions known as Meiosis I and Meiosis II.
The cell starts off containing two sets of genetic information (DNA) and by the end
four cells are produced each containing one set of the genetic information (DNA). The
same four stages used in mitosis are utilised during this process, the only difference
being that the process goes through them twice. In Meiosis 1, chromosomes in a
diploid cell resegregate, producing four haploid daughter cells. It is this step in
Meiosis that generates genetic diversity.
The phases of Meiosis 1:
Prophase I
DNA replication precedes the start of meiosis I. During prophase I,
homologous chromosomes pair and form synapses, a step unique to meiosis. The paired
chromosomes are called bivalents, and the formation of chiasmata caused by genetic
recombination becomes apparent. Chromosomal condensation allows these to be viewed in
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the microscope. Note that the bivalent has two chromosomes and four chromatids, with one
chromosome coming from each parent.
Metaphase I
Bivalents, each composed of two chromosomes (four chromatids) align at
the metaphase plate. The orientation is random, with either parental homologue on a side.
This means that there is a 50-50 chance for the daughter cells to get either the mother's or
father's homologue for each chromosome.
Anaphase I
Chiasmata separate. Chromosomes, each with two chromatids, move to
separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each
chromosome has two chromatids.
Telophase I
Nuclear envelopes may reform, or the cell may quickly start meiosis 2.
Cytokinesis
Analogous to mitosis where two complete daughter cells form.
Meiosis II starts out with prophase when the nuclear membrane disappears, the
chromosomes condense, and spindle fibres start to form. The chromosomes are still
made up of two sister chromatids. During metaphase II, the chromosomes line up
along the equator of the cell and in anaphase II the centromeres break. The spindle
fibres pull the sister chromatids apart, towards the opposite poles. Finally in telophase
II, the nuclear membrane reforms, the cytoplasm divides, and the spindles break
down. This results in four cells each carrying one copy of the genetic information in
their nucleus (haploid – 1n). The following diagram shows the differences between
mitosis and meiosis.
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THE DIFFERENCES BETWEEN MITOSIS AND MEIOSIS
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DNA: The Carrier of Genetic Information
Every cell, no matter how small, contains all the genetic information for the entire
organism. This genetic information is called Deoxyribonucleic Acid (DNA). DNA
contains the instructions for the development of an organism and for carrying out life
processes. Information encoded in DNA is transmitted from one generation to the
next. DNA codes for proteins which are important in determining the structure and
function of cells and tissues. Strands of DNA are long polymers built of millions of
nucleotides that are linked together. Individually, nucleotides are quite simple,
consisting of three distinct parts:
1. One of four nitrogen bases
2. Deoxyribose (a five-carbon sugar)
3. A phosphate group
The image below shows a simplified representation of a nucleotide. The P represents
the phosphate molecule, the S represents the sugar (deoxyribose), and B represents
one of the four nitrogen bases.
The structure of the phosphate group is shown below.
The four DNA nucleotides are adenine, guanine, cytosine, and thymine. These
will be referred to as A, G, C, and T respectively. Adenine and guanine are classified
as purines since they are double-ringed molecules. Cytosine and thymine are
pyrimidenes due to the fact that they are single-ringed molecules. A pyrine binds
with a pyrimidene in DNA to form a base pair. Adenine and thymine bind together to
form the A-T base pair. Likewise, guanine and cytosine come together to form the GC base pair. The bases are joined together by weak hydrogen bonds, and it is this
hydrogen bonding that produces DNA's familiar double helix shape. An image
illustrating the how two bases pair with hydrogen bonding is shown below (the blue
lines are the hydrogen bonds.)
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An example of a single strand of DNA is shown below.
Instead of always seeing a huge molecular diagram of a DNA strand, what one
often sees in a string of letters, such as "ATCTTAG." This string represents which
bases are in a certain side of a strand of DNA. The above string (ATCTTAG)
represents the string "adenine-thymine-cytosine-thymine-thymine-adenine-guanine."
DNA has two strands. Whatever nucleotides are in one strand, they rigidly fix the
sequence of nucleotides in the other strand due to the way base pairing occurs (A with
T, G with C). The two strands are complementary. In addition, it must be noted that
the two strands are antiparallel. That means that they run in opposite directions. One
strand goes in a 5' to 3' direction while the other goes in a 3' to 5' direction. By
convention, the strand which goes in the 5' to 3' direction is placed on the left in 2dimensional drawing. The figure below gives a visual example of this concept as well
as showing how the strands are complementary.
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In this next image, the double-helix shape of DNA is shown. The two strands
are clearly visible, one being coloured blue, and the other red.
DNA Replication:
The DNA is found in the nucleus of all cells. If the cell is going to divide to
make a new cell, then a copy of the DNA is needed for the nucleus of the new cell.
The process used to copy DNA is called DNA replication. To copy the DNA, the
double helix is unwound using enzymes to break the hydrogen bonds between the
nitrogen bases. Each strand could be used as a template to create the new
complementary strand. As the DNA is unzipped, free nucleotides from the
surrounding area in the nucleus bond to the single strands by base pairing. If the bond
between A-T is broken a new T will link up with the A forming a hydrogen bond
between the TWO and a new A will link with the T. Another enzyme will bond these
new nucleotides into a chain.
The process continues until the entire molecule has been unzipped and
replicated. Each strand formed is a complement of the original. A strand contains half
of the original and half of a new chain rewound in the double helix fashion. When the
entire DNA in the chromosomes of the cell has been copied there are two copies of
the genetic information. This allows the cell to pass on the extra copy of information
it has replicated.
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Biotechnology and Genetic Engineering
Scientists are now using all of the knowledge gained about cells and
combining scientific theory with technology to allow for advances in healthcare. This
emerging field is called Biotechnology. Biotechnology is the application of scientific
techniques to modify and improve plants, animals, and micro-organisms to enhance
their value. Agricultural biotechnology is the area of biotechnology involving
applications to agriculture. Agricultural biotechnology has been practiced for a long
time, as people have sought to improve agriculturally important organisms by
selection and breeding. An example of traditional agricultural biotechnology is the
development of disease-resistant wheat varieties by cross-breeding different wheat
types until the desired disease resistance was present in a resulting new variety.
Scientists in biotechnology are currently developing ways to modify bacteria
to produce human insulin. Insulin is a simple protein normally produced by the
pancreas. In people with diabetes, the pancreas is damaged and cannot produce
insulin. Since insulin is vital to the body's processing of glucose, this is a serious
problem. Many diabetics, therefore, must inject insulin into their bodies daily. Prior to
the 1980s, insulin for diabetics came from pigs and was very expensive.
To create insulin inexpensively, the gene that produces human insulin was added
to the genes in normal E. coli bacteria. Once the gene was in place, the normal
cellular machinery produced it just like any other enzyme. By culturing large
quantities of the modified bacteria and then killing and opening them, the insulin
could be extracted, purified and used very inexpensively. Other advances in
biotechnology include:
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Bacterial production of substances like human interferon, human insulin and
human growth hormone. That is, simple bacteria like E. coli are manipulated
to produce these chemicals so that they are easily harvested in vast quantities
for use in medicine. Bacteria have also been modified to produce all sorts of
other chemicals and enzymes.
Modification of plants to change their response to the environment, disease or
pesticides. For example, tomatoes can gain fungal resistance by adding
chitinases to their genome. A chitinase breaks down chitin, which forms the
cell wall of a fungus cell. The pesticide Roundup kills all plants, but crop
plants can be modified by adding genes that leave the plants immune to
Roundup.
Identification of people by their DNA. An individual's DNA is unique, and
various, fairly simple tests let DNA samples found at the scene of a crime be
matched with the person who left it. This process has been greatly aided by the
invention of the polymerase chain reaction (PCR) technique for taking a
small sample of DNA and magnifying it millions of times over in a very short
period of time.
Genetic Engineering:
In the 1970s, advances in the field of molecular biology provided scientists with
the ability to readily transfer DNA between more distantly related organisms. Today,
this technology has reached a stage where scientists can take one or more specific
genes from nearly any organism, including plants, animals, bacteria, or viruses, and
introduce those genes into another organism. This technology is called Genetic
Engineering. Genetic Engineering is the heritable, directed alteration of an organism.
A heritable alteration is a change that can be carried from one generation to the next.
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Genetic engineering is performed by modifying an organism's own DNA or
introducing new DNA to perform desired functions.
An organism that has been modified, or transformed, using modern
biotechnology techniques of genetic exchange is referred to as a genetically modified
organism (GMO). Genetic modification has been around for hundreds if not
thousands of years - deliberate crosses of one variety or breed with another result in
offspring that are genetically modified compared to the parents, and hybrid crosses
result in progeny with genetic combinations of closely related species.
Everything in life has its benefits and risks, and genetic modification is no
exception. Much has been said about potential risks of genetic modification
technology, but so far there is little evidence from scientific studies that these risks are
real. Transgenic organisms can offer a range of benefits above and beyond those that
emerged from innovations in traditional agricultural biotechnology. Following are a
few examples of benefits resulting from applying currently available genetic
modification techniques to agricultural biotechnology.
1. Biotechnology can help to increase crop productivity by introducing such
qualities as disease resistance and increased drought tolerance to the crops.
Researchers can select genes for disease resistance from other species and
transfer them to important crops. For example, researchers from the University
of Hawaii and Cornell University developed two varieties of papaya resistant
to papaya ringspot virus by transferring one of the virus’ genes to papaya to
create resistance in the plants. Seeds of the two varieties, named ‘SunUp’ and
‘Rainbow’, have been freely distributed to papaya growers since May of 1998.
2. Today there is increasing interest in improving the nutritional value, flavor,
and texture of foods. Transgenic crops in development include soybeans with
higher protein content, potatoes with more nutritionally available starch and an
improved amino acid content, beans with more essential amino acids, and rice
with the ability produce beta-carotene, a precursor of vitamin A, to help
prevent blindness in people who have nutritionally inadequate diets.
3. Genetic modification can result in improved keeping properties to make
transport of fresh produce easier, giving consumers access to nutritionally
valuable whole foods and preventing decay, damage, and loss of nutrients.
Transgenic tomatoes with delayed softening can be vine-ripened and still be
shipped without bruising. Research is under way to make similar
modifications to broccoli, celery, carrots, melons, and raspberry. The shelf-life
of some processed foods such as peanuts has also been improved by using
ingredients that have had their fatty acid profile modified.
4. When genetic engineering results in reduced pesticide dependence, we have
less pesticide residues on foods, we reduce pesticide leaching into
groundwater, and we minimize farm worker exposure to hazardous products.
With Bt cotton’s resistance to three major pests, the transgenic variety now
represents half of the U.S. cotton crop and has thereby reduced total world
insecticide use by 15 percent! Also, according to the U.S. Food and Drug
Administration (FDA), “increases in adoption of herbicide-tolerant soybeans
were associated with small increases in yields and variable profits but
significant decreases in herbicide use.”
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Some consumers and environmentalists feel that inadequate effort has been made
to understand the dangers in the use of genetic engineering, including their potential
long-term impacts. Some consumer-advocate and environmental groups have
demanded the abandonment of transgenic crop research and development. Many
individuals, when confronted with conflicting and confusing statements about the
effect of transgenic crops on our environment and food supply, experience a “dread
fear” that inspires great anxiety. This fear can be aroused by only a minimal amount
of information or, in some cases, misinformation. With people thus concerned for
their health and the well-being of our planetary ecology, the issues related to their
concerns need to be addressed. These issues and fears can be divided in to three
groups: health, environmental, and social.
1. Health-related issues:
I. People with food allergies have an unusual immune reaction when
they are exposed to specific proteins, called allergens, in food.
About 2 percent of people across all age groups have a food allergy
of some sort. The majority of foods do not cause any allergy in the
majority of people. Food-allergic people usually react only to one
or a few allergens in one or two specific foods. A major safety
concern raised with regard to genetic modification technology is
the risk of introducing allergens and toxins into otherwise safe
foods.
The Food and Drug Administration (FDA) checks to ensure that
the levels of naturally occurring allergens in foods made from
transgenic crops have not significantly increased above the natural
range found in conventional foods. And, transgenic technology is
being used to remove the allergens from peanuts, one of most
serious causes of food allergy.
II. Antibiotic resistance genes are used to identify and trace a trait of
interest that has been introduced into plant cells. This technique
ensures that a gene transfer during the course of genetic
engineering was successful. Use of these markers has raised
concerns that new antibiotic-resistant strains of bacteria will
emerge. The rise of diseases that are resistant to treatment with
common antibiotics is a serious medical concern of genetic
engineering opponents. The potential risk of transfer from plants
to bacteria is substantially less than the risk of normal transfer
between bacteria, or between us and the bacteria that naturally
occur within our alimentary tracts. Nevertheless, to be on the safe
side, FDA has advised food developers to avoid using marker
genes that encode resistance to clinically important antibiotics.
2.
Environmental and ecological issues
I. There is a belief among some opponents of genetic engineering that
the new crops might cross-pollinate with related weeds, possibly
resulting in “superweeds” that become more difficult to control. One
concern is that pollen transfer from glyphosate-resistant crops to
related weeds can confer resistance to glyphosate. While the chance
of this happening, though extremely small, is not inconceivable,
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resistance to a specific herbicide does not mean that the plant is
resistant to other herbicides, so affected weeds could still be
controlled with other products.
II. Another concern related to the potential impact of agricultural
biotechnology on the environment involves the question of whether
insect pests could develop resistance to crop-protection features of
transgenic crops. There is fear that large-scale adoption of Bt crops
will result in rapid build-up of resistance in pest populations. Insects
possess a remarkable capacity to adapt to selective pressures, but to
date, despite widespread planting of Bt crops, no Bt tolerance in
targeted insect pests has been detected.
The debate over biotechnology is far from over. The benefits and risks are
real and are caught up in political as well as financial battles. Biotechnology has no
only influenced plants and genetically engineered foods, but has spilled over into
human developments. As technology continues to emerge, we turn our attention to
biotechnology and the human genome.
The Human Genome Project
Since the beginning of time, people have yearned to explore the unknown,
chart where they have been, and contemplate what they have found. The maps we
make of these treks enable the next explorers to push ever farther the boundaries of
our knowledge - about the earth, the sea, the sky, and indeed, ourselves. On a new
quest to chart the innermost reaches of the human cell, scientists have now set out on
biology's most important mapping expedition: the Human Genome Project. Its
mission is to identify the full set of genetic instructions contained inside our cells and
to read the complete text written in the language of the hereditary chemical DNA.
As part of this international project, biologists, chemists, engineers, computer
scientists, mathematicians, and other scientists will work together to plot out several
types of biological maps that will enable researchers to find their way through the
labyrinth of molecules that define the physical traits of a human being.
Packed tightly into nearly every one of the several trillion body cells is a
complete copy of the human "genome" - all the genes that make up the master
blueprint for building a man or woman. One hundred thousand or so genes
sequestered inside the nucleus of each cell are parceled among the 46 sausage-shaped
genetic structures known as chromosomes. New maps developed through the Human
Genome Project will enable researchers to pinpoint specific genes on our
chromosomes. The most detailed map will allow scientists to decipher the genetic
instructions encoded in the estimated 3 billion base pairs of nucleotide bases that
make up human DNA.
Analysis of this information, likely to continue throughout much of the 21st
century, will revolutionize our understanding of how genes control the functions of
the human body. This knowledge will provide new strategies to diagnose, treat, and
possibly prevent human diseases. It will help explain the mysteries of embryonic
development and give us important insights into our evolutionary past. The
development of gene-splicing techniques over the past 20 years has given scientists
remarkable opportunities to understand the molecular basis of how a cell functions,
not only in disease, but in everyday activities as well. Using these techniques,
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scientists have mapped out the genetic molecules, or genes, that control many life
processes in common microorganisms.
Continued improvement of these biotechniques has allowed researchers to
begin to develop maps of human chromosomes, which contain many more times the
amount of genetic information than those of microorganisms. Though still somewhat
crude, these maps have led to the discovery of some important genes. By the mid1980s, rapid advances in chromosome mapping and other DNA techniques led many
scientists to consider mapping all 46 chromosomes in the very large human genome.
Detailed, standardized maps of all human chromosomes and knowledge about the
nucleotide sequence of human DNA will enable scientists to find and study the genes
involved in human diseases much more efficiently and rapidly than has ever been
possible. This new effort - the Human Genome Project - is expected to take 15 years
to complete and consists of two major components. The first - creating maps of the 23
pairs of chromosomes - should be completed in the first 5 to 10 years. The second
component - sequencing the DNA contained in all the chromosomes - will probably
require the full 15 years.
Although DNA sequencing technology has advanced rapidly over the past few
years, it is still too slow and costly to use for sequencing even the amount of DNA
contained in a single human chromosome. So while some genome project scientists
are developing chromosome maps, others will be working to improve the efficiency
and lower the cost of sequencing technology. Large-scale sequencing of the human
genome will not begin until those new machines have been invented.
Cloning:
In 1997, a 7-month-old sheep named Dolly became a celebrity. Dr. Ian
Wilmut, a Scottish scientist, announced to the world that he had created her using a
procedure called cloning. Cloning is a method that scientists use to produce a genetic
copy of another individual. In other words, Dolly is a clone of her mother. In
actuality, Dolly had three mothers. One mother gave Dolly her DNA, one mother
supplied an egg, and the third mother, her surrogate mother, gave birth to her.
Normally, an animal gets half of its DNA from its mother and half from its father.
Dolly is an identical twin of the mother who gave her her DNA. But Dolly is six years
younger.
However, Dolly and her mother are not identical in every way. Since Dolly
and her “DNA mother” have different experiences, they are different in many ways.
Like human twins, clones have unique personalities. It took scientists 277 tries to
succeed in cloning Dolly. To make her, Dr. Wilmut used a complicated method called
“nuclear transfer.” In this method, scientists remove a nucleus from one cell and
transfer, or move, it to a different cell. A diagram of Dolly’s cloning process is
described below:
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The goals and purposes for cloning range from making copies of those that
have deceased to better engineering the offspring in humans and animals. Cloning
could also directly offer a means of curing diseases or a technique that could extend
means to acquiring new data for the sciences of embryology and how organisms
develop as a whole over time. Currently, the agricultural industry demands nuclear
transfer to produce better livestock. Cloning could massively improve the agricultural
industry as the technique of nuclear transfer improves.
Currently, change in the phenotype of livestock is accomplished by
bombarding embryos of livestock with genes that produce livestock with preferred
traits. However, this technique is not efficient as only 5 percent of the offspring
express the traits. Scientists can easily genetically alter adult cells. Thus, cloning from
an adult cell would make it easier to alter the genetic material.
In agriculture, farmers want to produce transgenic livestock with ideal
characteristics for the agricultural industry and want to be able to manufacture
biological products such as proteins for humans. Farmers are attempting to produce
transgenic livestock already, but not efficiently, due to the minimal ability to alter
embryos genetically, as stated above. Researchers can harvest and grow adult cells in
large amounts compared to embryos. Scientists can then genetically alter these cells
and find which ones did transform and then clone only those cells.
A major problem with the use of cloning on a large is scale is the decline in
genetic diversity, and decline in gene pool. Think about it, if everyone has the same
genetic material, what happens if we lose the ability to clone. We would have to resort
to natural reproduction, causing us to inbreed, which will cause many problems. Also,
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if a population of organisms has the same genetic information, then the disease would
wipe out the entire population. Helping endangered species by cloning will not help
the problem.
Currently, zoologists and environmentalists trying to save endangered species
are not so much having trouble keeping population numbers up, but not having any
animals to breed that are not cousins. The technique of nuclear transfer is also early in
its developmental stages. Thus, errors are occurring when scientists carry out the
procedure. For instance, it took 277 tries to produce Dolly, and Roslin scientists
produced many lambs with abnormalities. If we tried to clone endangered species we
could possibly kill the last females integral to the survival of a species.
This may be the main reason science is holding out on cloning humans…or
have they already? That’s for you to find out.
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WORKS CITED
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/cells.htm
www.biology.arizona.edu/cell_bio/ tutorials/cell_cycle/cells3.html
Blaustein, D., Johnson, R., Mathieu, D., & Offner, S. Biology: the dynamics of life.
Blencoe/McGraw-Hill. 1995
http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page1.html
http://www.vuhs.org/apbio/clone/history.htm
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html
http://www.ctahr.hawaii.edu/gmo/risks/benefits.asp
http://www.accessexcellence.org/AB/IE/Intro_The_Human_Genome.html
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THE FOLLOWING IS A GLOSSARY OF
ANIMAL CELL TERMS:
cell membrane - the thin layer of protein and fat that surrounds the cell. The
cell membrane is semipermeable, allowing some substances to pass into the
cell and blocking others.
centrosome - (also called the "microtubule organizing center") a small body
located near the nucleus - it has a dense center and radiating tubules. The
centrosomes is where microtubules are made. During cell division (mitosis),
the centrosome divides and the two parts move to opposite sides of the
dividing cell. The centriole is the dense center of the centrosome.
cytoplasm - the jellylike material outside the cell nucleus in which the
organelles are located.
Golgi body - (also called the golgi apparatus or colgi complex) a flattened,
layered, sac-like organelle that looks like a stack of pancakes and is located
near the nucleus. It produces the membranes that surround the lysosomes.
The golgi body packages proteins and carbohydrates into membrane-bound
vesicles for "export" from the cell.
lysosome - (also called cell vesicles) round organelles surrounded by a
membrane that contain digestive enzymes. This is where the digestion of
cell nutrients takes place.
mitochondrion - spherical to rod-shaped organelles with a double
membrane. The inner membrane is infolded many times, forming a series of
projections (called cristae). The mitochondrion converts the energy stored in
glucose into ATP (adenosine triphosphate) for the cell.
nuclear membrane - the membrane that surrounds the nucleus.
nucleolus - an organelle within the nucleus - it is where ribosomal RNA is
produced. Some cells have more than one nucleolus.
nucleus - spherical body containing many organelles, including the
nucleolus. The nucleus controls many of the functions of the cell (by
controlling protein synthesis) and contains DNA (in chromosomes). The
nucleus is surrounded by the nuclear membrane.
ribosome - small organelles composed of RNA-rich cytoplasmic granules
that are sites of protein synthesis.
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rough endoplasmic reticulum - (rough ER) a vast system of
interconnected, membranous, infolded and convoluted sacks that are located
in the cell's cytoplasm (the ER is continuous with the outer nuclear
membrane). Rough ER is covered with ribosomes that give it a rough
appearance. Rough ER transport materials through the cell and produces
proteins in sacks called cisternae (which are sent to the Golgi body, or
inserted into the cell membrane).
smooth endoplasmic reticulum - (smooth ER) a vast system of
interconnected, membranous, infolded and convoluted tubes that are located
in the cell's cytoplasm (the ER is continuous with the outer nuclear
membrane). The space within the ER is called the ER lumen. Smooth ER
transport materials through the cell. It contains enzymes and produces and
digests lipids (fats) and membrane proteins; smooth ER buds off from rough
ER, moving the newly-made proteins and lipids to the Golgi body,
lysosomes, and membranes.
vacuole - fluid-filled, membrane-surrounded cavities inside a cell. The
vacuole fills with food being digested and waste material that is on its way
out of the cell.
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THE FOLLOWING IS A GLOSSARY OF
PLANT CELL TERMS:
amyloplast - an organelle in some plant cells that stores starch. Amyloplasts
are found in starchy plants like tubers and fruits.
ATP - ATP is short for adenosine triphosphate; it is a high-energy molecule
used for energy storage by organisms. In plant cells, ATP is produced in the
cristae of mitochondria and chloroplasts.
cell membrane - the thin layer of protein and fat that surrounds the cell, but
is inside the cell wall. The cell membrane is semipermeable, allowing some
substances to pass into the cell and blocking others.
cell wall - a thick, rigid membrane that surrounds a plant cell. This layer of
cellulose fiber gives the cell most of its support and structure. The cell wall
also bonds with other cell walls to form the structure of the plant.
centrosome - (also called the "microtubule organizing center") a small body
located near the nucleus - it has a dense center and radiating tubules. The
centrosomes is where microtubules are made. During cell division (mitosis),
the centrosome divides and the two parts move to opposite sides of the
dividing cell.
chlorophyll - chlorophyll is a molecule that can use light energy from
sunlight to turn water and carbon dioxide gas into sugar and oxygen (this
process is called photosynthesis). Chlorophyll is copper-based and is usually
green.
chloroplast - an elongated or disc-shaped organelle containing chlorophyll.
Photosynthesis (in which energy from sunlight is converted into chemical
energy - food) takes place in the chloroplasts.
christae - (singular crista) the multiply-folded inner membrane of a cell's
mitochondrion that are finger-like projections. The walls of the cristae are
the site of the cell's energy production (it is where ATP is generated).
cytoplasm - the jellylike material outside the cell nucleus in which the
organelles are located.
Golgi body - (also called the golgi apparatus or colgi complex) a flattened,
layered, sac-like organelle that looks like a stack of pancakes and is located
near the nucleus. The golgi body packages proteins and carbohydrates into
membrane-bound vesicles for "export" from the cell.
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granum - (plural grana) A stack of thylakoid disks within the chloroplast is
called a granum.
mitochondrion - spherical to rod-shaped organelles with a double
membrane. The inner membrane is infolded many times, forming a series of
projections (called cristae). The mitochondrion converts the energy stored in
glucose into ATP (adenosine triphosphate) for the cell.
nuclear membrane - the membrane that surrounds the nucleus.
nucleolus - an organelle within the nucleus - it is where ribosomal RNA is
produced.
nucleus - spherical body containing many organelles, including the
nucleolus. The nucleus controls many of the functions of the cell (by
controlling protein synthesis) and contains DNA (in chromosomes). The
nucleus is surrounded by the nuclear membrane.
photosynthesis - a process in which plants convert sunlight, water, and
carbon dioxide into food energy (sugars and starches), oxygen and water.
Chlorophyll or closely-related pigments (substances that color the plant) are
essential to the photosynthetic process.
ribosome - small organelles composed of RNA-rich cytoplasmic granules
that are sites of protein synthesis.
rough endoplasmic reticulum - (rough ER) a vast system of
interconnected, membranous, infolded and convoluted sacks that are located
in the cell's cytoplasm (the ER is continuous with the outer nuclear
membrane). Rough ER is covered with ribosomes that give it a rough
appearance. Rough ER transport materials through the cell and produces
proteins in sacks called cisternae (which are sent to the Golgi body, or
inserted into the cell membrane).
smooth endoplasmic reticulum - (smooth ER) a vast system of
interconnected, membranous, infolded and convoluted tubes that are located
in the cell's cytoplasm (the ER is continuous with the outer nuclear
membrane). The space within the ER is called the ER lumen. Smooth ER
transport materials through the cell. It contains enzymes and produces and
digests lipids (fats) and membrane proteins; smooth ER buds off from rough
ER, moving the newly-made proteins and lipids to the Golgi body and
membranes.
stroma - part of the chloroplasts in plant cells, located within the inner
membrane
of
chloroplasts,
between
the
grana.
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thylakoid disk - thylakoid disks are disk-shaped membrane structures in
chloroplasts that contain chlorophyll. Chloroplasts are made up of stacks of
thylakoid disks; a stack of thylakoid disks is called a granum.
Photosynthesis (the production of ATP molecules from sunlight) takes place
on thylakoid disks.
vacuole - a large, membrane-bound space within a plant cell that is filled
with fluid. Most plant cells have a single vacuole that takes up much of the
cell. It helps maintain the shape of the cell.
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