Cell Structure and Function - Stars



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.

The phases of Mitosis:

Interphase

[pic] 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

[pic] 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

[pic] 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

[pic] 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

[pic] 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

[pic] 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

[pic] 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 the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.

Metaphase I

[pic] 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

[pic] 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

[pic] Nuclear envelopes may reform, or the cell may quickly start meiosis 2.

Cytokinesis

[pic] 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.

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.

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The structure of the phosphate group is shown below.

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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 G-C 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.

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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 2-dimensional 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. 

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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:

• 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. 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.”

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, 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, 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 mid-1980s, 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, 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.

WORKS CITED



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









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.

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.

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.

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.

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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