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| My National Geographic |
| (c) 1999 Christin Fox-Perez |
| Genetic Engineering: Everything You never knew you needed to know |
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| By Dick Tionary Since the rediscovery of Mendel's laws in 1900, the science of genetics has advanced greatly in its technological capabilities. Theoretic and practical steps have taken place that affect society at large. The amazing rate of genetic technological advance has made it difficult (if not impossible) for the interested non-scientist to understand both what genetic technology is capable of, and the problems and issues that said capability has created. This article is intended to help correct this situation. It will do so in two ways: First, it will explain the basic units of life and basic life processes. Secondly, it will explain what genetic engineering and its components (cloning, transgenesis) are.3 After reading this article readers may find themselves agreeing or disagreeing with various points of the issue. Agreement or disagreement is unimportant-what is important is that you, as an individual, be informed and take some position. These issues are affecting our lives now, and only in an informed society can work to solve the many problems that will arise from this new technology. If society at large has no knowledge of these issues, then those few individuals who do know will have the power to judge the future of human genetics.3 The Basics The Cell The basic unit of life is the cell. The cell is a highly complex organization of matter in which the basic processes characteristic of life are performed. A thin membrane that usually allows only what the cell requires to pass through covers each cell. The membrane encloses the cytoplasm, which contains many organelles (small organs) including mitochondria. All cells also contain chromosomal material that, in many cases, is organized in the nucleus.73 The Nucleus The nucleus is usually the largest and most distinct organelle in a cell. It is roughly spherical and denser than the cytoplasm that surrounds it. The nucleus is held together by a two-layered nuclear membrane. Inside of the nucleus is a tangled mass of material called chromatin. Strands of chromatin join together during cell division to form threadlike bodies called chromosomes. Each species of organism has a distinct amount of chromosomes.73 Chromosomes carry the cell's heredity. They direct and guide the development of the organism and maintain its order and organization. They are made up of strands of deoxyribonucleic acid, or DNA, and proteins. DNA controls the hereditary characteristics of all living things and directs the production of proteins.73 A round body called a nucleolus is visible in the nucleus when the cell is not dividing. It is made up of granules rich in ribonucleic acid, or RNA, which is essential to the formation of proteins by the cell.73 Cells that contain nuclei and chromosomes are called eukaryotes. Bacteria are simpler in organization. While they contain DNA and RNA, they have no nuclei, nuclear membrane, or chromosomes. The heredity material of these prokaryotes floats in the cytoplasm.73 The Cytoplasm Cytoplasm is made mostly of water that has carbohydrates, proteins, lipids, and other small molecules suspended in it. Two types of organelles found within the cytoplasm are the endoplasmic reticulum, which contains ribosomes, and the mitochondria.73 Endoplasmic Reticulum Endoplasmic reticulum is a series of tubes that connect to the nuclear membrane. They provide channels from the nucleus through the cytoplasm to the outside of the cell membrane. Endoplasmic reticulum comes in two forms, smooth and rough. The texture of the rough form is made of ribosomes. Most of a cell's RNA is found in the ribosomes. The main functions of the endoplasmic reticulum are storage, separation, and transportation of substances within the cell.73 The Mitochondria Most of a body's heat and energy comes from the sausage-shaped mitochondria. They are the powerhouses of the cell, and create the high-energy yielding compounds that cells need to carry on their functions. They can also control the amounts of water, calcium, and other substances in the cell; breakdown and recycle proteins, fats, and carbohydrates; and make urea.73 Mitochondria have their own genetic structure, different from that of the cell's nucleus. Many scientists think that they may have evolved separately and live in symbiosis with the cell.73 DNA A single strand of DNA, stretched to its full length, would extend more than three feet. The tips of DNA strands are covered with telomeres, which protect them from harm, put them where they need to be in the nucleus, and tell the cell how old it is. Each time a cell divides, the telomeres get shaved down by a fixed amount, and signal the cell to how many more times it is to divide before it dies. On average, the older an organism becomes, the shorter it's telomeres get, despite the constant production of new cells by said organism.1 DNA is found only in the chromosomes. All the cells in an organism, with the exception of eggs and sperm, have the same amount of chromosomes and DNA. The sex cells have half the amount of chromosomes and DNA that are in the other cells. 73 DNA is a giant molecule formed from chains of smaller molecules. The chains are nucleotides, each made of similar components: a phosphate group, a 5-carbon sugar, deoxyribose, and one of 4 nitrogenous bases. The four bases are the purines- adenine and guanine- and the pyrimidines- cytosine and thymine. A bond between the phosphate group of one and the adjacent deoxyribose of the next connects nucleotides. 73 The structure of the DNA molecule is a double helix. A double helix is similar to a twisted ladder or zipper: the sides are composed of a linked deoxyribose-phosphate backbone, and the rungs are the bases. The sequence of the bases on one side of the helix determines the base sequence of the complementary side- adenine matches with thymine, and guanine matches with cytosine. The bases are joined by weak hydrogen bonds that make the double helix firm but still able to separate during mitosis. During mitosis, a DNA molecule "unzips," or separates, and each side becomes a mold for a new strand. The result is two DNA molecules, each identical to the original. 73 A gene is a section of DNA that carries the code for a string of amino acids. 9 Life-Cycle Fertilization occurs when an egg and sperm join together, combining their halves of DNA to create a single, unique cell. This cell is called a zygote. The first visible event of embryonic development is cleavage, the repeated mitotic division of the fertilized egg. (Mitosis occurs when all the organelles in a cell duplicate and the cell splits, creating two identical "daughter" cells.) As a result of these divisions, which proceed exponentially, the single cell becomes many celled. In many chordates (animals with a notochord), after a certain amount of cells is present they produce a fluid internally, and what was a solid mass becomes a hollow sphere, the blastula. Soon after that phase, cells start to migrate to take up their positions in the germ layers (ectoderm, mesoderm, and endoderm). The ectoderm eventually forms the skin, the mesoderm forms various internal organs, and the endoderm forms the digestive system. The endoderm is the origin of the term gastrula, which means 'little stomach' in Latin.74 Before gastrulation, all the cells of an embryo are very similar to one another. During gastrulation, they assort themselves into layers, and the cells still resemble each other. 74 After the completion of gastrulation, differentiation occurs, and cells become different from others, and join with cells similar to themselves. From this point on, these cells can only become certain body parts. After differentiation, the cells become further specialized to become tissues, organs, or bones. After this stage, the embryo is no longer an embryo and becomes a fetus. After complete development, the organism is born as an infantile version of its species.74 |
| Components of Genetic Engineering Cloning What is a clone? The word 'clone' is derived from the Greek word 'klon', meaning twig, or more specifically, a cutting from which an identical plant is produced.66 A clone is an organism identical to its 'parent'2 and is produced asexually.18 It can be a series of cells derived from a single cell 7, or an identical twin.72 Cancer is a clone of cells that are compelled to continue dividing. All living creatures are clones of their original fertilized egg. A clone is not a carbon copy of an organism, but rather a genetic replica, and a unique individual. 62 Another form of cloning, parthenogenesis, is natural and occurs in about one out of every1.6 million pregnancies. 16 Apparently a sudden shock can unmask the genetic information in a cell and it begins to divide and develop, and may result in the birth of a girl identical to her mother in every genetic respect, but only if the cell is in the uterus when this happens. 16 Parthenogenesis comes from the Greek parthenos, 'virgin'.16 How are clones made? Currently there are three ways to clone mammals. 12 Twinning The Roslin Technique The Honolulu Technique Twinning At any time between cleavage and differentiation, the embryo can be divided into individual cells that can all produce identical organisms. Most identical siblings occur from a single zygote that splits into more zygotes and separate from each other some how. 65 The Roslin Technique Previous to the cloning of Dolly, it had been believed that clones could not be made from the nuclei of adult somatic (body) cells. It was discovered that if the cell cycles were synchronized, the egg cell could accept the adult cell's nucleus. 68 The first thing done at Roslin was the selection of a cell (the donor cell) from the udder cells of a Finn Dorset sheep to provide the DNA for the clone. The researchers stimulated division of the cell to form a culture in vitro, or outside of an animal, usually in a petri dish. This made millions of copies of the donor cell. A cell is then removed from the culture and starved in a mixture that has only enough nutrients to keep the cell alive. This causes it to shut down it's active genes and go into the dormant cell stage known as the 'Gap Zero' or 'GO' stage. The egg cell of a black face ewe was then enucleated (the nucleus was removed or destroyed) and placed next to the donor cell. One to eight hours after the egg cell is placed with the donor cell, an electric impulse was used to fuse the two cells together, in an attempt to mimic the activation provided by sperm, and if it is successful, a viable embryo should result. 68 If the embryo survives, it is allowed to grow for 6 days, incubating in a sheep's oviduct. Finally, the embryo is placed into the uterus of a surrogate mother ewe. That ewe then carries the clone until it is ready to give birth. Assuming all things go well, a genetic duplicate of the donor animal is born. 68 This newborn sheep seems like a normal new born. It has yet to be seen if there are any adverse effects in any animals cloned with this method. 68 The Honolulu Technique Mice had long been considered to be one of the most difficult mammals to clone because the mouse's egg divides almost immediately after fertilization. Sheep were used in the Roslin Technique because their eggs don't divide for several hours after fertilization, giving scientists plenty of time to work with them. Even without the extra time, the Honolulu Technique has a much higher success rate (three out of every one hundred attempts) than Roslin (one in 277). 68 The problem of synchronizing cell cycles was approached differently by the Honolulu Technique. Roslin used udder cells, which had to be forced into the 'GO' stage. The Honolulu Technique initially used three types of cells; Sertoli cells, brain cells, and cumulus cells. Sertoli and brain cells are naturally found in the 'GO' stage and cumulus cells are almost always in the "GO" or "G1" stage. The most successful cells for the process were the cumulus cells, so research was concentrated on cells of that type. 68 The donor nuclei were taken from the cells within minutes of the cell's extraction from a mouse. They were immediately inserted into enucleated mouse egg cells, bypassing the Roslin Technique of creating more donor cells in a culture. After one hour, the egg cells accepted the new nucleus. The researchers let the new cell sit for five hours more, then placed it in a chemical culture to jump-start the cells growth, similar to fertilization in nature. The cells then developed into embryos and were transplanted into surrogate mothers and carried to term. 68 This new technique allows further research into how an egg reprograms a nucleus, since the cell functions of mice are well known, and because mice reproduce more rapidly than sheep. 68 The uses of Cloning Cloning can be used for many things. It can be used to make duplicates of prized livestock. It can be used to preserve endangered animals by creating clones of the ones that are left. With careful breeding programs, excessive inbreeding can be prevented. It can be used to give infertile couples children of their own. Or to make a twin of a child or loved one lost through an accident. Finally, cloning could be used to bring back animals that have recently gone extinct (within the last 50,000 years or so). It is impossible to create clones of animals extinct for longer than that because as the specimens where DNA might get obtained become older, the DNA is in increasingly smaller particles do to the fact that they decay over time. Genetic Engineering Genetic engineering is the artificial manipulation of life. This term means any of a wide range of techniques for the modification or manipulation of organisms through the hereditary and reproductive processes. It embraces artificial selection and other biomedical techniques; artificial insemination, invitro fertilization, sperm banks, cloning, and gene manipulation. 66 Thanks to genetic engineering there are eight different ways to make babies: 1)Artificial insemination of a wife by her husband 2) Artificial insemination of a woman by a donor 3) Egg transplant from a fertile woman to an infertile woman followed by artificial insemination by her husband or a donor 4) Fertilization of egg in vitro and subsequent implantation into a woman 5) Test-tube babies 6) Parthenogenesis 7) Cloning 8) Embryo fusion, where 2 fertilized eggs are brought together resulting in one infant with four parents instead of the traditional two. Given enough money, anything desired from genetic engineering can be accomplished.23 Transgenesis A transgenic organism is one that has DNA in its genome that is foreign to its species. Usually a plasmid is broken by chemical 'scissors' and a gene from the desired genome is placed in the empty space in the plasmid. The plasmid is then put into a bacterium that injects it into a cell, where the gene implants itself and starts to produce proteins. If the nucleus of that cell accepts the new gene, it can be implanted in an enucleated egg and the resulting embryo would be transgenic. Introducing a gene to a developing embryo usually results in a chimera, an organism that has the new genes in some of its cells, but not all. If the gene is present in the gametes, or sex cells, then the offspring of the chimera is said to be transgenic, because all of its cells have the new genetic information, or transgene. Transgenic animals offer many great advances to science in the future, from cows that make insulin in their milk to live-animal disease models to help cure human disease. |