A gene, from a basic biochemical standpoint, is a segment of deoxyribonucleic acid (DNA) inside every cell of an organism that carries the genetic code for assembling a particular protein product. On a more functional and dynamic level, genes determine what organisms – animals, plants, fungi and even bacteria – are and what they are destined to develop into.

While the behavior of genes is influenced by environmental factors (e.g., nutrition) and even by other genes, the composition of your genetic material overwhelmingly dictates almost everything about you, visible and unseen, from the size of your body to your response to microbial invaders, allergens and other external agents.

The ability to change, modify or engineer genes in specific ways would therefore introduce the option of being able to create exquisitely tailored organisms – including humans – using given combinations of DNA known to contain certain genes.

The process of altering an organism's genotype (loosely speaking, the sum of its individual genes) and hence its genetic "blueprint" is known as genetic modification. Also called genetic engineering, this kind of biochemical maneuvering has moved from the realm of science fiction into reality in recent decades.

Associated developments have trucked in both excitement at the prospect of bettering human health and quality of life and a host of thorny and inescapable ethical issues on various fronts.

Genetic modification is any process by which genes are manipulated, changed, deleted or adjusted in order to amplify, change or adjust a certain characteristic of an organism. It is the manipulation of traits at the absolute root – or cellular – level.

Consider the difference between routinely styling your hair a certain way and actually being able to control your hair's color, length and general arrangement (e.g., straight versus curly) without using any hair-care products, instead relying on giving unseen components of your body instructions concerning how to accomplish and ensure a desired cosmetic result, and you gain a sense of what genetic modification is all about.

Because all living organisms contain DNA, genetic engineering can be performed on any and all organisms, from bacteria to plants to human beings.

As you read this, the field of genetic engineering is burgeoning with new possibilities and practices in the areas of agriculture, medicine, manufacturing and other realms.

It is important to understand the difference between literally changing genes and behaving in a way that takes advantage of an existing gene.

Many genes do not operate independently of the environment in which the parent organism lives. Dietary habits, stresses of various kinds (e.g., chronic illnesses, which may or may not have a genetic basis of their own) and other things organisms routinely confront can affect gene expression, or the level to which genes are used to make the protein products for which they code.

If you come from a family of people who are genetically inclined to be taller and heavier than average, and you aspire to an athletic career in a sport that favors strength and size such as basketball or hockey, you can lift weights and eat a robust amount of food to maximize your chances of being as large and strong as possible.

But this is different from being able to insert new genes into your DNA that virtually guarantee a predictable level of muscle and bone growth and, ultimately, a human with all of the typical traits of a sports star.

Many types of genetic engineering techniques exist, and not all of them require the manipulation of genetic material using sophisticated laboratory equipment.

In fact, any process that involves the active and systematic manipulation of an organism's gene pool, or the sum of the genes in any population that reproduces by breeding (i.e, sexually), qualifies as genetic engineering. Some of these process, of course, are indeed on the cutting edge of technology.

Artificial selection: Also called simple selection or selective breeding, artificial selection is the choosing of parent organisms with a known genotype to produce offspring in quantities that would not occur if nature alone were the engineer, or at a minimum would only occur over far greater time scales.

When farmers or dog breeders select which plants or animals to breed in order to assure offspring with certain characteristics humans find desirable for some reason, they are practicing an everyday form of genetic modification.

Induced mutagenesis: This is the use of x-rays or chemicals to induce mutations (unplanned, often spontaneous changes to DNA) in specific genes or DNA sequences of bacteria. It can result in discovering gene variants that perform better (or if necessary, worse) than the “normal” gene. This process can help create new "lines" of organisms.

Mutations, while often harmful, are also the fundamental source of genetic variability in life on Earth. As a result, inducing them in large numbers, while certain to create populations of less-fit organisms, also increases the likelihood of a beneficial mutation, which can then be exploited for human purposes using additional techniques.

Viral or plasmid vectors: Scientists can introduce a gene into a phage (a virus that infects bacteria or their prokaryotic relatives, the Archaea) or a plasmid vector, and then place the modified plasmid or phage into other cells in order to introduce the new gene into those cells.

Applications of these processes include increasing resistance to disease, overcoming antibiotic resistance and improving an organism's ability to resist environmental stressors such as temperature extremes and toxins. Alternatively, the use of such vectors can amplify an existing characteristic instead of creating a new one.

Using plant breeding technology, a plant can be "ordered" to flower more often, or bacteria can be induced produce a protein or chemical that they normally wouldn’t.

Retroviral vectors: Here, portions of DNA containing certain genes are put into these special kinds of viruses, which then transport the genetic material into the cells of another organism. This material is incorporated into the host genome so that they can be expressed along with the rest of the DNA in that organism.

In plain terms, this involves snipping a strand of host DNA using special enzymes, inserting the new gene into the gap created by the snipping and attaching the DNA at both ends of the gene to the host DNA.

"Knock in, knock out" technology: As its name suggests, this type of technology allows for the complete or partial deletion of certain sections of DNA or certain genes ("knock out"). Along similar lines, the human engineers behind this form of genetic modification can choose when and how to turn on ("knock in") a new section of DNA or a new gene.

Injection of genes into nascent organisms: Injecting genes or vectors that contain genes into eggs (oocytes) can incorporate the new genes into the genome of the developing embryo, which are therefore expressed in the organism that eventually results.

Gene cloning is an example of the use of plasmid vectors. Plasmids, which are circular pieces of DNA, are extracted from a bacterial or yeast cell. Restriction enzymes, which are proteins that “cut” DNA in specific places along the molecule, are used to snip the DNA, creating a linear strand from the circular molecule. Then, the DNA for the desired gene is "pasted" into the plasmid, which is introduced into other cells.

Finally, those cells begin reading and coding the gene that was artificially added to the plasmid.

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Gene cloning includes four basic steps. In the following example, your aim is to produce a strain of E. coli bacteria that glows in the dark. (Ordinarily, of course, these bacteria do not possess this property; if they did, places like the world's sewer systems and many of its natural waterways would take on a distinctly different character, as E. coli are prevalent in the human gastrointestinal tract.)

1. Isolate the desired DNA. First, you need to find or create a gene that codes for a protein with the required property – in this case, glowing in the dark. Certain jellyfish make such proteins, and the gene responsible has been identified. This gene is called the target DNA. At the same time, you need to determine what plasmid you will use; this is the vector DNA.

2. Cleave the DNA using restriction enzymes. These aforementioned proteins, also called restriction endonucleases, are plentiful in the bacterial world. In this step, you use the same endonuclease to cut both the target DNA and the vector DNA.

Some of these enzymes cut straight across both strands of the DNA molecule, while in other instances they make a "staggered" cut, leaving small lengths of single-stranded DNA exposed. The latter are called sticky ends.

3. Combine the target DNA and the vector DNA. You now put the two types of DNA together along with an enzyme called DNA ligase, which functions as an elaborate kind of glue. This enzyme reverses the work of the endonucleases by joining the ends of the molecules together. The result is a chimera, or a strand of recombinant DNA.

• Human insulin, among many other vital chemicals, can be made using recombinant technology.

4. Introduce the recombinant DNA into the host cell. Now, you have the gene you need and a means of shuttling it to where it belongs. There are a number of ways to do this, among them transformation, in which so-called competent cells sweep up the new DNA, and electroporation, in which a pulse of electricity is used to briefly disrupt the cell membrane to allow the DNA molecule to enter the cell.

Artificial selection: Dog breeders can select for different traits, notably coat color. If a given breeder of Labrador retrievers sees a rise in demand for a given color of the breed, he or she can systematically breed for the color in question.

Gene therapy: In someone with a defective gene, a copy of the working gene can be introduced into that person's cells so that the required protein can be made using foreign DNA.

GM crops: Genetic modification agriculture methods can be used to create genetically modified (GM) crops such as herbicide-resistant plants, crops that yield more fruit compared to conventional breeding, GM plants that are resistant to cold, crops with an improved overall harvest yield, foods with a higher nutritional value and so on.

More broadly, in the 21st century, genetically modified organisms (GMOs) have blossomed into a hot-button issue in European and American markets owing to both food safety and business-ethics concerns surrounding the genetic modification of crops.

Genetically modified animals: One example of GM foods in the livestock world is breeding chickens that grow larger and more quickly to produce more breast meat. Recombinant DNA technology practices such as these raise ethical concerns because of the pain and discomfort it can cause to the animals.

Gene editing: An example of gene editing, or genome editing, is CRISPR, or clustered regularly interspaced short palindromic repeats. This process is "borrowed" from a method used by bacteria to defend themselves against viruses. It involves highly targeted genetic modification of different portions of the target genome.

In CRISPR, guide ribonucleic acid (gRNA), a molecule with the same sequence as the target site in the genome, is combined in the host cell with an endonuclease called Cas9. The gRNA will bind to the target DNA site, dragging Cas9 along with it. This genome editing can result in the "knocking out" of a bad gene (such as a variant implicated in causing cancer) and in some cases permit the bad gene to be replaced with a desirable variant.