Genetics

Genetics  a discipline of biology, is the science of genes, heredity, and variation in living organisms

What Is a Gene?

Look closely at the chromosomes and you'd see that each is made of bundles of looping coils. If you unraveled these coils, you'd have a six-foot long double strand of deoxyribonucleic acid-DNA.

A DNA molecule is a twisted ladder-like stack of building blocks called nucleotides. There are four types of DNA nucleotides-adenine, cytosine, guanine, and thymine-or A, C, G, and T, for short.

If you could peer into any one of your body's 50 trillion cells, you'd find a fantastically complex and busy world. At the center of this world you'd find a nucleus containing 46 molecules called chromosomes-23 from your mother and 23 from your father. These chromosomes are basically an instruction set for the construction and maintenance of... you.

These two long stacks of building blocks fit together like two sides of zipper, but there's a rule involved: adenine only pairs with thymine, and cytosine only pairs with guanine. So each rung in the DNA ladder is a pair of nucleotides, and each pair is either an A stuck to a T or a C stuck to a G.

You've got six billion of these pairs of nucleotides in each of your cells, and amongst these six billion nucleotide pairs are roughly 30,000 genes. A gene is a distinct stretch of DNA that determines something about who you are. (More on that later.) Genes vary in size, from just a few thousand pairs of nucleotides (or "base pairs") to over two million base pairs.

How Do Genes Work?

Genes are often called the blueprint for life, because they tell each of your cells what to do and when to do it: be a muscle, make bone, carry nerve signals, and so on. And how do genes orchestrate all this? They make proteins. In fact, each gene is really just a recipe for a making a certain protein.
And why are proteins important? Well, for starters, you are made of proteins. 50% of the dry weight of a cell is protein of one form or another. Meanwhile, proteins also do all of the heavy lifting in your body: digestion, circulation, immunity, communication between cells, motion-all are made possible by one or more of the estimated 100,000 different proteins that your body makes.
But the genes in your DNA don't make protein directly. Instead, special proteins called enzymes read and copy (or "transcribe") the DNA code. The segment of DNA to be transcribed gets "unzipped" by an enzyme, which uses the DNA as a template to build a single-stranded molecule of RNA. Like DNA, RNA is a long strand of nucleotides.
This transcribed RNA is called messenger RNA, or mRNA for short, because it leaves the nucleus and travels out into the cytoplasm of the cell. There, protein factories called ribosomes translate the mRNA code and use it to make the protein specified in the DNA recipe.
If all this sounds confusing, just remember: DNA is used to make RNA, then RNA is used to make proteins-and proteins run the show.

The Cell's Secret Code

All the proteins in your body are made from protein building blocks called amino acids. There are twenty different amino acids used to make proteins, but there are only 4 different nucleotides in DNA and RNA. How can a 4-letter code specify 20 different amino acids?

Actually, the DNA code is designed to be read as triplets. Each "word" in the code, called a codon, is three letters long. There are also special "start" and "stop" codons that mark the beginning and end of a gene. As you can see, the code is redundant, that is, most of the amino acids have at least two different codons.

Just about every living thing uses this exact code to make proteins from DNA.

"Junk" DNA

Scientists first studying DNA sequences were surprised to find that less than 2% of human DNA codes for proteins. If 98% of our genetic information (or "genome") isn't coding for protein, what is it for?

At first it wasn't clear, and some termed this non-coding DNA "junk DNA." But as more research is done, we are beginning to learn more about the DNA between the genes-intergenic DNA.

Intergenic DNA seems to play a key role in regulation, that is, controlling which genes are turned "on" or "off" at any given time.

For example, some intergenic sequences code for RNA that directly causes and controls reactions in a cell, a job that scientists originally thought only proteins could do.

Intergenic DNA is also thought to be responsible for "alternative splicing," a kind of mix-and-match process whereby several different proteins can be made from one gene.

In short, it now seems that much of the interest and complexity in the human genome lies in the stuff between the genes... so don't call it junk.

Why We are Different

Biologists use two fancy words to describe the relationship between your genes and your physical traits. The first word is genotype. Your genotype is your genes for a given trait. In most cases, you've got two copies of a gene - one from your mother and one from your father.
The second word is phenotype. Phenotype is what you actually turn out to be, the way these genes get expressed. Biologists have a saying involving these two fancy words: "Genotype determines phenotype."
Let's take eyelashes, for example. There are 2 kinds of eyelashes in people - long and short. Maybe you've got a short lashes version of a gene from your father, and a long lashes version of a gene from your mother. That's your genotype. And what length of lashes do you actually have? Long - that's your phenotype.
The eyelash example makes an important point. Some genes are dominant, and others are recessive. When you have two different genes for the same trait, and one is dominant (long lashes) while the other is recessive (short lashes), it's the dominant trait that wins out in the phenotype.
But not all genes follow this dominant/recessive model. For example, the gene for blood type is codominant; if you get a gene for type A blood from one parent and type B blood from the other, neither dominates. Instead, you wind up with type AB blood.
Other human traits are polygenic, which means that they are controlled by several genes that contribute in an additive fashion. Skin color is believed to be polygenic. Scientists also think that polygenic inheritance is responsible for inherited predispositions to certain diseases, such as heart disease, arteriosclerosis, and some cancers.

Pass the Peas, Please

Gregor Mendel was an Austrian monk who did extensive breeding research on pea plants. In doing so, he overturned our understanding of heredity.

At the time of Mendel's work, in the 1860's, most people believed in the blending theory of heredity, the idea that offspring were born with traits constituting a blend or average of the two parents. By this theory, when a red and a white flower are bred together, or crossed, their offspring should all be blended, that is, pink.

Mendel's results suggested otherwise. For example, when he crossed pure-bred tall pea plants with pure-bred short pea plants, he got all tall pea plants-no blending there. He concluded that every organism possesses two "factors" (we now call them genes) for a given trait, and passes on just one of these factors-at random-to its offspring.

Mutations and Disease

DNA is constantly subject to mutations, accidental changes in its code. Mutations can lead to missing or malformed proteins, and that can lead to disease.
We all start out our lives with some mutations. These mutations inherited from your parents are called germ-line mutations. However, you can also acquire mutations during your lifetime. Some mutations happen during cell division, when DNA gets duplicated. Still other mutations are caused when DNA gets damaged by environmental factors, including UV radiation, chemicals, and viruses.
Few mutations are bad for you. In fact, some mutations can be beneficial. Over time, genetic mutations create genetic diversity, which keeps populations healthy. Many mutations have no effect at all. These are called silent mutations.
But the mutations we hear about most often are the ones that cause disease. Some well-known inherited genetic disorders include cystic fibrosis, sickle cell anemia, Tay-Sachs disease, phenylketonuria and color-blindness, among many others. All of these disorders are caused by the mutation of a single gene.
Most inherited genetic diseases are recessive, which means that a person must inherit two copies of the mutated gene to inherit a disorder. This is one reason that marriage between close relatives is discouraged; two genetically similar adults are more likely to give a child two copies of a defective gene.
Diseases caused by just one copy of a defective gene, such as Huntington's disease, are rare. Thanks to natural selection, these dominant genetic diseases tend to get weeded out of populations over time, because afflicted carriers are more likely to die before reproducing.
Scientists estimate that every one of us has between 5 and 10 potentially deadly mutations in our genes-the good news is that because there's usually only one copy of the bad gene, these diseases don't manifest.
Cancer usually results from a series of mutations within a single cell. Often, a faulty, damaged, or missing p53 gene is to blame. The p53 gene makes a protein that stops mutated cells from dividing. Without this protein, cells divide unchecked and become tumors.

Sickle Cell

These are the sickle-shaped blood cells of someone with sickle cell anemia, a genetic disease common among those of African descent.

Sickle cell anemia is the result of a point mutation, a change in just one nucleotide in the gene for hemoglobin. This mutation causes the hemoglobin in red blood cells to distort to a sickle shape when deoxygenated. The sickle-shaped blood cells clog in the capillaries, cutting off circulation.

Having two copies of the mutated genes cause sickle cell anemia, but having just one copy does not, and can actually protect against malaria - an example of how mutations are sometimes beneficial.

Genetic Testing

Have you ever had your genes tested? Probably not. DNA testing is still pretty limited, although it is becoming more and more common, especially for fetuses and newborns.
Many prospective parents, especially those with a history of genetic disease in the family, seek genetic testing and counseling before having children. Genetic counselors can evaluate genetic tests and advise people of the risk of conceiving a child with recessive, inherited diseases like sickle cell anemia, Tay-Sachs disease, or cystic fibrosis.
Genetic tests can be performed on fetuses by taking cell samples from the womb. The two techniques available are called amniocentesis and chorionic villi sampling. Down syndrome, a condition caused by having an extra chromosome, is tested for this way. After birth, most newborns are given a blood test for phenylketonuria, a genetic disease that can cause mental retardation if it goes undiagnosed.
Not all genetic testing is focused on children. Testing is also done to match organ donors and recipients, to establish paternity or maternity, and in forensics, for identifying evidence from crime scenes. Testing can also help diagnose adult-onset inherited diseases, such as Huntington's disease.
Genetic tests are now available for a range of cancers. These tests don't test for cancer directly, but instead indicate an increased likelihood of developing a cancer. Likelihood is far from certainty, and cancer may or may not develop, since it must be triggered by additional mutations.
Meanwhile, many cancers develop in persons without so-called "cancer genes." For example, the two gene variants that have been linked with breast cancer, called BRCA1 and BRCA2, are involved in only 5% of breast cancer cases.
The decision to be tested can be loaded. What if a gene test told you that age 40 or so, you would start to lose control of your muscles and live a shorter lifespan? This is the case in Huntington's disease, which has no cure. Perhaps you would rather not know, but the information might also guide your life decisions, such as when or whether to have children.

Biochips

Biochips, also called DNA arrays or microarrays, are a new technology that promises to speed and simplify a wide range of genetic tests.

Each small glass slide or "chip" contains rows and rows of DNA probes, which test for the presence of a specific DNA sequence or mutation. If the mutation or sequence is present in the DNA being tested, a specific spot on the biochip will glow under special light. A biochip can test for thousands of mutations at once.

Biochips could be the first step towards genetic ID cards. Imagine something like a credit card, except this card would carry all of your genetic information. Your doctors could use this DNA data to tailor your medical care and choose the right drugs and dosages.

Making Medicines

Not long ago, if you were diabetic, the insulin your doctor prescribed would have come from a pig. If you required human growth hormone, it would have come from human cadavers, a source that is costly, not to mention a little creepy.
Now, these and other medicines can be made by specially-modified bacteria, called transgenic bacteria. These single-celled organisms have foreign genes along side their own DNA. They live and reproduce like ordinary bacteria, but they also do a bit of extra duty, and produce human proteins for medicines and vaccines.
Farm animals have also been put to work making drugs for humans. It's called gene pharming. For example, antithrombin III, which prevents blood clotting during surgery, is secreted in the milk of transgenic goats. To create these transgenic goats, scientists used a needle thinner than a human hair to inject the DNA sequence for antithrombin III into goat eggs. Then they transplanted the fertilized transgenic eggs into female goats.
Scientists also hope to use genetic technologies to make "magic bullets," drugs that are designed to target specific antigens (disease-causing substances) while leaving healthy tissue alone. "Magic bullets" would have few side effects, because they would consist of antibodies, the same kind of specific weapons that our own immune systems use to kill invaders like viruses, bacteria, or even cancer.

Drug Manufacturing Plants

Along with bacteria and farm animals, plants are being genetically engineered to make human hormones, antibodies, and blood-clotting factors. Bananas are being engineered to deliver vaccines.

To make a transgenic plant, scientists mix foreign DNA with protoplasts, plant cells that have had their tough cell walls removed. Then, they run a small electric current through the mixture. The current makes tiny holes in the cell, allowing the foreign DNA to enter. Once the protoplast develops into a plant, the medicine it has been tricked into making can be extracted from the plant's seeds.

Genetic engineering
Genetic engineering, also called genetic modification, is the direct manipulation of an organism's genome using biotechnology. New DNA may be inserted in the host genome by first isolating and copying the genetic material of interest using molecular cloning methods to generate a DNA sequence, or by synthesizing the DNA, and then inserting this construct into the host organism. Genes may be removed, or "knocked out", using a nuclease. Gene targeting is a different technique that uses homologous recombination to change an endogenous gene, and can be used to delete a gene, remove exons, add a gene, or introduce point mutations.
An organism that is generated through genetic engineering is considered to be a genetically modified organism (GMO). The first GMOs were bacteria in 1973; GM mice were generated in 1974. Insulin-producing bacteria were commercialized in 1982 and genetically modified food has been sold since 1994.
Genetic engineering techniques have been applied in numerous fields including research, agriculture, industrial biotechnology, and medicine. Enzymes used in laundry detergent and medicines such as insulin and human growth hormone are now manufactured in GM cells, experimental GM cell lines and GM animals such as mice or zebrafish are being used for research purposes, and genetically modified crops have been commercialized.
This article focuses on history and methods of genetic engineering, and on applications of genetic engineering and of genetically modified organisms (GMOs). The article on GMOs focuses on what organisms have been genetically engineered and for what purposes. The two articles cover much of the same ground but with different organizations (sorted by application in this article; sorted by organism in the other).

A genetically modified organism (GMO) is an organism whose genetic material has been altered using genetic engineering techniques. Organisms that have been genetically modified include micro-organisms such as bacteria and yeast, plants, fish, and mammals. GMOs are the source of genetically modified foods, and are also widely used in scientific research and to produce useful goods other than food.

Genetically modified crops (GM crops, or biotech crops) are plants, the DNA of which has been modified using genetic engineering techniques, which are then used in agriculture. Plants are also transgenically modified in scientific research; see genetically modified organism for discussion.
Genetic engineering techniques are much more precise than mutagenesis (mutation breeding) where an organism is exposed to radiation or chemicals to create a non-specific but stable change. Other techniques by which humans modify food organisms include selective breeding; plant breeding, and animal breeding, and somaclonal variation.
In most cases the aim is to introduce a new trait to the plant which does not occur naturally in this species. Examples include resistance to certain pests, diseases or environmental conditions, or the production of a certain nutrient or pharmaceutical agent.
Genetically modified foods (GM foods, or biotech foods) are foods derived from genetically modified organisms (GMOs), such as genetically modified crops or genetically modified fish. GMOs have had specific changes introduced into their DNA by genetic engineering techniques. These techniques are much more precise^[1] than mutagenesis (mutation breeding) where an organism is exposed to radiation or chemicals to create a non-specific but stable change.

Method of production

Genetically engineered plants are generated in a laboratory by altering their genetic makeup and are tested in the laboratory for desired qualities. This is usually done by adding one or more genes to a plant's genome using genetic engineering techniques. Most genetically modified plants are generated by the biolistic method (particle gun) or by Agrobacterium tumefaciens mediated transformation. Once satisfactory plants are produced, sufficient seeds are gathered, and the companies producing the seed need to apply for regulatory approval to field-test the seeds. If these field tests are successful, the company must seek regulatory approval for the crop to be marketed (see below). Once that approval is obtained, the seeds are mass produced, and sold to farmers. The farmers produce genetically modified crops, which also contain the inserted gene and its protein product. The farmers then sell their crops as commodities into the food supply market, in countries where such sales are permitted.
The genetically modified foods controversy is a dispute over the relative advantages and disadvantages of genetically modified food, genetically modified crops used to produce food and other goods, and other uses of genetically modified organisms in food production. The dispute involves consumers. biotechnology companies, governmental regulators, non-governmental organizations and scientists. The dispute is most intense in Japan and Europe, where public concern about GM food is higher than in other parts of the world such as the United States. In the United States, GM crops are more widely grown and the introduction of these products has been less controversial. These national differences have led to differing regulatory regimes - see regulation of the release of genetic modified organisms.
The key areas of controversy related to genetically modified (GM) food are: risk of harm from GM food, whether GM food should be labeled, the role of government regulators, the effect of GM crops on the environment, and GM crops' context as part of the industrial agriculture system.
There has never been a long-term study of the effects of a diet including GM food on humans, so no one can be certain whether such diet is as more, less, or as harmful as a diet without GM foods. Therefore, discussion of the safety of GM food is a matter of assessing the risk of harm. While it is not possible to make general statements on the safety of all GM foods, to date no adverse health effects in humans have been documented,^[1] and there is now broad scientific and regulatory consensus that GM food on the market is safe to eat.^[1][2] An OECD task force wrote in 2000: "Much experience has been gained in the safety assessment of the first generation of foods derived through modern biotechnology, and those countries that have conducted assessments are confident that those GM foods they have approved are as safe as other foods