Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.
Have you ever considered how you get your traits? What makes you have dark skin, or curly hair? Why are your eyes brown and not green? The answer is heritable information, or your DNA. DNA holds all the instructions for making you, you. Different species have different DNA that makes them unique. Today, scientists are able to manipulate, or change, heritable information to make cells and even entire organisms with desirable traits. Today, we're going to look at some of the techniques scientists use to do this and their applications.
A genetically modified organism is a living thing in which scientists have altered its DNA. Although genetically modified organisms have gotten some bad press lately, they actually have many uses both in research and consumer products. Depending on what organism is being modified, scientists use different techniques for modification. Next, we'll look at each one in detail.
Scientists use a process called transfection to introduce the foreign DNA into the cell. After cutting the desired DNA out of the original species and making copies, the cells are combined with the DNA. Cells are either treated with chemicals, or go through electroporation, where they are stimulated with electricity. The electricity pops holes in the membrane, making it easier for the new DNA to enter. Think of it like cutting open a pastry to fill it with creme. The pastry is the cell, and although damaging, the holes need to be cut to fill it.
Some of the new DNA will be incorporated into the cell's genome, and the cell will start to follow the instructions as if it were their own, making the desired protein.
One important example of transfection is in bacteria, single celled organisms that have their DNA floating freely in the cell. Bacteria can be used as factories to produce proteins that humans need for medicine. Insulin is one such protein produced like this.
Insulin is a medication used to treat diabetes. Diabetics don't make enough insulin or don't respond to insulin signals. These patients need to inject extra insulin to keep their blood sugar levels stable and healthy. Bacteria have been genetically manipulated to make the insulin protein using transfection. Bacteria make and secrete the insulin and scientists can collect and purify it for patients. Since bacteria grow so quickly, these organisms are powerhouses of production and make lots of insulin in a short amount of time.
Scientists also want to change the DNA in animal cells, like when researching diseases such as cancer. Scientists change genes in healthy cells and observe any changes that might lead to disease, shedding light on the original causes. Although transfection is used in animal cells, it can be hard on cells, causing excessive cell death, making the process inefficient. Sometimes, scientists use a virus to insert DNA inside the cell during a process called viral transduction. Transduction is also used for manipulating bacteria, although it is a more common technique for scientists studying human disease.
Scientists use a special type of virus for animal cells called retroviruses. Retroviruses like HIV enter cells and insert their DNA into the genome, or the DNA inside the host cell. Scientists can use these like a cargo ship to put their desired DNA into the cell. The virus is loaded with the genes that should be inserted and cells are infected. Some cells then express the new DNA and can be used in experiments.
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Genetic modification doesn't just involve single cells. Scientists have the technology to create entire genetically modified animals, such as Dolly the sheep, the first animal ever cloned. However, creation of a transgenic animal is more complicated than modifying a single cell.
Clones are genetically identical animals and have practical uses in agriculture and genetic research. To create a clone, scientists use a process called nuclear transfer. In this process, they extract a nucleus from a cell in the animal to be cloned, the nuclear donor. They then take an egg cell and remove the nucleus. The donor nucleus is implanted into the egg, which is then implanted into a surrogate mother. The egg develops into a full animal, just as during normal development. When the animal is born it has the same genetic makeup as the nuclear donor, producing a genetic clone.
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Animals can also be genetically modified on a smaller scale. Sometimes scientists just want to manipulate one gene, not change the entire genome like cloning. Sometimes, scientists want to prevent a gene from functioning properly, to see what will happen to the animal, and thus infer what the gene does. This type of genetic manipulation is called a knockout animal. When scientists add extra genes to an animal, they are called a knockin.
A famous knockout mouse is the leptin mouse. Scientists found that removing the gene that codes for a hormone called leptin led to a massive increase in weight that could not be modulated by diet alone, thus providing insight into weight related diseases such as obesity.
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To do this, scientists need embryonic stem cells from the animal, which can be obtained from the umbilical cord or embryos. They then use transfection or transduction to change the genes how they want, either turning genes off or adding different genes. The cells that have successfully changed are implanted into a surrogate mother where they develop and are birthed normally. Typically, scientists use some type of marker to help determine which mice have been genetically modified, such as a change in coat color in addition to the gene of interest.
DNA is heritable information and is responsible for all of our traits. Scientists can genetically modify organisms to change their DNA. Transfection is the process of introducing foreign DNA to a cell using chemicals or electroporation. This is successful with bacteria, but can be damaging to animal cells, so scientists sometimes use transduction with retroviruses instead. Entire animals can be cloned, or specific genes can be altered in knockout or knockin mice.
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