Title: Genetic Advances
Course: Biology (A)

Genetic Advances

Objectives and Reading Guide

Click here to open the reading guide

Lesson Objectives

- Explain how clones are made.
- Explain how vectors are made.
- Explain what sequencing a genome tells us.
- Describe how gene therapy works.

Vocabulary Defined

Vocabulary

cloning
Creating an identical copy of an individual with the same genes.

DNA ligase
Enzyme that joins DNA fragments together.

gene therapy
Treatment that provides a new gene to replace a defective gene; potentially "cures" a genetic disease.

Human Genome Project
International effort to sequence all the base pairs in human DNA.

plasmid
An accessory circle of DNA in bacteria.

recombinant DNA
DNA formed by the combination of DNA from two different sources, such as placing a human gene into a bacterial plasmid.

somatic cell
A body cell; not a gamete.

transformation
The process by which bacteria pick up foreign DNA and incorporate it in their genome.

vector
A vehicle, such as a plasmid, used to transfer foreign DNA into an organism.

Introduction

Introduction

Since Mendel’s time, there have been rapid advances in the understanding of genetics. As scientists understand better how DNA works, they can develop technologies that allow us to reveal the genetic secrets encoded in our DNA and even alter an organism’s DNA. Genetic engineering (or biotechnology or DNA technology) has helped us better understand and predict the inheritance of genetic diseases, produce new medicines, and even produce new food products. DNA technology has also made an impact on fighting crime. Because DNA is unique to an individual, the DNA in just a few hairs at a crime scene can help identify a criminal. This technology, known as DNA fingerprinting, has also helped innocent imprisoned people to appeal their case and clear their names. DNA technology has revolutionized not only criminal justice, but also many other aspects of our lives.

Recombinant DNA

Recombinant DNA

Recombinant DNA is the combination of DNA from two different sources. It is useful in gene cloning and in identifying the function of a gene, as well as producing useful proteins. Human insulin for treating diabetes has been produced through recombinant DNA methods. In this process, a gene of interest (or piece of DNA of interest) is placed into a host cell, such as a bacterium, so the gene can be copied (and cloned) and the protein that results from that gene can be produced.

To place the gene of interest into a host cell, a vector, or carrier molecule, is needed to the carry foreign DNA into the host cell. Bacteria have small accessory rings of DNA in the cytoplasm, called plasmids. When putting foreign DNA into a bacterium (a host cell), the plasmids are often used as a vector. Viruses can also be used as vectors.

The first step of making recombinant DNA involves a restriction enzyme that cuts the vector and the foreign (exogenous) DNA. Restriction enzymes cut DNA at specific sequences, such as GAATTC as shown in Figure below . There are more than 3,000 known restriction enzymes, most cutting the DNA at a unique sequence. This reaction results in the plasmid opening up a gap with “sticky ends,” which can attach with the complimentary base pairs on the sticky ends of the foreign DNA. Then the enzyme DNA ligase seals the foreign DNA in its new place inside the plasmid. These altered plasmids are introduced back into the bacteria, a process called transformation (Figure below). The bacteria will express the foreign gene.

Restriction enzymes cut DNA at specific sequences, in this example the sequence GAATTC." The enzyme cuts between the G and A on each strand, producing overhanging sticky ends.

This image shows a line drawing of a plasmid. The plasmid is drawn as two concentric circles that are very close together, with two large segments and one small segment depicted. The two large segments (1 and 2) indicate antibiotic resistances usually used in a screening procedure, and the small segment (3) indicates an origin of replication. The resulting DNA is a recombinant DNA molecule.

One application of recombinant DNA technology is producing the protein insulin, which is needed to treat diabetes. Previously, insulin had been extracted from the pancreases of animals. Through recombinant DNA technology, bacteria were created that carry the human gene which codes for the production of insulin. These bacteria become tiny factories that produce this protein. A step-by-step depiction of the cloning of the insulin gene is shown below in (Figure below).

A step-by-step depiction of the cloning of the insulin gene. The plasmid is opened up with restriction enzymes and the gene of interest (human cDNA) is inserted into the plasmid with complementary linkers, producing the recombinant plasmid. The plasmid is transfected into bacterial cells, where the human protein is produced.

Cloning

Cloning

Cloning is the process of creating an exact replica of an organism. The clone’s DNA is exactly the same as the parent’s DNA. Bacteria and plants have long been able to clone themselves through processes of asexual reproduction. In animals, however, cloning does not happen naturally.

Animals can now be cloned in a laboratory, however. In 1997, a sheep named Dolly was the first mammal ever to be successfully cloned. The process of producing an animal like Dolly starts with a single cell from the animal that is going to be cloned. In the case of Dolly, cells from the mammary glands were taken from the adult that was to be cloned. These cells are called somatic, meaning they come from the body and are not gametes like sperm or egg. Remember that somatic cells have a diploid number of chromosomes. Next, the nucleus was removed from this cell. The nucleus was placed in a donor egg that had already had the nucleus removed. The new cell then divided after the stimulation of an electric shock, and development proceeded normally just as if the embryo had formed naturally. The resulting embryo was implanted in a surrogate mother sheep, where it continued its development. This process is shown in Figure below.

To clone an animal, a nucleus from the animals cells are fused with an egg cell (from which the nucleus has been removed) from a donor.

Cloning is not always successful, though. Most of the time, this cloning process does not result in a healthy adult animal. The process has to be repeated many times until it works. In fact, 277 tries were needed to produce Dolly. This high failure rate is one reason that human cloning is banned in the United States. In order to produce a cloned human, many attempts would result in the surrogate mothers experiencing miscarriages, stillbirths, or deformities in the infant. There are also many additional ethical considerations related to human cloning.

Human Genome Project

Human Genome Project

A person’s genome is all of his or her genetic information; in other words, the human genome is all the information that makes us human. The Human Genome Project (Figure below) was an international effort to sequence all 3 billion bases that make up our DNA and to identify within this code the over 20,000 human genes. Scientists also completed a chromosome map, identifying where the genes are located on each of the chromosomes. The Human Genome Project was completed in 2003. Though the Human Genome Project is finished, analysis of the data will continue for many years.

To complete the Human Genome Project, all 23 pairs of chromosomes in the human body were sequenced. Each chromosome contains thousands of genes. This is a karyotype, a visual representation of an individuals chromosomes lined up by size.

There are many exciting applications of the Human Genome Project. The genetic basis for many diseases can be more easily determined, and now there are tests for over 1,000 genetic disorders. The National Institutes of Health, the United States government’s premiere biomedical research community, is also looking for ways to reduce the costs of sequencing so that people can have a map of their individual genome. Although some disorders are caused by a single gene, many other illnesses are caused by a combination of several genes and a person’s lifestyle. Analysis of your own genome could determine if you are at risk for specific diseases. Knowing you might be genetically prone to a certain disease would allow you to better seek preventive lifestyle changes and medical screenings.

A genetic map shows the location (or loci) of a gene on a chromosome. Genetic maps are important tools to help researchers understand genes and genetic diseases. Knowing where genes are in relation to other genes and knowing the order of genes on a chromosome is an important aspect of human genetics. The frequency of recombination (crossing-over during prophase I of meiosis) allows geneticists to estimate the distance between loci. Because crossing-over occurs relatively rarely at any location along the chromosome, the frequency of recombination between two locations depends on their distance. The farther apart genes are on the same chromosome, the more likely there is to be a cross-over event between them. The likelihood of a cross-over event between two closely located genes (said to be linked) is small.

Gene Therapy

Gene Therapy

Gene therapy is the insertion of genes into a person’s cells to cure a genetic disorder. There are two main types of gene therapy; one done inside the body and one done outside the body. In ex vivo gene therapy, done outside the body, cells are removed from the patient and the proper gene is inserted using a virus as a vector. Then the modified cells are placed back into the patient. One of the first uses of this type of gene therapy was in the treatment of a young girl with a rare genetic disease, Adenosine deaminase deficiency, or ADA deficiency. People with this disorder are missing the ADA enzyme, which breaks down a toxin called deoxyadenosine. If the toxin is not broken down, it accumulates and destroys immune cells. As a result, individuals with ADA deficiency do not have a healthy immune system to fight off infections. In the gene therapy treatment for this disorder, bone marrow stem cells were taken from the girl’s body and the missing gene was inserted in these cells outside the body. Then the modified cells were put back into her bloodstream. This treatment proved sufficient to restore the function of her immune system, but only with continual repeated treatments.

During in vivo gene therapy, done inside the body, the vector with the gene of interest is introduced directly into the patient and taken up by the patient’s cells. The vector is inserted where the gene product is needed. For example, cystic fibrosis gene therapy is targeted at the respiratory system, so a solution with the vector can be sprayed into the patient’s nose. Recently in vivo gene therapy was also used to partially restore the vision of three young adults with a rare type of retinal disease that is congenital, meaning present at birth.

Biotechnology

Biotechnology in Medicine and Agriculture

There are many applications of genetic information, including applications in medicine and agriculture. These applications show daily the significance of biotechnology, and the impact biotechnology has on our society.

Medicine

As mentioned above, one application of recombinant DNA technology is producing the protein insulin. Using biotechnological techniques, the specific gene sequence that codes for human insulin was introduced into the bacteria E. coli. The transformed gene altered the genetic makeup of the bacterial cells, such that in a 24 hour period, billions of E. coli containing the human insulin gene resulted, producing human insulin to be administered to patients. Recombinant DNA technology has allowed mass quantities of insulin to be produced, treating the growing population that relies on this protein.

Though the production of human insulin by recombinant DNA procedures is an extremely significant event, many other aspects of DNA technology are beginning to become reality. In medicine, modern biotechnology provides significant applications in such areas as pharmacogenomics, genetic testing (and prenatal diagnosis), and gene therapy. These applications use our knowledge of biology to improve our health and our lives. Many of these medical applications are based on the findings of the Human Genome Project.

Agriculture

Biotechnology has also led scientists to develop useful applications in agriculture and food science. These include the development of transgenic crops - the placement of genes into plants to give the crop a beneficial trait. Benefits include:

- Improved yield from crops.
- Reduced vulnerability of crops to environmental stresses.
- Increased nutritional qualities of food crops.
- Improved taste, texture or appearance of food.
- Reduced dependence on fertilizers, pesticides and other agrochemicals.

Crops are obviously dependent on environmental conditions. Drought can destroy crop yields, as can too much rain or floods. But what if crops could be developed to withstand these harsh conditions? Biotechnology will allow the development of crops containing genes that will enable them to withstand harsh conditions. For example, drought and excessively salty soil are two significant factors affecting crop productivity. But there are crops that can withstand these harsh conditions. Why? Probably because of that plant's genetics. So scientists are studying plants that can cope with these extreme conditions, trying to identify and isolate the genes that control these beneficial traits. The genes could then be transferred into more desirable crops, with the hope of producing the same phenotypes in those crops.

Thale cress (Figure below), a species of Arabidopsis (Arabidopsis thaliana), is a tiny weed that is often used for plant research because it is very easy to grow and its DNA has been extensively characterized. Scientists have identified a gene from this plant, At-DBF2, that gives the plant resistance to some environmental stresses. When this gene is inserted into tomato and tobacco cells, the cells were able to withstand environmental stresses like salt, drought, cold and heat far better than ordinary cells. If these preliminary results prove successful in larger trials, then At-DBF2 genes could help in engineering crops that can better withstand harsh environments. Researchers have also created transgenic rice plants that are resistant to a rice virus. In Africa, this virus destroys much of the rice crops and makes the surviving plants more susceptible to fungal infections.

Thale cress.

Lesson Summary

Lesson Summary

- Using recombinant DNA technology, a foreign gene can be inserted into an organism’s DNA.
- Cloning of mammals is still being perfected, but several cloned animals have been created by implanting the nucleus of a somatic cell into a cell in which the nucleus has been removed.
- The Human Genome Project produced a genetic map of all the human chromosomes and determined the sequence of every base pair in our DNA.
- Gene therapy involves treating an illness caused by a defective gene through the use of a vector to integrate a normal copy of the gene into the patient.