Although the sequencing of the bases of the human genome has been completed, the task of identifying all of the genes is ongoing. As these genes are found, it would be possible for a person to be screened for particular genes that could aff ect them – for example, by increasing their susceptibility to cancer or the likelihood that they will develop Alzheimer’s disease in later life.
Questions to consider
1 Does simply knowing the sequence of the three billion base pairs of the human chromosomes tell us anything about what it means to be human?
2 Should third parties such as health insurance companies have the right to see genetic test results or demand that a person is screened before off ering insurance cover or setting the level of premiums?
3 If treatment is unavailable, is it valuable to provide
knowledge of a genetic condition that a person may carry?
4 Knowledge of an individual’s genome has implications for other members of their families. Should their rights be protected?
4 GENETICS 1 91 Gene transfer is possible because the genetic code is universal. No
matter what the species, the genetic code spells out the same information and produces an amino acid sequence in one species that is exactly the same in any other species.
Usually, in gene transfer, only single genes are used – for example, the gene for producing human blood-clotting factor XI has been transferred to sheep, which then produce the factor in their milk.
The technique of gene transfer
One of the fi rst important uses of gene transfer was to produce insulin for diabetic patients who do not produce insulin properly. Many years ago, insulin was obtained from cow or pig pancreases but the process was diffi cult and the insulin was likely to be contaminated. Today, diabetics inject themselves with human insulin that has been made by modifi ed E. coli bacteria (Figure 4.14, overleaf).
There are key three steps in the process:
•
obtaining the desired human insulin gene in the form of a piece of DNA•
attaching this DNA to a vector, which will carry it into the host cell (E. coli) – the vector used is the plasmid found inside the bacterium•
culturing E. coli bacteria so that they translate the DNA and make insulin, which is collected.Genetically modifi ed organisms (GMOs)
By 2009, almost 100 plant species had been genetically modifi ed and many trials have taken place to assess their usefulness. In comparison, there are very few examples of genetically modifi ed animal species.
Most genetic engineering has involved commercial crops such as maize, potatoes, tomatoes and cotton. Plants have been modifi ed to make them resistant to pests and disease and tolerant to herbicides. Genetically engineered animals, on the other hand, have mainly been farmed for the products of the inserted genes – most common are proteins such as factor XI and α1 antitrypsin, which are needed for the treatment of human diseases.
Herbicide tolerance
Herbicides are used to kill weeds in crop fi elds but they are expensive and can aff ect local ecosystems as well as cultivated areas. One commonly sprayed and very powerful herbicide is glyphosate, which is rapidly broken down by soil bacteria. For maximum crop protection, farmers needed to spray several times a year. But now, the genes from soil bacteria have been successfully transferred into maize plants making them resistant to the herbicide.
Farmers can plant the modifi ed maize seeds, which germinate along with the competing weeds. Spraying once with glyphosate kills the weeds and leaves the maize unaff ected. The maize then grows and out-competes any weeds that grow later when the glyphosate has broken down in the soil. Yields are improved and less herbicide has to be used.
New developments in genetic modifi cation
While strawberries, pineapples, sweet peppers and bananas have all been genetically modifi ed to remain fresh for longer, a new variety of golden-coloured rice has been genetically modifi ed so that it contains high levels of a substance called beta-carotene. Beta-carotene gives carrots their orange colour but, more importantly, the body converts it to vitamin A, which is essential for the development of pigments in the retina of the eye.
Three genes had to be introduced into rice so it could produce beta-carotene. Two of these came from daff odils and the third was taken from a bacterium.
Enriched Golden RiceTM is a valuable dietary supplement for people whose diet is low in vitamin A and who might otherwise suff er vision problems or blindness. Since Golden RiceTM was developed in the 1990s, research has continued into its use and help in enriching diets. Some interesting and controversial issues have arisen about the ownership of the rights to the seeds, the fl avour of the rice and the publication of research data from experimental studies. You may like to read more about this in Plant Physiology, March 2001, vol. 125, pages 1157–1161, www.plantphysiol.org.
Isolation of human gene
Formation of recombinant DNA, identification and cloning
Manufacture
Preparation of vector
human pancreas bacteria
restriction enzyme
Plasmids from bacteria are cut with the same restriction enzyme to produce sticky ends.
The gene is cloned by growing the bacterium.
The transformed bacteria, containing the insulin gene, can produce insulin. These bacteria are now grown at an industrial scale in large fermenters. Insulin is extracted from the bacteria.
fermenter
pure human insulin extraction and purification of insulin A transformed
bacterium is identified
The recombinant plasmid is introduced into bacteria.
ligase enzyme
The insulin DNA and plasmid DNA are mixed together with a ligase enzyme. The sticky end bases form hydrogen bonds. The ligase joins the DNA backbone and a recombinant plasmid is produced.
plasmids from bacteria
free nucleotides DNA polymerase
DNA
complementary to the mRNA strand
cDNA
restriction enzyme reverse
transcriptase enzyme
mRNA from β cells, some with the code for human insulin
mRNA with the code for human insulin identified and isolated
mRNA is taken from β cells and used to make DNA with the help of reverse transcriptase. It is then cut to leave sticky ends.
β cells
Figure 4.14 Stages in producing a transgenic bacterium.
4 GENETICS 1 93 Reducing pollution
Pigs fed on grains and soybean meal produce a lot of phosphate in their manure. Phosphate causes pollution and eutrophication in the environment. Genetically modifi ed pigs have been developed with a gene from the bacterium E. coli. The bacteria make an enzyme, phytase, which releases the digestible phosphorus found in grains and soybeans.
Genetically modifi ed pigs produce this enzyme in their saliva and so digest their food better. More phosphorus becomes available to them and less goes undigested. The pigs absorb the nutrients into their blood, so they grow better, and much less phosphate is released in their manure.
Potential benefi ts and possible harm from genetic modifi cation
Genetic modifi cation of plants and animals is potentially enormously helpful to the human race but it raises ethical and social questions, which are the source of heated debate. Some of the possible benefi ts for the future are listed below.
•
As our population increases and more people need feeding, modifying plants and animals to increase yield or to be able to grow in places where they previously could not, will provide more food. Plants can be made tolerant to drought or salt water so that food can be grown in diffi cult areas.•
Crop plants that are disease resistant not only increase yields but also reduce the need for applying potentially harmful pesticides.•
Many substances, such as human growth hormone, a blood-clotting factor, antibodies, and vitamins, are already being made by genetically modifi ed organisms to improve human health.On the other hand, there are those who are greatly concerned by the use of genetically modifi ed plants and animals.