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Biotechnology: Class 12 Biology
Traditional and modern biotechnology, genetic engineering tools, recombinant DNA, PCR and gel electrophoresis, cloning, transgenic organisms, gene therapy, applications in medicine and agriculture, and ethical issues.
Key points
Biotechnology is the use of living organisms or their parts to make useful products or to solve problems. Traditional biotechnology is very old: making yoghurt and cheese with bacteria, bread and alcohol with yeast, and selective breeding of crops and animals. Modern biotechnology uses genetic engineering, tissue culture and fermenters.
Genetic engineering is the direct change of the genes of an organism by humans. A gene from one organism is cut out and put into another, which makes recombinant DNA. This is possible because the genetic code is almost universal, so a human gene can work in a bacterium.
Tools of genetic engineering:
- Restriction enzymes: cut DNA at specific base sequences. Many cut in a staggered way, leaving short single stranded 'sticky ends'. EcoRI is an example.
- DNA ligase: joins DNA pieces by forming bonds in the sugar phosphate backbone.
- Vectors: carriers that take the gene into a host cell. Plasmids, small circular DNA of bacteria, are commonly used. Viruses can also be vectors.
- Host cells: such as the bacterium Escherichia coli or yeast, which copy the gene as they divide.
Steps in making a recombinant plasmid. The gene is isolated, or made from mRNA with the enzyme reverse transcriptase. The plasmid and the gene are cut with the same restriction enzyme so that they have matching sticky ends. They are mixed and DNA ligase joins them. The plasmid is put into bacteria. Bacteria that took it up are selected, often with a marker gene such as antibiotic resistance, and then grown in a fermenter to make the product.
Human insulin was the first genetically engineered medicine to be sold. The human insulin gene is put in E. coli, which then make insulin in large amounts. It replaces insulin taken from animals, causes fewer allergic reactions and is available in large quantities. Other products include human growth hormone, clotting factor VIII for haemophilia and some vaccines such as the hepatitis B vaccine.
The polymerase chain reaction (PCR) makes millions of copies of a DNA segment in a few hours. A cycle has three steps: heating to about 95 °C to separate the strands (denaturing), cooling to about 55 °C so that short primers attach (annealing), and about 72 °C when heat stable Taq polymerase builds new strands (extension). Each cycle doubles the DNA, so 10 cycles give 2¹⁰ = 1024 copies per original molecule.
Gel electrophoresis separates DNA fragments by size. DNA is negatively charged, so in an electric field the fragments move through a gel towards the positive electrode. Short fragments move faster and farther. The pattern of bands is the DNA profile (fingerprint), used in crime investigation, paternity tests and diagnosing disease. Related people share more bands.
Transgenic organisms contain a gene from another species. Bt cotton and Bt maize carry a gene from the bacterium Bacillus thuringiensis that makes a protein poisonous to certain insect pests, so less pesticide is needed. Golden rice has genes that make beta carotene (a source of vitamin A). Other crops are made resistant to herbicides, drought or disease. Pakistan grows Bt cotton.
Cloning produces genetically identical copies. In plants, tissue culture grows a whole plant from a small piece of tissue on a sterile medium with nutrients and hormones, and it is used for rapid production of banana, potato and ornamental plants. In animals, Dolly the sheep (1996) was the first mammal cloned from an adult cell: the nucleus from a udder cell was put into an egg whose nucleus had been removed, and the embryo was placed in a surrogate mother.
Gene therapy tries to treat a genetic disease by placing a healthy copy of a gene in the cells of a patient, often with a virus as the vector. It has been tried for conditions such as severe combined immunodeficiency. Stem cells, which can become many types of cell, are also studied for treating injuries and disease.
Other uses: microorganisms make antibiotics, enzymes, biogas and single cell protein. Bacteria are used to treat sewage and clean oil spills (bioremediation). Biotechnology also raises issues. There are concerns about safety to health and the environment, spread of genes to wild plants, ownership of genes by companies, and the ethics of cloning, which societies must discuss and regulate.
Study tip: Learn the steps of making a recombinant plasmid in order, with the role of each enzyme named. Practise a short comparison of PCR and electrophoresis and be ready to give one benefit and one concern of GM crops.
Definitions and flashcards
Say the meaning first, then open the card to check.
Biotechnology
The use of living organisms or their products for human benefit.
Recombinant DNA
DNA made by joining pieces from different sources.
Restriction enzyme
An enzyme that cuts DNA at a particular base sequence.
Plasmid
A small circular DNA molecule found in bacteria, used as a vector.
Transgenic organism
An organism that carries a gene from another species.
PCR
A technique that makes many copies of a piece of DNA using repeated heating and cooling.
Tissue culture
Growing cells or tissues on a sterile nutrient medium.
Clone
A group of genetically identical organisms or cells.
Gene therapy
The treatment of a genetic disease by giving the patient a working gene.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
What are restriction enzymes? (2 marks)
Restriction enzymes are enzymes, found in bacteria, that cut DNA at specific recognition sequences. They often leave sticky ends, which allows DNA pieces from different sources to be joined.
Why are plasmids used as vectors? (2 marks)
Plasmids are small, circular and easily cut and joined, and they can enter bacteria. They replicate independently of the main chromosome, so the inserted gene is copied many times.
Outline how human insulin is made by genetic engineering. (3 marks)
The human insulin gene is cut out with a restriction enzyme. A plasmid is cut with the same enzyme and the gene is joined into it with DNA ligase. The recombinant plasmid is put into E. coli. The bacteria are grown in a fermenter, where they copy the gene and make insulin, which is then collected and purified.
Describe the steps of PCR. (3 marks)
The DNA, primers, Taq polymerase and nucleotides are placed in a machine. At about 95 °C the two strands separate. At about 55 °C primers attach to the ends of the target sequence. At about 72 °C Taq polymerase adds nucleotides and copies each strand. The cycle is repeated many times, and the amount of DNA doubles each time.
Explain how gel electrophoresis separates DNA fragments. (3 marks)
DNA cut into fragments is placed in wells at the negative end of a gel. Because DNA is negatively charged, it moves toward the positive electrode when current passes. Small fragments move through the gel more easily and travel farther, while large ones move slowly. This separates the fragments in bands according to size, which can be seen with a stain.
Discuss the advantages and the concerns of genetically modified crops. (5 marks)
Advantages: crops such as Bt cotton resist insect pests, so farmers use less pesticide and lose less yield. Other crops resist drought or herbicides, and some, such as golden rice, have more nutrients, which can help against deficiency diseases. Concerns: the gene may spread to wild relatives, insects may become resistant to the toxin, some people fear health effects or allergies, seeds can be expensive and controlled by companies, and long term effects on the environment are not fully known. Careful testing and regulation are therefore needed.
Common mistakes
- Saying ligase cuts DNA. Restriction enzymes cut and ligase joins.
- Confusing the stages of PCR or the temperatures at each step.
- Writing that DNA moves to the negative electrode in electrophoresis. It moves to the positive.
- Calling every plasmid a virus, or saying vectors are always viruses.
- Treating cloning and genetic engineering as the same. Cloning copies a whole organism or cell, while genetic engineering changes genes.