3.5 Genetic Modification Flashcards
Polymerase Chain Reaction
way of producing large quantities of a specific target sequence of DNA.
Useful when only a small amount of DNA is available for testing e.g. crime scene samples of blood, semen, tissue, hair, etc.
steps in Polymerase Chain Reaction (PCR)
- Denaturation: DNA sample is heated to separate it into two strands (~95*C)
- Annealing: DNA primers attach to opposite ends of the target sequence (~50*C)
- Elongation: A heat-tolerant DNA polymerase (Taq) copies the strands (~72*C)
Gel Electrophoresis & DNA Profiling
Through gel electrophoresis, fragments of DNA are moved through an electric field and separated based on their size.
- DNA samples are taken and amplified with PCR.
- Restriction enzymes cut DNA into fragments at specific base sequences in each sample.
- A fluorescent marker binds to a triplet in the DNA fragments, so that results can be seen.
- Samples are added to a gel electrophoresis chamber. Electric current is passed through, pushing the fragments along.
- Heavier fragments stay closer to the origin and smaller fragments go further.
- A banding pattern shows up for each DNA sample and can be analysed or compared (DNA profiling).
DNA profiling in forensic investigations
DNA is often left behind at a crime scene. It is present in all kinds of evidence, including blood, hair, skin, saliva, and semen.
Look for a full match between the bands of the DNA sample and the bands of the potential suspects
DNA profiling in paternity investigations
DNA samples are needed from the mother, (potential) father and child in question.
Since offspring inherits a mix of DNA from parents, the child will show bands unique to each parent
Each band in the child’s profile must match either a band in the mother’s profile OR a band in the father’s profile (usually 50-50 split)
Genetic modification
Also known as genetic engineering, gene transfer or transgenics.
All living things use the same bases and the same genetic code.
Why Genetic modification?
Able to make large quantities of a protein usually made in small quantities in the original organism’s body
e.g. human insulin made by bacteria, spider silk made by goats
Introduce new characteristics to an organism
e.g. salt resistance in tomatoes
Introduce new varieties to an organism
e.g. purple potatoes from snapdragon gene, golden-coloured rice from daffodil and bacteria genes (B-carotene)
Gene Transfer
- Restriction enzymes ‘cut’ the desired gene from the genome.
- E. coli bacteria contain small circles of DNA called plasmids. These can be removed
- The same restriction enzyme cuts into the plasmid.
- Because it is the same restriction enzyme the same bases are left exposed, creating ‘sticky ends’
- Ligase joins the sticky ends, using complementary base pairing, fixing the gene into the E. coli plasmid.
- The recombinant plasmid is inserted into the host cell. It now expresses the new gene. An example of this is human insulin production.
- Fermenters are used to produce large quantities of bacteria. The human insulin is then separated from the bacteria and purified.
Alternative to using restriction enzymes to “cut” the gene of sequence from the genome, mRNA transcripts of the gene can be used:
mRNA can treated with reverse transcriptase to produce short DNA segments of the mRNA called cDNA (complementary DNA)
Why use mRNA?
- Easier to extract than DNA
- mRNA is already spliced to remove introns
Gene transferred from bacterium Bacillus thuringiensis that codes for…
Bt toxin
- Toxin is a protein that kills insect orders that contain butterflies, moths, flies, beetles, bees and ants
Concerns about Bt corn affecting:
non-target insects
Species of concern is the monarch butterfly
- Feed on leaves of milkweed
- Milkweed grows close to Bt corn, gets dusted with corn pollen (Bt) 🡪 monarchs become poisoned by toxin from GM corn crops
Claims about environmental benefits of
GM crops:
Pest-resistant crop varieties can be produced by transferring a gene for making a toxin to the plants. Less insecticide then has to be sprayed on to the crop so fewer bees and other benefcial insects are harmed.
Use of GM crop varieties reduces the need for plowing and spraying crops, so less fuel is needed for farm machinery.
The shelf-life of fruit and vegetables can be improved, reducing wastage and reducing the area of crops that have to be grown.
Claims about the health benefits of
GM crops:
The nutritional value of crops can be improved, for example by increasing the vitamin content.
Varieties of crops could be produced lacking allergens or toxins that are naturally present in them.
GM crops could be engineered that produce edible vaccines so by eating the crop a person would be vaccinated against a disease.
Claims about agricultural benefts of
GM crops:
Varieties resistant to drought, cold and salinity can be produced by gene transfer, expending the range over which crops can be produced and increasing total yields.
A gene for herbicide resistance can be transerred to crop plants allowing all other plants to be killed in the growing crop by spraying with herbicide. With less weed competition crop yields are higher. Herbicides that kill all plants can be used to create weed-free conditions or sowing non-GM crops but they cannot be used once the crop is growing.
Crop varieties can be produced that are resistant to diseases caused by viruses. These diseases currently reduce crop yields signifcantly and the only current method of control is to reduce transmission by killing insect vectors of the viruses with insecticides.
Claims made about health risks of GM crops:
Proteins produced by transcription and translation of transferred genes could be toxic or cause allergic reactions in humans or livestock that eat GM crops.
Antibiotic resistance genes used as markers during gene transer could spread to pathogenic bacteria.
Transferred genes could mutate and cause unexpected problems that were not risk- assessed during the development of GM crops.
