IB Biology SL

Revision Notes

3.4.3 Genetic Modification

Principles of Genetic Engineering

  • Genetic engineering is a term usually used to refer to the manipulation of the DNA sequences of an organism
  • The key feature of the genetic code that makes this possible is that it is universal, meaning that almost every organism uses the same four nitrogenous bases – A, T, C & G. There are a few exceptions
    • Additionally the same codons code for the same amino acids in all living things (meaning that genetic information is transferable between species)
  • Thus scientists have been able to artificially change an organism’s DNA by combining lengths of nucleotides from different sources (typically the nucleotides are from different species)
  • The altered DNA, with the introduced nucleotides, is called recombinant DNA (rDNA)
  • If an organism contains nucleotide sequences from a different species it is called a transgenic organism
  • Any organism that has introduced genetic material is a genetically modified organism (GMO)
  • The mechanisms of transcription and translation are also universal which means that the transferred DNA can be translated within cells of the genetically modified organism
  • Genetic engineering is being used in the new field of science called synthetic biology
    • This is an area of research that studies the design and construction of different biological pathways, organisms and devices, as well as the redesigning of existing natural biological systems

Recombinant DNA, downloadable AS & A Level Biology revision notes

Illustration of a maize plant that has recombinant DNA (DNA from Bacillus thuringiensis)

Recombinant DNA technology

  • This form of genetic engineering involves the transfer of fragments of DNA from one organism/species into another organism/species
  • The resulting genetically engineered organism will then contain recombinant DNA and will be a genetically modified organism (GMO)

Uses of genetic engineering

  • Some of the key uses of genetic engineering include:
    • Genetic modification of crops to increase crop yield through resistance to drought, disease, pesticides and herbicides; or to provide increased nutritional value (e.g. golden rice)
    • Genetic modification of livestock to give disease and pest resistance and increased productivity
    • Genetic modification of bacteria to produce medicines e.g. insulin. Additionally bacterial can be modified to decompose toxic pollutants or carry out large scale chemical production

Genetic Engineering Techniques

  • In order for an organism to be genetically engineered the following steps must be taken:
    • Identification of the DNA fragment or gene
    • Isolation of the desired DNA fragment (either using restriction enzymes, a gene machine or reverse transcriptase)
    • Multiplication of the DNA fragment (using polymerase chain reaction – PCR)
    • Transfer into the organism using a vector (e.g. plasmids, viruses, liposomes)
    • Identification of the cells with the new DNA fragment (by using a marker), which is then cloned
  • Genetic engineers need the following ‘tools’ to modify an organism:
    • Enzymes
      • Restriction endonucleases – used to cut genes at specific base sequences (restriction sites). Different restriction enzymes cut at different restriction site
      • Ligase – used to join together the cut ends of DNA by forming phosphodiester bonds
      • Reverse transcriptase – Used to build double stranded DNA from single stranded RNA
    • Vectors – used to deliver DNA fragments into a cell
      • Plasmids – transfer DNA into bacteria or yeast
      • Viruses – transfer DNA into human cells or bacteria
      • Liposomes – fuse with cell membranes to transfer DNA into cells
    • Markers – genes that code for identifiable substances that can be tracked
      • Florescent markers e.g. green fluorescent protein (GFP) which fluoresces under UV light
      • Enzyme markers e.g. β-glucuronidase (GUS) enzyme which transforms colourless or non-fluorescent substrates into products that are coloured or fluorescent
      • Antibiotic resistance marker genes – The required gene sequence is inserted into a gene for antibiotic resistance. This inactivates the antibiotic resistance gene and therefore means that successfully transformed bacteria will be wiped out if exposed to the antibiotic. A replica plating method is then used to isolate the successfully transformed bacteria

Genetic engineering explained (1), downloadable AS & A Level Biology revision notes Genetic engineering explained (2), downloadable AS & A Level Biology revision notes Genetic engineering explained (3), downloadable AS & A Level Biology revision notes Genetic engineering explained (4), downloadable AS & A Level Biology revision notes

An overview of the steps taken to genetically engineer an organism (in this case bacteria are being genetically engineered to produce human insulin)

Exam Tip

When answering questions about genetic engineering you should remember to include the names of any enzymes (restriction endonucleases, reverse transcriptase, ligase) involved and mention that markers (genes which can be identified) and vectors (transfer the desired gene) are also used.

Uses of Genetic Engineering

  • When a biotechnology company genetically modifies an organism in a specific way they will patent the genetic modification
    • A patent gives the owner the legal right to prevent others from replicating their invention for a limited time period

GM microorganisms

  • Recombinant proteins can be generated using microorganisms such as bacteria, yeast, or animal cells in culture 
    • They are used for research purposes and for treatments (eg. diabetes, cancer, infectious diseases, haemophilia)
  • Most recombinant human proteins are produced using eukaryotic cells (eg. yeast, or animal cells in culture) rather than using prokaryotic cells, as these cells will carry out the post-translational modification (due to the presence of Golgi Apparatus and/or enzymes) that is required to produce a suitable human protein
  • The advantages of genetic engineering microorganisms to produce recombinant human proteins are:
    • More cost-effective to produce large volumes (i.e. there is an unlimited availability)
    • Simpler (with regards to using prokaryotic cells)
    • Faster to produce many proteins
    • Reliable supply available
    • The proteins are engineered to be identical to human proteins or have modifications that are beneficial
    • It can solve the issue for people who have moral or ethical or religious concerns against using cow or pork produced proteins

The production of human insulin

  • In 1982, insulin was the first recombinant human protein to be approved for use in diabetes treatment
  • Bacteria plasmids are modified to include the human insulin gene
    • Restriction endonucleases are used to cut open plasmids and DNA ligase is used to splice the plasmid and human DNA together
  • These recombinant plasmids are then inserted into Escherichia coli by transformation (bath of calcium ions and then heat or electric shock)
  • Once the transgenic bacteria are identified (by the markers), they are isolated, purified and placed into fermenters that provide optimal conditions
  • The transgenic bacteria multiply by binary fission, and express the human protein – insulin, which is eventually extracted and purified
  • The advantages for scientists to use recombinant insulin are:
    • It is identical to human insulin unless modified to have different properties (eg. act faster, which is useful for taking immediately after a meal or to act more slowly)
    • There is a reliable supply available to meet demand (no need to depend on the availability of meat stock)
    • Fewer ethical, moral or religious concerns (proteins are not extracted from cows or pigs)
    • Fewer rejection problems or side effects or allergic reactions
    • Cheaper to produce in large volumes
    • That it is useful for people who have animal insulin tolerance

GM plants and animals

  • Although plants and animals have been genetically engineered to produce proteins used in medicine, the main purpose for genetically engineering them is to meet the global demand for food
  • The benefits of using genetic engineering rather than the more traditional selective breeding techniques to solve the global demand for food are:
    • Organisms with the desired characteristics are produced more quickly
    • All organisms will contain the desired characteristic (there is no chance that recessive allele may arise in the population)
    • The desired characteristic may come from a different species/kingdom

GM crops

  • Crop plants have been genetically modified to be:
    • Resistant to herbicides – increases productivity / yield
    • Resistant to pests – increases productivity / yield
    • Enriched in vitamins – increases the nutritional value
  • Scientists have genetically modified crops such as maize (to be resistant to insect attacks) and rice (to produce β-carotene to provide vitamin A)
  • GM crops could reduce the impact of farming on the environment due to there being less need to spray pesticides (eg. less beneficial insects being harmed)

Insect resistance in soya

  • Soya bean plants are susceptible to a number of insect pests that cause billions of dollars of damage every year
    • E.g. the fall army worm and soybean podworm
  • In response to these large losses in revenue, a biotechnology company has genetically modified the already herbicide-resistant variety of soybean (Roundup Ready™, RR1) by inserting a gene for the Bt toxin
    • This gene is taken from the bacterium Bacillus thuringiensis
  • Soya plants modified with the Bt toxin gene produce their own insecticide
    • When an insect ingests parts of the soya plant, the alkaline conditions in their guts activate the toxin (the toxin is harmless to vertebrates as their stomach is highly acidic), killing the insect
  • After 11 years of testing and development, the new variety of soybean (INTACTA RR2 PRO™) was introduced in Brazil in 2013
  • Note that insect populations have developed resistance to the genes for Bt toxin, reducing its effectiveness as a means of protecting crops

GM livestock

  • Some farmed animals have been genetically modified to grow faster
    • It is rarer for animals to be modified for food production due to ethical concerns associated with this practice
  • Scientists have also genetically modified livestock to produce pharmaceutical drugs in a process known as pharming
    •  These “biopharm” sheep and goats have been genetically modified to produce a number of useful human proteins in their milk
      • E.g. the human blood protein known as AAT in sheep milk
      • E.g. the human protein antithrombin (stops blood clotting) in goat milk

GM salmon

  • In 2015 AquaAdventure Salmon was approved by the US Food and Drug Authority (FDA) for human consumption
  • This salmon has been genetically modified (GM) to grow more rapidly than non-GM salmon as a result of growth hormone being produced in the salmon throughout the year, instead of just in spring and summer. The producer, therefore, has a product to sell in half the time, which increases their yield
  • Scientists combined a growth hormone gene from a chinook salmon with the promoter gene from an ocean pout, a cold-water fish. The ocean pout fish can grow in near-freezing waters, thus the promoter gene ensured the growth hormone was continually being expressed
  • To prevent the GM salmon from reproducing in the wild, all the salmon are female and sterile

GM pathogens

  • Many animal and plant pathogens have been studied using the techniques of genetic engineering
    • Pathogens can be modified to shed light on their metabolism, drug resistance as well as how it causes damage to its host
    • The development of effective vaccines and drugs can also be aided by this research
  • Adenoviruses can be genetically altered to act as vectors in gene therapy
    • These viruses are ideal vectors as they are not cell-specific or species-specific; they can infect the cells of many mammals
    • Specific genes are removed from the virus so that it can not replicate once inside host cells, creating space for the insertion of other desired genes

Tobacco mosaic virus (TMV)

  • The gene that codes for the hormone TMOF can be inserted into the cells of crop plants via a genetically modified tobacco mosaic virus
  • Modified TMV is sprayed onto the surface of the crops where it can invade the plant cells
  • The host plant cells transcribe the gene to produce the hormone TMOF
    • TMOF inhibits the production of the enzyme trypsin within insect pests
    • It has no negative effects on the host plant
  • The leaves of GM crops that have been exposed to the GM virus can be collected after harvest and ground into a powder to create a repellent spray against mosquitoes etc.

Arguments Against the Use of GMOs

  • Biotech companies charge farmers more money for GM seeds vs non-GM seeds to try and make back the money they have invested in their product
    • Seeds can not be kept from GM crops to regrow the crop the following year because GM crops do not “breed true”
    • Buying seeds year upon year can be a major struggle for farmers in developing countries
  • Many people object to the use of GMOs in food production due to a lack of long-term research on the effects on human health
    • It is unknown whether it will cause allergies or be toxic over time (although there has been no evidence to suggest this would occur to date)
  • Some state that without appropriate labelling the consumer cannot make an informed decision about the consumption of GM foods and so choices are being made for them
  • Organic farmers have complained that the pollen from GM crops may contaminate nearby non-GM crops that have been certified as organic
  • Environmentalists are concerned about the reduction in biodiversity for future generations
    • Crops with less genetic diversity are more vulnerable to extinction
    • GM crops may become weeds or invade the natural habitats bordering the farmland
  • Herbicide-resistance genes could transfer to weed plants resulting in “superweeds
  • GM crops that produce toxins may cause harm to non-target species like the Monarch butterflies)
  • The antibiotic-resistance genes that are commonly used as marker genes in genetic engineering could transfer to pathogenic organisms that would then be untreatable with antibiotics – “superbug
  • Tampering with viral genomes could result in a completely novel animal virus that can affect humans or cause existing ones to become more harmful to the host
    • This is only an issue if the pathogens are able to escape the lab and enter the wild
  • Over time mutations may occur in the inserted genes that cause them to have unwanted effects in organisms
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