Gene Editing and CRISPR
You'll be able to explain how CRISPR-Cas9 works in simple terms.
In 2023, the US FDA approved Casgevy, the first CRISPR-based medicine for sickle cell disease. It edits blood stem cells to increase fetal haemoglobin; trial results show major clinical benefit, not a proven permanent cure.
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Q1 · What do you already know about gene editing? Have you heard of CRISPR?
Think about what you have read or seen about scientists editing genes in plants, animals or humans.
Q2 · If scientists could safely edit human embryos to prevent genetic diseases, should they be allowed to do so? Why or why not?
Consider both the potential to eliminate suffering and the ethical concerns about altering human DNA.
● Know
- That gene editing means making precise changes to DNA
- That CRISPR-Cas9 is a targeted gene-editing tool
- Some potential applications of gene editing in medicine and agriculture
- That gene editing has limitations and ethical concerns
● Understand
- How CRISPR-Cas9 allows scientists to target specific genes
- Why gene editing is more precise than older genetic technologies
- The difference between editing somatic cells and germline cells
● Can do
- Explain how CRISPR-Cas9 works in simple terms
- Evaluate one benefit and one limitation of a CRISPR application
- Compare gene editing to selective breeding and genetic modification
In laboratory research, scientists have used CRISPR-Cas9 to target and experimentally correct the sickle-causing variant in HBB, the gene for the beta-globin part of haemoglobin, in blood stem and progenitor cells. In this approach, a guide RNA directs Cas9 to the target DNA; a supplied donor template can guide the cell to copy a corrected sequence during repair. This direct HBB-repair strategy is experimental and is not the mechanism of the approved treatment Casgevy. Casgevy edits a regulatory region of BCL11A in a patient's blood stem cells, which increases fetal haemoglobin. A primary study of direct sickle-variant correction in patient-derived cells describes the research approach.
CRISPR-Cas9 was adapted from a bacterial immune system. A piece of guide RNA is designed to match a specific DNA sequence. This RNA attaches to the Cas9 protein and leads it to the target site in the genome, where Cas9 cuts both DNA strands. The cell's natural repair mechanisms then act; depending on the method and repair template, scientists can disrupt a target, correct a mutation or insert genetic material.
CRISPR can be programmed to target different DNA sequences, making it useful in research and medicine. Editing outcomes depend on the guide, the cell's repair processes and the editing method, and each proposed use needs evidence about its effectiveness and risks.
Inspect: Separate targeting from editing: what determines where Cas9 cuts, and what determines the final DNA sequence after the cut?
In 2023, the US FDA approved Casgevy, a CRISPR-based treatment for eligible people with sickle cell disease. Doctors collect a patient's blood stem cells, edit the erythroid-specific enhancer of BCL11A, then return the cells after conditioning treatment. The edit increases fetal haemoglobin (HbF), which helps red blood cells resist sickling; it does not correct the sickle variant in HBB. In the FDA's 2023 approval analysis, 29 of 31 evaluable participants had no protocol-defined severe vaso-occlusive crises for at least 12 consecutive months within the 24-month follow-up period. This was a time-limited trial outcome, not evidence that every patient was permanently cured. FDA approval analysis.
Australian research: Scientists at the Children's Medical Research Institute in Sydney use CRISPR to study and potentially treat muscular dystrophy. By correcting the genetic mutation in patient cells grown in the lab, they hope to develop therapies that could one day be delivered directly to muscles, sparing children from progressive muscle wasting.
Many students think gene editing is still science fiction or only happens in movies. In reality, CRISPR therapies are already in clinical trials and one has been approved for human use. Gene-edited crops are being grown, and gene drives are being tested to control mosquito-borne diseases. The technology is here now, and society must grapple with its implications in real time.
CRISPR gene editing was adapted from a bacterial immune system. Before reading on, predict: what do bacteria use CRISPR to defend against?
How close was your prediction?
Nice calibration, your intuition is good for this kind of problem.
Good, being surprised is the point. This answer is worth remembering.
Gene editing can be applied to two types of cells, with radically different consequences. Somatic editing changes the DNA of body cells, skin, blood, muscle or nerve cells. The changes help the treated individual but are not passed to their children. Germline editing changes the DNA of sperm, eggs or embryos. These changes are inherited by every descendant, permanently altering the human gene pool.
Most scientists and ethicists support somatic editing for serious diseases, provided safety and consent are assured. Germline editing is far more controversial. The changes are irreversible, the long-term effects are unknown, and the individuals affected cannot consent. Many countries have banned or restricted germline editing, though the technology to do it exists. The debate is not about whether we can edit embryos, we can, but whether we should.
In 2018, a Chinese scientist announced the birth of twin girls whose embryos had been edited using CRISPR to resist HIV infection. The experiment was widely condemned as unethical, illegal and scientifically premature. The edits may have caused unintended changes elsewhere in the genome, and the girls had no ability to consent to becoming the world's first gene-edited humans.
Australian policy: Australia prohibits heritable germline modification under the Prohibition of Human Cloning for Reproduction Act 2002. Somatic gene therapy is permitted under strict regulatory oversight by the Office of the Gene Technology Regulator and the Therapeutic Goods Administration. This framework aims to allow beneficial therapies while preventing reckless experiments.
Beyond medicine, CRISPR is transforming agriculture and conservation. Scientists are developing crops with higher yields, better nutrition and resistance to drought and pests by editing their native genes rather than inserting foreign DNA. These 'gene-edited' crops are regulated differently from transgenic GMOs in many countries because they do not contain DNA from other species.
In conservation, gene drives are being explored as a way to control invasive species or suppress disease vectors. A gene drive ensures that a particular gene is inherited by nearly all offspring rather than the usual 50%. In theory, a gene drive could spread infertility through a population of invasive mice or render mosquitoes unable to transmit malaria. The power is enormous, and so are the ecological risks if something goes wrong.
The proposal argued over most in this country is a gene drive against Aedes aegypti, the mosquito that carries dengue fever in north Queensland. The drive would spread a gene stopping females reproducing until the wild population collapses. The hard part is not whether it would work. It is that nobody can promise to undo it. An ordinary GM organism released and regretted can usually be wound back by not releasing more, because ordinary inheritance halves its share each generation and selection tends to clear it if it costs the organism anything, though that takes time and is not guaranteed. A gene drive is built to do the opposite: it pushes itself into nearly every offspring, so it keeps spreading without further releases. That does not make it unstoppable in principle. Mosquitoes can evolve resistance to the drive, a drive that leaves females badly weakened may be selected against, isolated populations may never be reached, and researchers are working on reversal drives designed to overwrite an earlier one. What none of those offers is a reliable way of calling the drive back once it is out. The property that makes a drive effective is the same property that makes the decision very hard to undo, and the honest scientific position is that reversal is uncertain rather than impossible.
Researchers in Australia are developing a gene drive to control invasive cane toads, which devastate native wildlife. The idea is to release gene-edited toads that produce only male offspring, causing the population to crash over time. Before any release, scientists must model the ecological consequences carefully, because an accidental spread beyond the target area could have unpredictable effects.
Australian biosecurity: CSIRO is investigating gene editing as a tool to protect Australian agriculture from pests and diseases. By editing the genes of crop plants to resist pathogens, or by developing genetic biocontrols for invasive species, they aim to reduce reliance on chemical pesticides while preserving Australia's unique ecosystems.
Australian regulatory oversight of gene technology is among the most rigorous in the world. The Office of the Gene Technology Regulator (OGTR), based in Canberra, assesses all genetically modified organisms before they can be released. CSIRO, Australia's national science agency, conducts extensive gene-editing research on crops and livestock but must obtain OGTR approval for any field trials. In 2019, Australian researchers at the University of Queensland used CRISPR to develop a rapid diagnostic test for COVID-19, demonstrating how the same technology can be applied outside of gene editing itself. The National Health and Medical Research Council (NHMRC) oversees ethical guidelines for human gene therapy research, ensuring that Australian scientists operate within internationally accepted boundaries.
Wrong: "CRISPR can perfectly edit any gene with no mistakes."
Right: CRISPR is precise but not perfect. Off-target effects can occur, so every edit must be carefully validated before clinical or environmental use.
Wrong: "Because CRISPR is guided by a matching sequence, it can only ever cut the gene you aimed at."
Right: CRISPR is highly precise, but off-target effects remain a challenge. Scientists must validate edits and consider unintended consequences.
Wrong: "Gene editing and genetic modification are exactly the same thing."
Right: Genetic modification typically involves inserting foreign DNA from another species. Gene editing makes precise changes within an organism's existing DNA without necessarily adding foreign genes.
Wrong: "Gene editing must be safer than genetic modification, because it does not add foreign DNA."
Right: Not adding foreign DNA removes one worry; it does not make an edit safe. Safety depends on what the edit actually does: whether Cas9 cut anywhere it was not aimed at, and whether changing that gene has effects on the organism, or on the ecosystem it is released into, that nobody predicted. Several countries regulate gene-edited crops more lightly than transgenic ones, but that is a rule about origin of DNA, not a finding that the product is harmless. Each application has to be tested on its own evidence.
Sickle Cell Disease and the First CRISPR Therapy
Sickle cell disease affects approximately 100,000 people in the United States and millions worldwide. It is commonly caused by a DNA variant in HBB that changes one amino acid in beta-globin.
In 2023, regulators in the UK and USA approved Casgevy (exagamglogene autotemcel), the first CRISPR-based medicine. The treatment edits an erythroid-specific enhancer of BCL11A in a patient's blood stem cells. Lower BCL11A activity in red-cell precursors increases fetal haemoglobin (HbF), which helps reduce sickling; Casgevy does not repair the sickle variant in HBB. In the FDA's 2023 approval analysis, 29 of 31 evaluable participants had no protocol-defined severe vaso-occlusive crises for at least 12 consecutive months during a 24-month follow-up period. This is a significant study outcome, but it does not establish a permanent cure for every patient. FDA approval analysis.
Evaluate CRISPR Applications
1 Scientists use CRISPR to edit blood stem cells from a patient with sickle cell disease, enabling them to produce healthy red blood cells. Identify one benefit and one limitation of this approach.
2 Australian researchers develop a gene-edited wheat variety that survives with 30% less water. Explain one benefit to Australian farmers and one potential concern about releasing this wheat into the environment.
3 A research team proposes using CRISPR to make Australian feral cats sterile, so their population declines over time. Identify one potential benefit and one significant risk of this approach.
Compare Genetic Technologies
1 Explain how selective breeding and CRISPR gene editing both aim to improve traits, but achieve this in fundamentally different ways.
2 Genetic modification (GM) often involves inserting a gene from one species into another. Gene editing can make precise changes without adding foreign DNA. Explain why some people view gene editing as more acceptable than GM, even though both alter an organism's DNA.
3 A scientist claims that because gene editing is more precise than selective breeding, it should replace traditional breeding methods entirely. Evaluate this claim, considering at least one advantage of selective breeding that CRISPR cannot easily replicate.
Copy Into Your Book
▼Gene Editing Basics
- Gene editing = precise changes to DNA
- CRISPR-Cas9 = guide molecule + cutting enzyme
- Guide RNA finds the target sequence
- Cas9 cuts DNA; cell repairs it
Applications
- Disease treatment: sickle cell, cancers
- Agriculture: drought-resistant crops
- Conservation: gene drives for pest control
- Australia: CSIRO wheat and barley research
Limitations
- Off-target effects: cutting wrong DNA
- Ethical concerns with human embryos
- Ecological risks of gene drives
- Regulatory oversight required
Comparing Technologies
- Selective breeding = choosing parents
- GM = inserting foreign DNA
- Gene editing = precise internal changes
- Germline editing affects future generations
At the start of this lesson you heard that scientists at Colossal Biosciences are using CRISPR to edit thylacine genes into living marsupial cells, trying to bring back an animal that has been extinct since 1936. That example was chosen to show you that CRISPR can do something that seemed like pure science fiction just a decade ago.
Now that you understand how CRISPR-Cas9 finds a target sequence, cuts the DNA and allows edits to be made, explain how that thylacine project could work in principle. What does knowing the mechanism make you think differently about, the possibility, the risks, or the ethics?
Q1. Describe how CRISPR-Cas9 works in simple terms. What makes it different from older genetic technologies? 4 MARKS
Q2. Choose one application of CRISPR (disease treatment, agriculture, or conservation). Explain the potential benefit and one limitation. 4 MARKS
Q3. Compare CRISPR to selective breeding and genetic modification. How are they similar and how do they differ? 4 MARKS
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you predict that gene editing could treat diseases by fixing faulty DNA?
- Has your view on whether scientists should edit human embryos changed after learning about germline editing?
- Write one sentence explaining why CRISPR is described as a "molecular scissors" tool.
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Evaluate CRISPR Applications
1. Sickle cell therapy: Benefit, a CRISPR treatment can greatly reduce severe pain crises for some patients during the period studied [1 mark]. Limitation, treatment requires specialised medical facilities and is not accessible to everyone who could benefit [1 mark].
2. Drought-resistant wheat: Benefit, Australian farmers could maintain yields during droughts, improving food security and farm income [1 mark]. Environmental concern, the edited wheat might cross-breed with wild relatives or alter soil microbiome interactions in unpredictable ways [1 mark].
3. Sterile feral cats: Benefit, could reduce the devastating impact of feral cats on Australia's native wildlife, which kill billions of animals annually [1 mark]. Risk, the gene drive could spread to non-target species or have cascading ecological effects that are difficult to predict or reverse [1 mark].
Activity 2, Compare Genetic Technologies
1. Selective breeding chooses parents with desirable traits and relies on natural genetic recombination during reproduction [1 mark]. CRISPR directly alters DNA sequences in a laboratory, bypassing generations of breeding and achieving precise, predictable changes [1 mark].
2. Gene editing makes changes within an organism's own DNA without necessarily adding foreign genetic material [1 mark]. This means the resulting organism might have a genetic profile that could theoretically arise through natural mutation, which some people find more acceptable than combining DNA from different species [1 mark].
3. The claim is overstated [1 mark]. Selective breeding has safely improved crops and livestock for thousands of years without laboratory intervention, and it often improves multiple traits simultaneously through natural polygenic variation [1 mark]. CRISPR is powerful for single-gene changes but does not easily replicate the complex, coordinated improvements that selective breeding can achieve across many genes [1 mark]. Both tools have roles in modern agriculture.
Multiple Choice
1. B CRISPR-Cas9 is a tool for making precise changes to DNA. It is not an antibiotic, sequencing method or cloning technique.
2. C CRISPR is being used to treat genetic diseases such as sickle cell disease. For example, Casgevy edits a regulatory region to increase fetal haemoglobin; it does not correct the sickle variant in HBB.
3. A An off-target effect occurs when CRISPR cuts DNA at an unintended location, potentially disrupting another gene.
4. D Editing embryos (germline editing) is controversial because the genetic changes would be inherited by all descendants.
5. B Releasing gene-edited mosquitoes could have unexpected effects on food webs, predator-prey relationships and ecosystem balance.
Short Answer Model Answers
Q1 (4 marks): CRISPR-Cas9 uses a guide RNA molecule to locate a specific DNA sequence [1 mark]. The Cas9 enzyme then cuts the DNA at that precise location [1 mark]. The cell's natural repair machinery fixes the break, and scientists can direct this repair to make a desired change [1 mark]. Unlike older technologies such as selective breeding (which is slow and indirect) or genetic modification (which inserts foreign DNA), CRISPR can make precise, targeted changes directly at a specific gene [1 mark].
Q2 (4 marks): Example answer, disease treatment: Casgevy edits a regulatory region of BCL11A in a patient's blood stem cells, increasing fetal haemoglobin rather than correcting the sickle variant in HBB [1 mark]. A potential benefit is fewer severe pain crises for some patients during the period studied [1 mark]. One limitation is that the treatment requires specialised medical facilities and is not accessible to everyone who could benefit [1 mark]. Additionally, treatment and gene editing have risks that need careful clinical monitoring [1 mark].
Q3 (4 marks): All three technologies aim to improve the traits of organisms [1 mark]. Selective breeding achieves this by choosing which organisms reproduce, without directly altering DNA [1 mark]. Genetic modification inserts genes from other species into an organism's genome [1 mark]. CRISPR gene editing makes precise changes within an organism's existing DNA, allowing specific genes to be altered, removed or corrected without necessarily adding foreign genetic material [1 mark].