IB Biology HLTopic 4 — Genetics, Inheritance & ChangePaper 1 & 2Nature of science~10 min read
Gene Editing
Knowing the sequence of the human genome was only the start. The harder question was what each gene actually does — and the neatest way to find out is to switch one off and see what breaks. From there it is a short step to fixing genes rather than just studying them.
📘 What you need to know
The genome is the entire set of genetic material — all the DNA — found inside a cell.
The Human Genome Project, completed in 2003, determined the DNA sequence of the whole human genome.
Gene knockout removes a gene or makes it unusable, so scientists can study what changes. The organism is a knockout organism, often a laboratory mouse.
Gene editing inserts, deletes or replaces DNA at specific sites. It modifies existing DNA, unlike genetic engineering, which inserts DNA from another organism.
CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) uses a guide RNA to locate a sequence and the enzyme Cas9 to cut it.
Gene therapy is the treatment of a genetic disease by altering a person’s genotype.
Gene editing raises ethical issues around consent, insurance and legal control of genetic data, overseen by ethics committees and international bodies.
From sequence to function
The Human Genome Project was an international, collaborative effort to read the entire human DNA sequence and record every human gene. Finishing it in 2003 was a huge breakthrough, but it left an obvious gap: knowing the letters does not tell you what any of them do.
That is what the work since has been about — finding out what each gene codes for and how it affects an organism, so that diseases can be treated and prevented more effectively.
Gene knockout: learn by removing
One reliable way to work out what a gene does is to take it away. Gene knockout means removing a gene from the genome or making it unusable, and then looking at what changes in the organism. It is classed as a genetic engineering technique.
The organism produced is called a knockout organism. Laboratory mice are the most common.
Whole libraries of knockout organisms exist — for example for the fungus Saccharomyces cerevisiae — and are used to understand how a drug works and to target specific biological processes or deficiencies.
Conditions studied this way include obesity, diabetes, cancer likelihood, addiction and cardiovascular disease.
This is a controlled experiment: one variable changed, everything else kept constant. The same logic you use in any practical.
You are not expected to know how a knockout organism is actually made. What you do need is the reasoning: remove one gene, compare with a control, and the difference tells you what that gene was contributing.
Gene editing and how it differs from genetic engineering
Gene editing lets scientists insert, delete or replace DNA at specific sites in a genome — usually sites known to cause disease. The important distinction:
The difference in one line
gene editing = changing the DNA already there • genetic engineering = adding DNA from another organism
The older techniques, and why they were limited
Modified viruses were used to carry DNA into cells, for example into a gene causing a disease. The problem was control: the DNA sometimes ended up inserted into other genes, with unforeseen consequences.
Liposomes — small spheres of lipid molecules — containing a normal copy of a gene were sprayed into the nose. This only ever worked short-term, because the epithelial cells lining the nasal passageway are short-lived and are replaced.
Both worked, up to a point. Neither was precise, and that lack of precision is exactly what CRISPR solved.
CRISPR
CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. The technique borrows a natural defence mechanism that bacteria (and some archaea) evolved to cut up the DNA of invading viruses.
It has two working parts:
a guide RNA, whose sequence determines exactly where the cut is made — scientists design their own to match the target;
the enzyme Cas9, attached to the guide RNA, which cuts the DNA strands at that point.
Once the DNA has been cut, faulty sequences can be inserted, deleted or replaced with normal DNA.
Think of the guide RNA as the postcode and Cas9 as the pair of scissors. Change the postcode and the same scissors will cut somewhere else entirely.
Gene therapy. Gene editing is the tool; gene therapy is one use of it — treating a genetic disease by altering the person’s genotype. Treatments for cystic fibrosis and sickle cell anaemia are being developed this way.
Progress depends on more than the editing tool itself. As researchers learn more about the human genome and the proteome, and use computational biology to handle enormous data sets, they can work out which genes cause which diseases, where those genes sit, and exactly which bases need changing.
Nature of science: the ethics of CRISPR
Some potential uses of gene editing raise ethical questions that have to be settled before the technology is used, not afterwards.
Consent — who agrees to genetic data being collected and used, especially when that data also reveals things about relatives?
Insurance — companies may want access to genetic test results, which could affect what people are charged or offered.
Legal control — laws are needed over how genetic data is used, particularly data on human genomes.
Decisions on these questions tend to be made on a worldwide scale rather than country by country:
Ethics committees must approve experiments, taking advice from world-leading experts.
Individual countries have laws protecting participants in genetic technology research.
International committees, such as the International Commission on the Clinical Use of Human Germline Genome Editing, debate the issues and make recommendations to governments and scientists.
The World Health Organisation issues guidance and best-practice guidelines.
The ongoing challenge is getting all policymakers and countries to coordinate their regulations, so the rules apply consistently to every gene editing process, CRISPR included.
In an ethics question, do not just say “it is wrong” or “it is good”. Give the specific issue — consent, insurance, regulation — and say who is responsible for handling it. That is what separates a top answer from an opinion.
Worked examples
WORKED EXAMPLE
Interpreting a knockout experiment
Mice with gene X knocked out gain far more body mass than normal mice on the same diet. State what this suggests about gene X, and give one limitation of the conclusion.
Step 1: Identify the variable that changed
Only gene X differs between the two groups; diet is controlled.
Step 2: Draw the conclusion
Gene X is involved in controlling body mass, and its normal product must limit mass gain.
Step 3: Give a limitation
It shows what the gene affects, not the mechanism, and results in mice may not apply directly to humans.
Gene X helps regulate body mass; the mechanism is still unknown“Suggests” is doing work in the question — match it in your answer.
WORKED EXAMPLE
Naming the parts of CRISPR
State the two components of the CRISPR system used in gene editing and give the role of each.
Component 1Guide RNA — its base sequence determines which DNA sequence is targeted, so it decides where the cut happens.
Component 2Cas9 enzyme — attached to the guide RNA, it cuts the DNA strands at that point.
What follows
Once cut, bases can be inserted, deleted or replaced.
Guide RNA locates; Cas9 cutsTwo components, two roles — this is a classic two-mark question.
WORKED EXAMPLE
Editing versus engineering
A scientist replaces three faulty bases in a patient’s own gene. Another inserts a bacterial gene into a crop plant. Identify which is gene editing and justify your answer.
Step 1: Check the source of the DNA
First case: the DNA changed is already in the organism.
Second case: DNA comes from another organism.
Step 2: Apply the definitions
Modifying existing DNA is gene editing; inserting foreign DNA is genetic engineering.
The first is gene editing; the second is genetic engineeringThe give-away word is “own” or “existing”. Look for it in the stem.
💡 Exam tip
Define genome precisely: all of the DNA, or genetic information, inside a cell.
Give the Human Genome Project date, 2003, if the question asks about it.
For knockout, explain the logic: disable one gene, compare with a control, and infer the function from the difference.
Learn the one-line distinction between gene editing and genetic engineering. It is asked frequently.
Give both CRISPR components with their roles: guide RNA locates, Cas9 cuts.
For ethics, name a specific issue and a specific body (ethics committee, WHO) rather than writing vaguely about “morals”.
⚠ Common mix-up
Treating gene editing and genetic engineering as the same thing. Editing modifies DNA already present; engineering inserts DNA from elsewhere.
Saying Cas9 chooses the target. The guide RNA does. Cas9 only cuts.
Thinking a knockout tells you the mechanism. It tells you what the gene affects, which is not the same thing.
Saying the Human Genome Project told us what every gene does. It gave the sequence, not the functions.
Describing gene therapy as curing all genetic disease. Treatments are still being developed and are not routine cures.
Confusing liposomes with viruses. Liposomes are lipid spheres; viruses were modified to carry DNA.
Up next: Conserved Sequences — if mutations happen everywhere at random, why are some stretches of DNA almost identical in humans, yeast and bacteria? The answer says a lot about what those sequences do.
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