# Gene editing

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u19-gene-editing/

This sub-topic covers the mechanism, steps and applications of the CRISPR-Cas9 gene editing system, the most widely used modern gene editing tool, and compares it to earlier genetic modification techniques for CIE A-Level Biology.

**Prerequisites:** Basic understanding of DNA structure and function; [Recombinant DNA technology](https://www.owlsprep.com/study/cie-9700-u19-recombinant-dna/)

## Learning objectives

- Describe the mechanism of the CRISPR-Cas9 gene editing system
- Distinguish between the two main DNA repair pathways after Cas9 cutting
- Outline key applications of gene editing in agriculture and medicine
- Compare gene editing to traditional genetic modification techniques

## Mechanism of CRISPR-Cas9

**CRISPR-Cas9** — A naturally occurring adaptive immune system in bacteria and archaea that has been repurposed as a precise gene editing tool for eukaryotic genomes

*Example:* Used to edit the CCR5 gene in human cells to confer HIV resistance

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) are short repeated DNA sequences in bacterial genomes that originate from fragments of DNA from previous viral infections. When the same virus re-infects the bacteria, the CRISPR system transcribes guide RNA (gRNA) that recognizes the matching viral DNA sequence.

The Cas9 enzyme is a DNA nuclease that cuts DNA at the target sequence complementary to the gRNA. A short sequence called the PAM (Protospacer Adjacent Motif) must be present next to the target sequence for Cas9 to bind and cut.

**Worked example:** Describe how CRISPR-Cas9 targets a specific 20 base pair sequence in the human genome to create a double-strand break.

1. A synthetic guide RNA (gRNA) is designed to be complementary to the 20 base pair target sequence, which is located adjacent to the PAM sequence (5'-NGG-3' for most commonly used Cas9).
2. The gRNA binds to the Cas9 enzyme to form an active Cas9-gRNA complex.
3. The complex diffuses through the nucleus and base pairs with the complementary target DNA sequence.
4. Cas9 confirms the presence of the PAM sequence, then creates a double-strand break 3 base pairs upstream of the PAM sequence.

## DNA Repair Pathways and Editing Outcomes

After Cas9 creates a double-strand break at the target site, the cell repairs the break using one of two main pathways, which produce very different editing outcomes.

**Non-Homologous End Joining (NHEJ)** — An error-prone DNA repair pathway that ligates the two broken DNA ends together without a repair template

*Example:* Most commonly introduces small insertions or deletions (indels) that shift the reading frame of a gene, knocking out its function

The second pathway, homology-directed repair (HDR), only occurs during the S and G2 phases of the cell cycle, when a homologous template is available. If a synthetic donor DNA template with the desired edit is provided, HDR will use it to make a precise edit.

**Worked example:** A researcher wants to insert a specific single nucleotide mutation into a target gene. Which repair pathway should they use, and what steps are required?

1. The researcher uses HDR, because NHEJ is error-prone and produces random mutations, not the specific desired edit.
2. Alongside Cas9 and gRNA that cut the target site, the researcher adds a synthetic donor DNA template. The template contains the desired single nucleotide mutation, flanked by sequences homologous to the regions on either side of the break.
3. The cell's endogenous HDR machinery uses the homologous donor template to repair the break, incorporating the desired mutation into the genome.
4. Edited cells are then selected and validated to confirm the correct mutation is present.

## Applications of Gene Editing

Gene editing has a wide range of current and potential applications in agriculture, medicine, and basic biological research.

- **Agriculture**: Editing crop genomes to improve yield, drought resistance, pest resistance, or nutritional content (e.g. non-browning mushrooms, higher-yield oilseed rape).
- **Medicine**: Editing somatic human cells to treat genetic disorders (e.g. editing hematopoietic stem cells to treat sickle cell anemia, editing T cells for cancer immunotherapy).
- **Research**: Creating knockout cell lines or animal models to study the function of unknown genes.

**Worked example:** Outline how gene editing is used to produce hornless (polled) cattle, to avoid the need for painful dehorning.

1. Some breeds of cattle naturally carry a dominant allele for the polled (hornless) trait.
2. Guide RNA is designed to target the polled locus in zygotes from horned cattle.
3. Cas9 cuts the target locus, and a donor DNA template containing the natural polled allele is introduced to the zygote.
4. HDR incorporates the polled allele into the genome, producing edited cattle that are born hornless and do not require dehorning.

## Gene Editing vs Traditional Genetic Modification

CIE examiners regularly ask candidates to distinguish between modern gene editing and older traditional genetic modification (GM) techniques, so it is important to remember the key differences.

**Comparing methods**

Key differences between the two approaches are outlined below:

- **Traditional Genetic Modification** — Inserts an entire functional foreign gene (e.g. the Bt toxin gene from *Bacillus thuringiensis*) into the host genome at a random location
  - Pros: Well-established, easy to implement for most organisms
  - Cons: Random insertion can disrupt native genes, permanent insertion of foreign DNA

- **Gene Editing** — Makes targeted, precise changes to the host's own existing endogenous genes; most edits do not involve insertion of foreign DNA
  - Pros: Low risk of insertional mutagenesis, predictable changes to the genome
  - Cons: More technically challenging, rare off-target cuts can still occur

**Exam command terms**

Common exam command terms for this topic have specific marking expectations:

- **Compare** — You must give at least one similarity and one difference *(Similarity: both change the genome of the organism; difference: targeted vs random insertion)*

- **Describe** — You must outline all key steps of the process *(For CRISPR-Cas9, you must mention gRNA, Cas9 cutting, and the two repair pathways)*

## Common pitfalls

- **Wrong:** Stating that Cas9 cuts the guide RNA instead of the target genomic DNA
  - Why it fails: Confuses the role of gRNA as a guide, rather than a substrate for cutting
  - Correct: Cas9 is a nuclease that cuts the target genomic DNA at the sequence complementary to the guide RNA
- **Wrong:** Claiming that all gene editing inserts foreign DNA into the genome
  - Why it fails: Confuses gene editing with traditional GM, where foreign DNA is always added
  - Correct: Many gene edits are small knockouts made via NHEJ, and do not involve insertion of any foreign DNA
- **Wrong:** Mixing up NHEJ and HDR, stating NHEJ is the precise repair pathway
  - Why it fails: Common memory error that costs easy marks in exams
  - Correct: Homology-directed repair (HDR) is the precise pathway that uses a donor template; NHEJ is error-prone and produces random mutations
- **Wrong:** Forgetting that CRISPR is originally a bacterial immune system
  - Why it fails: CIE regularly asks for the origin of the CRISPR system, and this mark is often missed
  - Correct: CRISPR-Cas9 is a naturally occurring adaptive immune system found in bacteria and archaea

## Cheatsheet

| Component/Process | Role | Key Outcome |
| --- | --- | --- |
| Guide RNA | Guides Cas9 to target sequence | Targets correct cut site |
| Cas9 | Nuclease enzyme | Creates double-strand break |
| PAM sequence | Required Cas9 binding site | Confirms correct target |
| NHEJ repair | Repairs break without template | Random indels, gene knockout |
| HDR repair | Repairs break with donor template | Precise targeted edits |
| Traditional GM | Inserts gene at random | Adds new function, insertion risk |
| Gene Editing | Targets existing DNA | Precise changes, low insertion risk |

## What's next

Gene editing is a core topic in the genetic technology unit of CIE A-Level Biology, and is regularly tested in extended response questions on Paper 2. It connects to broader concepts including genetic variation, the ethics of biotechnology, and agricultural improvements that support global food security. Understanding how gene editing works is also key for evaluating the benefits and risks of modern genetic technologies, a common exam question theme. Building on this knowledge allows you to answer compare and contrast questions between different genetic engineering techniques, and evaluate ethical arguments for and against its use in different contexts.

- [Biotechnology applications](https://www.owlsprep.com/study/cie-9700-u19-biotechnology-applications/)

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