Gene targeting and gene editing are both techniques for removing or modifying a particular gene, each of which can produce the same ultimate goal. What is the main technical difference in how DNA is modified that differs between these approaches?

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Understand that both gene targeting and gene editing aim to modify specific DNA sequences within the genome, but they use different molecular mechanisms to achieve this.

Gene targeting typically relies on homologous recombination, a natural cellular process where a piece of introduced DNA with sequences homologous to the target site replaces or modifies the endogenous gene through crossover events.

Gene editing, on the other hand, often uses engineered nucleases (such as CRISPR-Cas9, TALENs, or ZFNs) to create targeted double-strand breaks (DSBs) at specific genomic locations.

After the DSB is introduced in gene editing, the cell's repair machinery fixes the break either by non-homologous end joining (NHEJ), which can introduce insertions or deletions, or by homology-directed repair (HDR) if a repair template is provided, allowing precise modifications.

Therefore, the main technical difference lies in gene targeting using homologous recombination without necessarily inducing breaks, while gene editing actively induces targeted DNA breaks to stimulate repair pathways for modification.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Gene Targeting

Gene targeting is a technique that uses homologous recombination to introduce specific changes into a gene by replacing or disrupting it with a designed DNA sequence. This method relies on the cell's natural DNA repair mechanisms to incorporate the introduced DNA at a precise location.

Gene Editing

Gene editing involves directly cutting the DNA at a specific site using engineered nucleases like CRISPR-Cas9, TALENs, or ZFNs, followed by the cell's repair processes to introduce mutations or insertions. It allows precise and efficient modification without necessarily relying on homologous recombination.

DNA Repair Mechanisms

Cells repair DNA breaks primarily through homologous recombination (HR) or non-homologous end joining (NHEJ). Gene targeting depends on HR for precise DNA replacement, while gene editing often exploits NHEJ or HR after nuclease-induced double-strand breaks to achieve desired modifications.

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One complication of making a transgenic animal is that the transgene may integrate at random into the coding region, or the regulatory region, of an endogenous gene. What might be the consequences of such random integrations? How might this complicate genetic analysis of the transgene?

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When disrupting a mouse gene by knockout, why is it desirable to breed mice until offspring homozygous (−/−) for the knockout target gene are obtained?

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The CRISPR-Cas system has great potential but also raises many ethical issues about its potential applications because, theoretically, it can be used to edit any gene in the genome. What do you think are some of the concerns about the use of CRISPR-Cas on humans? Should CRISPR-Cas applications be limited for use on only certain human genes but not others? Explain your answers.

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What is the difference between a knockout animal and a transgenic animal?