IB Biology Sub-topic D1.3 Notes

Gene knockout

Now that you are aware of mutations and how to lead to genetic diseases and cancers, you can appreciate that considerable research has gone into medical genetics to solve these problems.

However, this first requires scientists to determine which genes perform which functions. This is done by gene knockout, which is a form of genetic engineering used to remove or inactivate one or more specific genes from an organism. Gene knockout can appear as two types:

1. Complete - the gene is permanently deactivated.
2. Conditional - the gene can be turned on and off by command. 

The ability to perform gene knockouts is useful because a library of organisms with gene knockouts can be produced. These are then used as models in research to:

1. Understand the impact of the gene on the organism's development and life.
2. Discover the pathways of metabolic processes and the proteins involved.
3. Develop treatments for genetic conditions that are otherwise untreatable.

You are expected to understand that gene knockout is typically performed via one of three methods:

1. Homologous recombination - forced crossing over with an inactivated gene during meiosis, introducing that gene to the organism's genome.
2. TALENs - nucleases that binds to a DNA domain and cleaves it to introduce a frameshift mutation and inactivate the gene.
3. CRISPR-Cas9 - the Cas9 enzyme cleaves DNA to introduce various mutations for gene inactivation. 

CRISPR-Cas9

Whilst you are not expected to know details of the first two methods, you need to know more about CRISPR-Cas9. It performs DNA insertion, deletion, or alteration to edit genomes via deliberate mutations. To accomplish this, two molecules are used:

1. A guide RNA (gRNA) targets a specific DNA sequence in a gene and delivers Cas9.
2. Cas9 cleaves the two DNA strands at the desired spot or location to add or remove DNA.
3. It then allows the cell to repair DNA by itself, introducing a mutation in the process.

CRISPR Cas9 offers hope in the development of treatments for conditions that have a genetic component, such as certain cancers and hypercholesterolaemia and in certain viruses such as hepatitis B.

However, editing the genes of germline cells of humans raises questions of ethics. Changes made to such cells will be inherited by future generations, and can be potentially dangerous. As a result, it is currently an illegal practice in most countries.