Genetic editing has become a pretty hot topic in recent years as scientists understand more about what makes each one of us different. Every person, animal, and plant is made up of tens of thousands of genes that all work together to create the complex organisms we see. Understanding exactly how genes work could be key to some extraordinary future feats, from preventing cancer or eradicating pests that transmit diseases to producing allergy-free food or biofuels.
However, genes are extremely complex. Billions of base pairs make up the human genome, all arranged in a specific order to carry out their function – one wrong base pair, and you can end up with disastrous consequences. Single gene disorders are the most common genetic disorders, in which just one gene is mutated, creating the wrong type of protein, leading to disease. Common examples include cystic fibrosis, sickle-cell disease, and Huntington’s disease.
By mapping and understanding what each gene does within the human genome, we can identify key genetic regions that are linked directly to each disease. It is not always simple – complex diseases like heart disease may have tens or even hundreds of genes that play a pathogenic role. However, if we are certain that one mutation plays a key role in causing a genetic disorder, we could fix that mutation and the gene should function normally.
But how can we accomplish this? How can scientists find specific mutations among the billions of base pairs and specifically fix that mutation without changing any of the other bases? Well, that is where PCR and CRISPR-Cas9 come in, and these techniques are rapidly revolutionizing medicine.
To identify a gene within a DNA sequence and then produce enough of it to study in detail, scientists use Polymerase chain reaction (PCR). PCR is a technique that allows scientists to amplify specific DNA sequences to increase the amount of DNA present in a sample, and is the main technology currently used in Covid-19 testing. From a tiny concentration of DNA, scientists can produce millions to billions of sequence copies in a very short period. PCR involves multiple different stages. First, the DNA sample is heated until the double-stranded structure is split apart (denatured), giving many short pieces of DNA. Specific sequences, called primers, are then introduced that attach to the desired sequence, and a heat-specific enzyme called Taq polymerase extends this sequence. This is then repeated up to 50 times, producing a sample with huge quantities of the copied desired sequence.
PCR is one of the most important and relevant technologies utilized in genetics to date. Watch the video linked below for an introduction to the process of PCR and how it works.
Once the gene has been identified, scientists can get to work on fixing the mutation. CRISPR-Cas9 is a gene-editing technology that allows scientists to edit specific DNA sequences within a genome by acting as genetic “scissors”. The system requires two parts: CRISPR, which seeks out the desired DNA sequence to edit; and Cas9, which snips the precise section of DNA and alters the sequence to whatever the researchers wish. CRISPR is an RNA sequence, derived from bacteria and archaea, that is attached to the enzyme Cas9 and acts as a homing beacon, seeking out the area that the researchers want to edit. Once identified, Cas9 is deployed to split open the double helix, which triggers the body to repair the damaged DNA. Scientists can then hijack that self-repair mechanism to insert genetic material into the double-stranded break, resulting in direct genetic editing.
CRISPR is an important tool in modern medicine, but fully understanding how it works can be difficult. Luckily, LabXchange has created a simple simulation that allows you to discover exactly how CRISPR-Cas9 works to edit DNA.