Imagine a world where cancer isn’t deadly. Where diseases like sickle cell anemia, muscular dystrophy, and Huntington’s disease are all things of the past.
Now imagine a world where an elite group are smarter, stronger, and more physically attractive than the rest. Governments have armies which can feel no pain. Children are designed by their parents.
Both of these futures have become possible with CRISPR (an acronym for clustered regularly interspaced short palindromic repeats; more on that in a bit), a revolutionary gene editing technology which has enabled researchers to easily and cheaply edit the DNA of any living organism. But… what is CRISPR? And what is gene editing?
Meet the Gene
To understand CRISPR and gene editing, it’s important to understand what a gene is. A gene is a segment of deoxyribose nucleic acid (DNA) that most frequently codes for proteins, which are the workers of your cell.
Together, all your genes make up your genome, and your genome is what makes you, well, you. Genes determine your height, your hair color, and even your blood type. However, in some cases the different versions of genes can cause harm.
Many diseases, such as sickle cell anemia, are caused by mutations in a gene. For those diseases, there has never been a cure; it’s ingrained in all of the person’s cells. However, gene editing provides a solution.
Now that we know what the gene is, gene editing is straightforward; it’s modifying your genes. The DNA that makes up your genes contains four bases, and by changing the order and amount of those bases through gene editing, you can change your genes.
Gene editing has been around longer than you would think; the first genetically modified organism, a bacterium, was generated in 1973. But for many years, it wasn’t easy; previous methods, like zinc finger nucleases and transcription activator-like effector nucleases, could take years to develop.
Then CRISPR was modified as a gene editing tool, and everything changed.
CRISPR in Bacteria
Before CRISPR was a groundbreaking gene editing tool, it was part of archaea and bacteria’s immune system.
In these organisms’ DNA, there is a stretch which contains a repeating code that is interspaced with other, unique segments of DNA. (This is where the full name of CRISPR, clustered regularly interspaced short palindromic repeats, comes from).
So what does this have to do with the bacteria’s immune system? The unique segments of DNA are taken from bacteriophages (a virus but for bacteria) that the organism has been infected by before.
This CRISPR section of DNA is first transcribed into RNA. RNA (ribonucleic acid) is similar to DNA, but DNA is double stranded and RNA is single stranded. They share three of the same bases, but then have different fourth ones. In transcription, the DNA is read base by base, and a complementary RNA strand is created (which would match the other strand of DNA).
After transcription, the CRISPR RNA forms a complex with a type of protein called CRISPR-associated protein (or cas for short) and another type of RNA called tracrRNA, one complex for each new section of virus DNA. If the same type of virus infects the bacterium a second time, the DNA it injects matches the CRISPR RNA in the complex, and the cas protein cuts it, preventing it from being transcribed.
CRISPR in Gene Editing
Now for the fun part. A cas protein, commonly cas9, can be combined with a programmed guideRNA (a simplified combination of CRISPR DNA and tracrRNA) to cut DNA at a selected spot.
After that, a researcher has two options.
Leave it to the Cell
After cutting the DNA, researchers can leave the cell on its own for repairing the break. This means the repair is probably going to be really messy.
It doesn’t sound like a great idea, but when you want to just silence a gene the process works, because afterwards it’s highly likely the gene won’t be functional.
If you want to change a gene, you can provide a DNA template to the cell. The template is like instructions for the cell; it tells the cell which bases to use and in what order to repair the DNA.
Pros and Cons of CRISPR
Sounds great, doesn’t it? And it is; CRISPR has created a gene editing revolution, making gene editing easy and quick.
But there are drawbacks. CRISPR can cause edits in genes which weren’t supposed to be edited, which could potentially cause cells to become cancerous.
Most importantly, the ease with which CRISPR can be used raises many ethical concerns, especially in the case of genetic changes which could be inherited by future generations.
Remember the two worlds back in the beginning? Well, it’s up to us to make wise decisions to maximize CRISPR’s positive impact.
- Gene editing is the practice of modifying an organism’s genes, which are what determine the characteristics of the organism.
- CRISPR is part of a bacterial immune system, but was modified to become a gene editing tool.
- CRISPR cuts the DNA at a selected spot, and then researchers can silence the gene by leaving the cell to try and fix it, or control the new gene by providing a template.
- CRISPR raises a lot of ethical questions, and it’s up to us to make wise decisions.