Molecular Scissors: CRISPR Cas9 Explained

If a bacterium is attacked by a bacteriophage (virus), then it must defend itself. If the bacterium saves snippets of viral DNA to recognize and cut that virus in the future, then the system functions as an adaptive immune response.

If an enzyme is a nuclease, then its job is to break the phosphodiester bonds of a nucleic acid. If Cas9 is a programmable nuclease, then it acts as the "scissors" that create a double-strand break in the DNA.

If the Cas9 protein needs to find one specific sequence among billions of bases, then it requires a map. If a short RNA sequence is complementary to the target DNA, then it serves as the "guide" RNA (gRNA) to lead the protein to the right spot.

If researchers found repetitive DNA sequences and Cas proteins in bacteria and archaea before using them in labs, then the system is naturally occurring in prokaryotes rather than being a human invention from scratch.

If the Cas9 protein needs to distinguish between "self" and "non-self" DNA to avoid cutting the bacterium's own genome, then it looks for a specific motif. If that motif (like NGG) is only found in the viral target, then it is the required "PAM" sequence.

If the bacterial genome contains clusters of identical DNA repeats separated by unique "spacer" sequences, then the acronym CRISPR describes that specific genomic architecture.

If the cell's repair machinery tries to jam two broken DNA ends back together without a template, then it often makes mistakes. If these "indels" shift the reading frame, then the gene is broken or "knocked out."

If the system needs to function, then it requires the cutter (Cas9) and the two-part RNA guide (crRNA and tracrRNA) that binds to it. DNA polymerase and ribosomes are involved in other cellular processes like replication and translation.

If simplifyng the system makes it easier to deliver into cells, then scientists will link the two necessary RNA parts. If they are linked into one strand, then it is termed a single-guide RNA.

If older tools required designing a brand-new protein for every new DNA target, then they were slow. If CRISPR only requires changing a 20-base RNA sequence to target a new gene, then it is vastly more efficient and easier to use.

If the CRISPR array consists of repeats and non-repeats, then the non-repeats must be the viral signatures. Since these signatures "space out" the repeats, they are logically named spacers.

If the guide RNA sequence accidentally matches a different part of the genome with similar bases, then the Cas9 protein will bind and cut there. If this unintended cut occurs, then it is defined as off-target activity.

If CRISPR can change specific DNA sequences, then it can fix mutations or improve traits in organisms. While it can treat blood disorders, fight cancer, and help agriculture, it cannot physically transform humans into mechanical robots.

If a mutation is corrected in the reproductive cells using crispr cas9, then every cell in the resulting offspring will have that correction. If that offspring has children, then they will also carry the edited gene, making it a permanent change in the lineage.

If the bacterium has already acquired the virus's DNA and made guide RNAs, then it is ready for the final step. If the gRNA-Cas9 complex finds a matching virus and cuts it, then it is performing the interference phase.

If the Nobel committee wanted to recognize the transformation of a bacterial immune system into a universal gene-editing tool, then they awarded the scientists who first proved it could work in a test tube.

If we can edit human embryos, then there are concerns about picking traits or creating permanent genetic shifts. If the technology is expensive, then it may only benefit wealthy groups, creating social gaps; however, DNA cannot turn into gold.

If CRISPR is a defining feature of prokaryotes (bacteria and archaea), then it is not found naturally in eukaryotes (plants, animals, fungi). If humans use it in eukaryotic cells, then they must artificially introduce the components.

If the goal is to make a gene non-functional so its absence can be observed, then the process is defined as knocking out the gene. This is often the first step in basic crispr cas9 research.

If different Cas enzymes are specialized for different tasks, then their targets will vary. If Cas9 and Cas12 cut double-stranded DNA, but Cas13 is specifically designed to recognize and cleave single-stranded RNA, then that is their primary difference.