La réponse courte
DNA is the master copy and stays safely in the nucleus. When a cell needs a protein it makes a working copy of the relevant gene in RNA, sends that copy out to be read, and then destroys it. RNA is the photocopy you write on and throw away — which is why treatments that target RNA are temporary, and why CRISPR uses an RNA guide.
Ribonucleic acid is single-stranded, uses ribose rather than deoxyribose, and substitutes uracil for thymine. Messenger RNA carries coding sequence from nucleus to ribosome; transfer and ribosomal RNAs perform translation; and many non-coding RNAs regulate expression. In CRISPR, the guide RNA provides the sequence specificity — the system is RNA-programmed, which is exactly why it is easy to reprogram.
Why CRISPR uses RNA to aim
Earlier editing tools — zinc fingers, TALENs — recognised DNA using protein, so aiming at a new target meant engineering a new protein: months of work. CRISPR aims using RNA, and RNA can be ordered from a supplier overnight for a few dollars. That single difference is why CRISPR spread through biology in about a year while its predecessors never left specialist laboratories.
Sources
- National Human Genome Research Institute
Talking Glossary of Genomic Terms ↗ - MedlinePlus Genetics, U.S. National Library of Medicine
Help Me Understand Genetics ↗