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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 and 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 ↗