Definition
The genotype is the DNA sequence of an organism — the genetic description. The phenotype is the resulting organism: structure, behavior, physiology. The mapping from genotype to phenotype is enacted by the cellular machinery (ribosomes, polymerases, regulatory proteins), which is itself built by the same DNA. Hofstadter's Topic 6 uses this as a central example of a strange loop in biology.
Why it matters
How it works
DNA stores sequences encoding RNA and protein. Transcription copies DNA to mRNA. Translation reads mRNA codons and assembles amino acids into proteins via ribosomes. Proteins fold, catalyze reactions, and form structures — producing the phenotype. DNA polymerase copies DNA for the next generation. The whole machinery is itself built by proteins encoded in the DNA, so the DNA is both the message and (indirectly) the instructions for the messenger. This is the canonical strange loop in living systems.