Final answer:
Three letters or nucleotides of an RNA molecule form a codon, which codes for a single amino acid during protein synthesis.
Step-by-step explanation:
It takes three 'letters' of an RNA molecule, in sequence, to provide the code for a single amino acid. This three-letter code is known as a codon. Each codon corresponds to a specific amino acid or a stop signal in the process of protein synthesis. The genetic information is encoded in the DNA and transcribed into messenger RNA (mRNA), which then translates this code into amino acids with the help of transfer RNA (tRNA) during the process of translation in the ribosome.