The genetic code refers to the sequence of three nucleotides in RNA that determines the amino acid sequence of proteins. Evidence showed that changes in nucleotides led to changes in amino acids, leading to the proposal of a genetic code. The genetic code was deciphered by determining that codons are triplets of three bases that code for amino acids. Scientists like George Gamow, Har Gobind Khorana, and Marshall Nirenberg helped discover that there are 64 possible codons that code for 20 amino acids. The genetic code is nearly universal, with some exceptions, and has key features like being triplet-based, non-overlapping, and read sequentially from the 5' to 3' end of mRNA.