What are codons and how many amino acids do they encode?

Codons are three-nucleotide sequences in mRNA that specify particular amino acids during translation. With 4 nucleotides possible at each position, there are 64 possible codons (4^3). These encode 20 standard amino acids plus 3 stop signals (UAA, UAG, UGA), making the genetic code degenerate (redundant) - multiple codons can specify the same amino acid. AUG serves as both the start codon and codes for methionine. The code is nearly universal across all organisms with minor variations in mitochondria and some organisms.