# Codon --- A **codon** is a group of three [[nucleotide|nucleotides]] on a DNA or RNA strand that represent a single [[amino acids]] in order to make [[protein|proteins]]. By convention, the codons are written in the 5' -> 3' direction ![[codon chart - RNA.png]] ## Redundancy With two exceptions (methionine and tryptophan), all amino acids have multiple "synonymous" codes. In these cases, it is usually only the third base that differs. Conversely, each codon only makes exactly one amino acid. (The exception of course is for for the stop/start codons, which do not translate to amino acids). ### The Evolutionary Advantage of Redundancy Codon redundancy can provide an **[[evolution|evolutionary]] advantage** for organisms that live in different environments. For example, GC base pairs fare better in cold environments than TA pairs on account of their three bonds. For this reason, organisms that are adapted to colder environments evolve to have more GC pairs as they are more resistant to cold than TA pairs. This way, even though two baceria might have much different DNA, but produce the same exact protein. ## Reading Frame The **reading frame** refers to the particular sequence of neucleotides that make up the codon. Altering the frame by one (or two), such as after a [[point mutation#Framshift Mutation|frameshift mutation]] results in genetic gibberish. ## Special Codons The **start codon** is the AUG codon. This is always the first codon of an [[mRNA]] and codes for the methionene [[amino acid.png|amino acid]]. The methionene can be latered swapped out if need be. The **stop codons** are the UAA, UAG and UGA codons. The **anticodon** is a triplet of nucleotides located on a [[tRNA]] strand that forms complemantary pairs with the codon that is located on an [[mRNA]] strand. (Side note: As both codons and anti codons are read in the 5' -> 3' direction, and as they are antiparallel, technically they should be read in reverse of each other. But for simplicity let's not instead.) ## Why *This* Code? It just sort of happened this way. There are examples of DNA (like mitochondrial DNA) that use a different code. ___