mRNA is read in sets of three nucleotides in the reading frame designated by the first start codon read.
AUG is the start codon, so once that sequence is read the ribosome will begin to synthesize the polypeptide chain. Mitochondrial and prokaryotic genomes can have other start codons as well.
tRNAs (transfer RNA) have complementary sequences to the codons, called anti-codons. These sequences match up with the appropriate codon being read.
Each codon (three nucleotide RNA sequence) can only code for one amino acid, but multiple codons can code for an amino acid. For example, UCU always codes for serine, but UCC, UCA, UCG, AGU, and AGC also code for serine.
AUG, the start codon, always codes for methionine in eukaryotes and a modified methionine in prokaryotes.
There are three stop codons that signal the end of a protein and termination of translation. These codons are UAG, UGA, and UAA. These codons signal release factor binding which cause ribosomal sub-units to disassociate and release the amino acid.
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