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ATG (adenine-thymine-guanine) or AUG (adenine-uracil-guanine) is a codon, a sequence of three nucleotides in DNA or RNA that codes for a specific amino acid during protein synthesis. Specifically, the codon ATG (or AUG in RNA) is the start codon, which signals the beginning of protein translation and initiates the formation of the protein chain.

During protein synthesis, the genetic information stored in DNA is transcribed into RNA, then translated into protein. The start codon ATG (or AUG) is recognized by the ribosome, a molecular machine synthesizing proteins and initiating the protein chain assembly.

In addition to its role as a start codon, ATG can also serve as a regulatory element in gene expression. In some cases, an ATG codon in the 5' untranslated region (UTR) of an mRNA can promote translation of the downstream coding sequence. Alternatively, an upstream open reading frame (uORF), initiated by an ATG codon, can interfere with translating the downstream main ORF (open reading frame).

It is important to note that the genetic code is highly conserved across all living organisms, meaning that the same codons code for the same amino acids in all organisms. For example, the codon ATG (or AUG in RNA) always codes for the amino acid methionine, regardless of the organism.

ATG (or AUG) is also frequently used as a molecular marker in molecular biology experiments. For example, introducing a mutation that changes the start codon ATG to another codon can result in the loss of protein expression. This can be used to study the function of specific proteins or to develop new therapies for genetic diseases caused by mutations in the start codon.

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