Question

Why is ATG a start codon?

Answer 1

Hint: During protein synthesis, a codon is a three-nucleotide sequence of DNA or RNA that corresponds to a specific amino acid or stop signal. Proteins are written in a 20-amino-acid language, whereas DNA and RNA molecules are written in a four-nucleotide language.
The first codon that a ribosome will translate is the start codon of a messenger RNA (mRNA) transcript.

Complete answer:
In most cases, protein translation begins with a Methionine codon (even though the codon can be seen elsewhere in the protein sequence too). In eukaryotic DNA, the sequence is ATG, but in RNA, it is AUG.
The first AUG in the mRNA is normally where translation begins, followed by an open reading frame, which is a sequence of three nucleotides that codes for amino acids in a protein. Only when the protein-coding region is complete do stop codons appear.
In eukaryotes and Archaea, the start codon always codes for methionine, while in bacteria, mitochondria, and plastids, it codes for N-formylmethionine (fMet). The most widely utilised start codon is AUG (i.e., ATG in the corresponding DNA sequence).
"AUG" is the first codon in practically all reading frames because it codes for methionine and acts as the start codon. Despite differences in the encoded amino acid, practically all cells use the same start codon.

Note:
Alternative start codons differ from the normal AUG codon in both prokaryotes (bacteria and archaea) and eukaryotes. When alternate start codons arrive at the beginning of a protein, the translation is still completed (even if the codon encodes a different amino acid otherwise). This is the case due to the usage of a separate transfer RNA (tRNA) during initiation.