AP BIOLOGY

Detailed explanation-1: -The standard start codon for translation of a gene is “AUG, ” which encodes the amino acid methionine (Met or M), and establishes the reading frame for the ribosome to follow, adding corresponding amino acids in a polypeptide chain.

Detailed explanation-2: -The start codon sets the reading frame: instead of continuing to move down the mRNA transcript one base at a time, the ribosome now reads the mRNA codons consecutively, three bases at a time (Fig. 3.18). The sequence of the triplet codon determines which amino acid is added next to the growing protein.

Detailed explanation-3: -A nucleotide sequence that contains a start codon (initiation codon, typically AUG) and a stop codon (termination codons, UAA, UAG or UGA) is called an open reading frame (ORF). An ORF is hence a portion of an organism’s genome which contains a sequence of bases that could potentially encode an individual protein.

Detailed explanation-4: -AUG is the most commonly used start codon (i.e., ATG in the corresponding DNA sequence). Because it codes for methionine and serves as the start codon, “AUG” is the first codon in almost all reading frames. Regardless of the encoded amino acid, nearly all cells employ the same start codon.