How does a ribosome translate mRNA into a protein, and what is the start codon?

• A. Ribosome reads mRNA codons, tRNAs bring amino acids; start codon is AUG.
• B. DNA is transcribed into RNA by ribosome; start codon is TAA.
• C. Proteins are translated directly from mRNA without ribosomes; start codon is UAA.
• D. The ribosome uses rRNA to bind tRNA; start codon is ATG.

Answer: (A)

During translation, the ribosome reads the mRNA codons three nucleotides at a time. Each codon pairs with a complementary tRNA anticodon, bringing its specific amino acid. The ribosome then links these amino acids together, forming the growing protein. The start signal on the mRNA is AUG, which codes for methionine and establishes the reading frame for the entire message. In bacteria, the initiator tRNA carries formyl-methionine, but AUG is the universal start codon for beginning protein synthesis. Transcription, not translation, converts DNA into RNA, and DNA codons (like ATG or stop codons like TAA) are read as RNA (AUG or UAA) during translation; the start codon is the RNA AUG, not DNA ATG.