What is a codon?

• A. A three-nucleotide sequence in mRNA that specifies an amino acid or stop signal.
• B. A promoter sequence where RNA polymerase binds.
• C. The linear type of ribonucleic acid that encodes genetic information from DNA and conveys it to ribosomes.
• D. A type of RNA that interprets translation and has anticodons.

Answer: (A)

A codon is a three-nucleotide sequence in mRNA that specifies an amino acid or a stop signal. During translation, the ribosome reads mRNA three bases at a time from the 5' end, and each codon pairs with a complementary anticodon on a transfer RNA that brings the corresponding amino acid. This is how genetic information is translated into a protein. The start codon, usually AUG, marks where translation begins and also codes for methionine, while the stop codons (UAA, UAG, UGA) signal termination. The genetic code is redundant, meaning several codons can encode the same amino acid, but a codon never codes for more than one amino acid. Codons exist in mRNA, not in DNA directly, and the other descriptions refer to different molecules or roles: promoter sequences for transcription, the mRNA molecule that carries information, and tRNA that reads codons with anticodons.