Final answer:
The true statement about a codon is D. Both B and C, as codons include start codons like AUG and stop codons such as UAA, UAG, and UGA. Codons are part of the universal genetic code, specifying the incorporation of amino acids into polypeptides during translation or signaling its termination.
Step-by-step explanation:
The question "Which of the following is true of a codon?" relates to the role of codons in the synthesis of proteins during the process of translation. A codon is a sequence of three nucleotides on messenger RNA (mRNA) that corresponds to a specific amino acid or termination signal during protein synthesis. The correct answer to the question is D. Both B and C because codons include both start codons such as AUG, which signify the beginning of translation and codes for methionine, and stop codons like UAA, UAG, and UGA, which signal the end of translation, releasing the newly formed polypeptide chain from the translation machinery.
The genetic code is both unambiguous and redundant. Unambiguous means that a specific codon will code for only one amino acid or a start or stop function, ensuring precision in protein synthesis. Redundancy means that multiple codons can code for the same amino acid, which provides a buffer against certain types of mutations; if one nucleotide of a codon is changed, it might still encode the same amino acid. However, one codon does not encode for more than one amino acid. In the context of redundancy, glycine is coded for by four different codons, which all share the same first two nucleotides but differ in the third nucleotide, showcasing the 'wobble' position of codons, where variation typically occurs without altering the amino acid that is produced.
Moreover, codons are universal across different organisms; for example, the codons for tyrosine in plants are the same as those in humans. This universality underscores the commonality of life's genetic language. When a stop codon is reached, translation ends and the protein is released. If the stop codon is mutated to code for another amino acid, the translation process could continue, leading to longer proteins with possibly harmful effects.