Which of the following is not true of a codon?
A It extends from one end of a tRNA molecule.
B It may code for the same amino acid as another codon does:
C It consists of three nucleotides
D It never codes for more than one amino acid,
E It is the basic unit of the genetic code
The correct answer and explanation is:
The correct answer is A: It extends from one end of a tRNA molecule.
A codon is a sequence of three nucleotides found on mRNA that codes for a specific amino acid during protein synthesis. Codons do not extend from tRNA molecules, but rather are found on the mRNA. On the other hand, anticodons, which are complementary to codons, extend from one end of a tRNA molecule.
Here’s a breakdown of the other options:
- B. It may code for the same amino acid as another codon does: This statement is true because the genetic code is degenerate. This means that multiple codons can code for the same amino acid. For example, both UUU and UUC code for the amino acid phenylalanine.
- C. It consists of three nucleotides: This is true because a codon is always made up of three nucleotides, and this triplet of nucleotides is what specifies which amino acid is added during translation.
- D. It never codes for more than one amino acid: This is true because each codon specifies only one amino acid. The genetic code is read in triplets, and each codon is specific to one amino acid or a stop signal. There is no ambiguity in how codons are read during translation.
- E. It is the basic unit of the genetic code: This is correct because codons are the fundamental units that determine the sequence of amino acids in a protein. The sequence of codons in mRNA determines the sequence of amino acids in a polypeptide chain.
So, the only option that does not accurately describe a codon is A, as codons are part of the mRNA, not the tRNA.