RUTH WILLIAMSStop codons are not always genetic stop signs, according to a study published in Science today (May 22). A large proportion of bacteria and viruses have reassigned at least one stop codon—a signal marking the end of protein-coding sequences—to instead encode for an amino acid.
“This is the first real data point that looks at [stop codon reassignment] in an objective way to ask how common is it,” said Laura Landweber, a professor of ecology and evolutionary biology at Princeton University, who was not involved in the study.
Protein-coding DNA is written as a chain of trinucleotide words, or codons. There are 64 codons—the total possible combinations of the four DNA nucleotides, G, C, A, and T—and 61 of them encode amino acids. The remaining three—TGA, TAA, and TAG—are used at the end of protein-coding DNA sequences, and tell the cell’s translation machinery to stop adding amino acids.
This genetic code was believed to be universal to all life on the planet. That is, it was thought that TGA, TAA, ...
