A Crack In Creation: A Nobel Prize Winner's Insight into the Future of Genetic Engineering

by Jennifer A. Doudna

First, a temporary copy of DNA, called messenger RNA (or mRNA), is made in the cell’s nucleus. Like a strand of DNA, the mRNA is a chain of letters, and its sequence matches the sequence of the DNA it copied (the only major exception being that T gets replaced by U). The mRNA is exported out of the cell’s nucleus and delivered to a protein-synthesizing factory called a ribosome, which translates the four-letter language of RNA (A, G, C, and U) into the twenty-letter language of proteins (the twenty amino acids). This translation proceeds according to the genetic code, a cipher in which every three-letter RNA combination, called a codon, instructs the ribosome to add one specific amino acid. (With sixty-four possible codons but only twenty amino acids, many codons code for the same amino acid, and three codons serve as stop signs to terminate protein synthesis.) The ribosome begins at one end of the mRNA and reads one consecutive codon after another, adding the corresponding amino acids to the growing protein chain until it reaches the other end of the mRNA; the process is much like building a car on an assembly line. A critical feature of this system is that the ribosome must remain in the correct three-letter reading frame; even the slightest hiccup can dramatically and calamitously affect the entire translation.