17: The Genetic Code
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17.1 Deciphering the Genetic Code: tRNA as Adaptor
17.2 The Rules of the Code
17.3 Cracking the Code
17.4 Exceptions Proving the Rules
The origin of life has long been an interest of mine, particularly the evolution of nucleic acids and the reason that ribose was selected as the sugar used in RNA. In the 1950s, Stanley Miller and others showed that ribose can be produced abiotically (without enzymes), but Miller and others had noted that ribose is not very stable. This is because, in the lab, ribose and other five-
Although we weren’t actually studying this particular problem, a comment from a colleague about “solving the ribose problem” coincided with a trip I took to Death Valley to collect rocks. While there, I began musing about the borate-
When I returned to the lab, it only took about a day and a half to show experimentally that ribose could be made stably at high pH in the presence of borate. Because borate is abundant in nature, it seems likely that it stabilized the prebiotic production of ribose, providing a simple and logical explanation for the presence of ribose on the early Earth. It was satisfying to make this discovery, but also humbling to realize that borate-
—Steve Benner, on discovering that borate minerals stabilize ribose
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The discoveries that DNA is composed of complementary strands and that it holds the instructions for all the proteins in an organism were huge advances in our understanding of the flow of biological information. However, proteins and nucleic acids are very different types of chemicals, and after the structure of DNA was solved, how the sequence in a chain of nucleotides determines the sequence of amino acids in a protein was not immediately apparent. The next 10 years brought several discoveries that revealed the fascinating processes by which DNA is decoded to produce proteins.
The linear nucleotide sequence of mRNA is translated into protein by tRNA molecules that carry amino acids and contain nucleotide sequences (called anticodons) that pair with complementary sequences (codons) in the mRNA. Different amino acid–
Amino acids and nucleotide bases have no obvious chemical relationship, and therefore it is not at all obvious how given amino acids became matched to particular trinucleotide sequences. Yet all organisms—
This chapter presents an overview of the genetic code and how it works. We first look at how the tRNA molecule functions in decoding, and how it is exquisitely designed to take advantage of the “degeneracy” of codons, enabling one tRNA to decipher more than one codon. We also examine how the genetic code can resist the harmful effects of single-