(d) Quaternary structure

15.7 Proteins have several levels of structural organization.

The Genetic Code

In 1953, Watson and Crick solved the structure of DNA and identified the base sequence as the carrier of genetic information. However, the way in which the base sequence of DNA specified the amino acid sequences of proteins (the genetic code) was not immediately obvious and remained elusive for another 10 years.

One of the first questions about the genetic code to be addressed was: How many nucleotides are necessary to specify a single amino acid? This basic unit of the genetic code—the set of bases that encode a single amino acid—is a codon (Chapter 14). Many early investigators recognized that codons must contain a minimum of three nucleotides. Each nucleotide position in mRNA can be occupied by one of four bases: A, G, C, or U. If a codon consisted of a single nucleotide, only four different codons (A, G, C, and U) would be possible, which is not enough to code for the 20 different amino acids commonly found in proteins. If codons were made up of two nucleotides each (i.e., GU, AC, etc.) there would be 4 X 4 = 16 possible codons—still not enough to code for all 20 amino acids. With three nucleotides per codon, there are 4 X 4 X 4 = 64 possible codons, which is more than enough to specify 20 different amino acids. Therefore, a triplet code requiring three nucleotides per codon is the most efficient way to encode all 20 amino acids. Using mutations in bacteriophage, Francis Crick and his colleagues confirmed in 1961 that the genetic code is indeed a triplet code.

Concepts 9

The genetic code is a triplet code, in which three nucleotides code for each amino acid in a protein. An electronic table of codons and the amino acids they specify

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