Nirenberg and Matthaei Experiment - Background

Background

Oswald Avery discovered that the substance responsible for producing inheritable change in the disease-causing bacteria was neither a protein nor a lipid, rather deoxyribonucleic acid (DNA). He and his colleagues Colin MacLeod and Maclyn McCarty suggested that DNA was responsible for transferring genetic information. Later, Erwin Chargaff discovered that the makeup of DNA differs from one species to another. These experiments helped pave the way for the discovery of the structure of DNA. In 1953, with the help of Maurice Wilkins and Rosalind Franklin’s X-ray crystallography, James Watson and Francis Crick proposed DNA is structured as a double helix.

In the 1960s, one main DNA mystery scientists needed to figure out was in transcription how many bases would be in each code word, or codon. Scientists knew there was a total of four bases (guanine, cytosine, adenine, and thymine). They also knew that were 20 known amino acids. George Gamow suggested that the genetic code was made of three nucleotides per amino acid. He reasoned that because there are 20 amino acids and only four bases, the coding units could not be single (4 combinations) or pairs (only 16 combinations). Rather, he thought triplets (64 possible combinations) were the coding unit of the genetic code. However, he proposed that the triplets were overlapping and non-degenerate.

Seymour Benzer in the late 1950s had developed an assay using phage mutations which provided the first detailed linearly structured map of a genetic region. Crick felt he could use mutagenesis and genetic recombination phage to further delineate the nature of the genetic code. In the Crick, Brenner et al. experiment, using these phages, the triplet nature of the genetic code was confirmed. They used frameshift mutations and a process called reversions, to add and delete various numbers of nucleotides. When a nucleotide triplet was added or deleted to the DNA sequence the encoded protein was minimally affected. Thus, they concluded that the genetic code is a triplet code because it did not cause a frameshift in the reading frame. They correctly concluded that the code is degenerate (triplets are not overlapping) and that each nucleotide sequence is read from a specific starting point.

Marshall Nirenberg and Johann Matthaei both longed to understand how information gets transmitted from DNA to protein. At this time there was a race to crack the code of the DNA language. At the same time, Severo Ochoa was busy working on the coding problem with the help of Leon Heppel, a skillful biochemist capable of making artificial RNAs of defined compositions. Ochoa had a big staff, and Nirenberg was worried he would not be able to keep up. Many NIH scientists helped Nirenberg in deciphering the mRNA codons for amino acids.Nirenberg and his post doctoral fellow Matthaei started their experiments in a lab in Germany and completed them in a National Institutes of Health (NIH) laboratory campus in Maryland.