Milestones in the rules of life--from genes to proteins
The National Science Foundation (NSF) plans to support convergence of research across multiple disciplines including molecular biology, computer science, engineering and behavioral science to conduct research into how life functions. This bold initiative could, for example, enhance the ability to predict the observable characteristics of a living creature from genetic and environmental information. "Understanding the Rules of Life," is one of NSF’s 10 Big Ideas that were unveiled in 2016. It is the next frontier in the life sciences, the conquest of which will provide new knowledge about plants, animals and humans that could help predict disease risk, improve agriculture and advance national health.
The DNA to protein riddle
The quest to understand the fundamental rules of life started when James Watson and Francis Crick, aided by an X-ray diffraction image from Rosalind Franklin, described the double helix of deoxyribonucleic acid (DNA) in 1953 and declared that DNA carried the genetic code. The long double strands of DNA consisted of nucleotide bases and, it was proposed, that three pairs of bases on the double helix were a code (codon) for an amino acid. Amino acids are the building blocks that are linked together by peptide bonds to form peptides and the much longer and more complex proteins in all living things.
Caption: François Jacob and Jacques Monod with André Lwoff (1965).
Image: Jacob (in white lab coat), Monod and Lwoff (in white lab coat), recipients of the 1965 Nobel Prize in Physiology or medicine, standing and chatting in a science laboratory in the Pasteur Institute.
Credit: Institut Pasteur/Archives Agnès Ullmann
Our structural features including skin, hair and muscles are made up of proteins. Our internal organs and tissues such as the heart and liver, as well as the enzymes, hormones and receptors that carry out our bodies’ physiological functions, are all proteins or peptides. Basically, peptides and proteins make us who we are as living creatures and as individuals. So, after understanding the role of DNA in the rules of life, the next question was: How did DNA direct a living cell to assemble the requisite amino acids and synthesize a specific peptide or protein?In the late 1950s and early 1960s, NSF catalyzed the convergence of research in biology, biochemistry, organic chemistry and physics to uncover the discrete steps involved in peptide and protein synthesis. This contributed to the birth of molecular biology as a scientific discipline and earned the Nobel Prize for four researchers whose work was directly supported by NSF.
The protein synthesis machinery
By 1955, scientists had figured out that proteins were synthesized by specialized structures in the cell called ribosomes. Ribosomes were made up of ribonucleic acids (RNA) and did not contain DNA. Therefore, it was unlikely that they used DNA directly as templates to make proteins.
Caption: Steps in the rules of life from DNA through mRNA to protein.
Image: Diagrammatic representation of Monod’s discovery of the transcription of the genetic code from DNA to messenger RNA, Jacob’s finding that each transcribed code is translated in the ribosome as a specific amino acid, Holley’s description of transfer-RNA and its role in transporting specific amino acids to the ribosome and, the formation of a peptide bond between a growing peptide and a specified amino acid. It also shows the cracking of the genetic code by Nirenberg and synthesis of DNA and RNA by Khorana. Red octagons identify Monod, Jacob, Holley and Khorana as NSF-supported researchers who were awarded the Nobel Prize in Physiology or Medicine.
Credit: Paul Lartey, AAAS-STPF/NSF
So, in 1961, Jacques Monod of the Pasteur Institute and his colleague François Jacob, whose research had been supported by NSF since 1959, proposed the existence of a transient intermediary or "messenger" that carried information from DNA to the ribosome for "translation" into protein. They later demonstrated that the message coding for amino acids that made up the structures of peptides and proteins was first transcribed from DNA onto an RNA template they called messenger RNA (mRNA). The message was subsequently carried by mRNA to ribosomes and translated to make the specified proteins.
Monod and Jacob were awarded the 1965 Nobel Prize in Physiology or Medicine jointly with André Lwoff, whose work was on bacterial viruses "for their discoveries concerning genetic control of enzyme and virus synthesis."
Cracking the genetic code
A couple of key questions remained: What did a message look like and how were amino acids assembled to make a protein? These questions triggered a flurry of research activity in a race to "crack the genetic code."
Caption: Robert Holley (L) and a team of Agricultural Research Service and Cornell University researchers (Ca. 1964-1968).
Image: Photograph of Holley (in white shirt) with five other researchers (in white lab coats) in a laboratory, holding and examining a long paper tape representing a strand of ribonucleic acid.
Credit: Sol Goldberg
The big breakthrough came when Marshall Nirenberg of the National Institutes of Health conducted an experiment using a synthetic mRNA strand composed entirely of the nucleic acid base uracil (U). He used the strand to direct peptide synthesis in a cell extract and demonstrated that the newly produced peptide was made up entirely of the amino acid phenylalanine. This observation, coupled with the hypothesis that the genetic code was a series of three DNA bases, led to the deduction that the mRNA codon for phenylalanine was UUU. In this manner, Nirenberg systematically deciphered the genetic codes for all 20 amino acids found in peptides and proteins.
Meanwhile, the research of former NSF postdoctoral fellow Robert Holley at Cornell University had revealed that extracts from bakers’ yeast or rat liver cells contained 20 distinct but related RNAs that could be catalyzed by specific enzymes to react with amino acids. Holley postulated that the reaction activated amino acids to enable them to form peptide bonds. He proposed that these special RNAs "transfer" specific amino acids to their codons on the mRNA template and, hence, described them as transfer-RNA (tRNA). By the end of 1964, with NSF support, Holley had isolated and determined the chemical structure of the specific t-RNA whose function was to deliver the amino acid alanine to the ribosome when coded for by mRNA.
Putting it all together
Gobind Khorana of the University of Wisconsin pulled all of this together by focusing his NSF-supported research on the chemical synthesis of DNA and RNA.
Khorana synthesized several DNA templates to confirm that codons indeed consisted of three DNA base pairs. He demonstrated that cell extracts could use synthetic DNA to make DNA-like double helices and produce peptides using synthetic mRNA as templates. Khorana later synthesized the gene for yeast alanyl tRNA, which transfers alanine to the ribosome. This was the first total synthesis of a gene originally found in nature. In 1968, Khorana shared the Nobel Prize in Physiology or Medicine with Holley and Nirenberg "for their interpretation of the genetic code and its function in protein synthesis."
Thus, in the short dizzying period in science between 1953 and 1964, when the disciplines converged, the fundamental life processes of DNA replication, its transcription to mRNA and tRNA, and translation of mRNA to protein were unveiled.
Caption: Gobind Khorana in his laboratory (Ca. 1950-1960).
Image: Photograph of Khorana in a white lab coat, standing behind a laboratory bench and working with a chromatography column.
Credit: University of Wisconsin-Madison Collection
The next major milestone in "Understanding the Rules of Life" could similarly be delivered in short order as NSF again supports the convergence of powerful cutting-edge disciplines to that end.