Читать книгу Welcome to the Genome - Michael Yudell - Страница 22

Nearly 50 years passed between the discovery of the double helix and the sequencing of the human genome. Some of the earliest techniques developed by scientists working on the problems of genetic heredity so closely resemble methods used by contemporary genome scientists that it may seem surprising that it took so long to complete the human gene sequence. But molecular biology was still in its infancy in the 1950s, and the technological advances necessary to sequence a whole genome would still take decades to come to fruition.

The first big step forward for sequencing technology took place at Cambridge University, England, in the mid‐1950s in the laboratory of biologist Frederick Sanger. Well before gene sequences, in the earliest stages of our understanding of how genes function, Sanger discovered how to take a protein, break it down into its component parts, and, piecing the puzzle back together, determine the order of amino acids along a protein. His ingenious approach to understanding the sequencing of proteins eventually won him his first of two Nobel Prizes and was the conceptual precursor to contemporary DNA sequencing. (7) An understanding of proteins was also important because of the role these complex molecules play in an organism. Proteins receive their instructions from genes to carry out such diverse tasks as food digestion, production of energy in a cell, transmission of impulses in the nervous system, and the ability to smell, see, and hear. If genes and DNA are the material that perpetuate heredity and help determine an organism’s form and function, then proteins are the cell’s workhorses, carrying out the varied instructions inscribed in an individual’s DNA. Proteins can also play a harmful role in an organism. Genetic defects can cause the absence or overabundance of a particular protein, which in both cases can cause devastating illnesses. For example, phenylketonuria, or PKU, is a metabolic disease caused by a genetic defect that leaves individuals without a protein that breaks down the amino acid phenylalanine. A buildup of phenylalanine causes severe mental retardation. Babies diagnosed with the disease as part of newborn screening programs can have their diets altered to keep levels of phenylalanine low and avoid PKU’s dreadful effects. (8)

The method developed by Sanger exploited the chemistry of amino acids and proteins that had been well known for over 10 years. Just as nucleotides are the building blocks of DNA, amino acids are the building blocks of proteins. Sanger himself wrote in the journal Science:

In 1943 the basic principles of protein chemistry were firmly established. It was known that all proteins were built up from amino acid residues bound together by peptide bonds to form long polypeptide chains. Twenty different amino acids are found in most mammalian proteins, and by analytical procedures it was possible to say with reasonable accuracy how many residues of each one was present in a given protein. (9)

Figure 2.1 This figure shows the way in which amino acids are the building blocks of proteins. In this case, we can see how a hemoglobin molecule is made up of a string of amino acids.

Credit: Wiley Publishers

Figure 2.2 Frederick Sanger played a critical role in the development of molecular biology and in the technologies that enabled the sequencing of the human genome.

Credit: https://commons.wikimedia.org/wiki/File:Frederick_Sanger2.jpg

Sanger’s challenge was to figure out a way to read the order of the amino acids that determine a protein. For his experiments Sanger chose to use bovine, or cow, insulin because of its important medical significance and its relatively short length—only 105 amino acids. Sanger set out to find ways to read the unwieldy molecule, which by his method could be deciphered only by breaking the protein apart, looking at small stretches of four or five amino acids, and then conceptually putting the molecule back together like a puzzle to determine the full sequence.

Sanger determined that the exposure of insulin to certain chemicals could break the peptide bonds in a protein chain. Sanger was able to identify the kinds of amino acids these broken‐down parts contained. He then created groups of small chains of amino acids that could be “tiled,” or pieced together, to give a full‐length sequence of a protein. (10)

Sanger was considered to be “reticent, even shy, a man who worked with his hands, at the laboratory bench.” (11) Yet he also recognized the impact that his work would have on science and medicine.

In his address to the Nobel committee in 1958 Sanger underlined the importance of understanding the chemical nature of proteins. “These studies are aimed,” he said, “at determining the exact chemical structure of the many proteins that go to make up living matter and hence understand how these proteins perform their specific functions on which the processes of life depend.” He also hoped that his work “may reveal changes that take place in disease, and that our efforts may be of more practical use to humanity.” (12) This connection between proteins, genes, and medicine, uncovered in part by Sanger and his techniques, is at the heart of what lies ahead in genomics. Fred Sanger died at the age of 95 in 2013. His legacy is immense including an institute in the UK named after him and two Nobel Prizes. He was, as he said about himself, “a chap who messed about in his lab,” but he was also a chap who really made a difference to humankind. (13)