Jacob's Ladder by Henry Gee

Author:Henry Gee
Language: eng
Format: epub
Publisher: W. W. Norton & Company

9

Operon

The first inklings that the interactions between genes were at least as important as genes themselves in framing the form and disposition of living things came as long ago as 1908, when an English physician named Archibald Garrod (1857-1936) noticed something curious about a collection of rare diseases in humans. The diseases were curious because they did not seem to be the result of infection or contagion. They did seem to occur more often in some families than in others, and this tendency was most marked when families were inbred. At a time when Mendelian genetics was still quite new, Garrod applied the lessons of Mendel’s peas to humans, describing the inheritance of recessive traits – in this case, a tendency to contract certain rare diseases, which Garrod called ‘inborn errors of metabolism’.1

One of these diseases is now known as alkaptonuria. Its symptoms include mottling of the skin and a progressive arthritis, and it afflicts approximately one person in every 200,000. Another disease in the same ‘family’ is phenylketonuria. This disease, which afflicts infants and young children, has a range of symptoms ranging from eczema to tremors and mental retardation. Fortunately it is easily treatable, by strict adherence in childhood to a diet that does not contain the amino acid phenylalanine. One in 10,000 babies is born with phenylketonuria, making it one of the commoner inherited diseases. This frequency – and the need for sufferers to keep to a strict diet – explains otherwise mysterious slogans on jars and packets in supermarkets proclaiming that the contents ‘contain a source of phenylalanine’. Garrod’s insight was that these seemingly disparate diseases were the result of defects in inheritance, and that they were somehow connected. We now know that they are caused by inherited deficiencies in certain substances called enzymes.

Enzymes are examples of catalysts – substances that facilitate chemical reactions which might otherwise not occur at all. This constraint applies to virtually every active change within the body, from the growth of your hair and nails to the very first cell division of the embryo and all subsequent cell divisions; from the breakdown of food in your intestines to the exhalation of carbon dioxide in your breath. Enzymes are what builds your body and what breaks it down again. Without enzymes, the strands of DNA cannot be copied, nor can their cargo of genetic information be read or acted upon. But of what manner of substance are these everyday miracles that make life possible, these commonplace fixers of the marvellous? Enzymes – like the cells and bodies whose lives they control – are proteins, made of chains of amino acids. And herein lies a conundrum. If enzymes are part of the body and are responsible for its creation and function, then what makes the enzymes? This is where genetic information enters the story.

We now know that alkaptonuria is the result of a single genetic mutation, a misprint in the DNA. Normally, this piece of DNA – this gene – contains the instructions needed to make an enzyme called homogentisate 1,2-dioxygenase.

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