Free Radicals - Michael Brooks [78]
Large spots were produced when a pigmentation gene turned on early in the cell division process, giving time for the colourful mutation to be repeated. Small spots came from a mutation appearing late in the growth cycle of the kernel. And the relative prevalence of large, small and medium-sized spots made it very clear that this was not a random process. Something was controlling the mutations – but McClintock knew that this was biological heresy. It took three years of further breeding, observation and examination of the cells under the microscope before she eventually found a reason to believe that she wasn’t fooling herself.
Years of studying the speckling of kernels had led McClintock to study the variegation, or stripiness, of the plant’s leaves. In 1946 she had noticed that there was a pattern to variations in the variegation: if there were ten stripes per centimetre on average, she would often find that a region of the leaf that was two stripes above the average sat next to a region that was two stripes below average. To McClintock, it looked as if one cell had gained what an adjacent cell had lost. Somewhere along the line from gene to cell to leaf, something had been passed from one thing to another – and in a tightly controlled way. The agent, she eventually found, was in the chromosomes.
The science writer Matt Ridley has an interesting way to describe chromosomes. Chromosomes are made of DNA, and the fundamental units of DNA are four molecules known as bases – adenine, cytosine, guanine and thymine – which we represent by the letters A, T, C and G. These letters can be strung together to make words that form paragraphs that will eventually form a chapter. That chapter, Ridley suggests, is the chromosome. Put all the chromosome chapters together, and you have a book that perfectly describes the instructions for making an organism.
You and I are described by twenty-three pairs of chromosomes in our cells, one of each pair coming from each parent. Maize cells contain ten pairs of chromosomes. McClintock’s heresy took place on chromosome nine. She found that entire paragraphs were moving around within this chapter. She called the process transposition, and suggested that this is what causes the variegation in the leaves and the speckle in the kernels. The analogy is not strictly correct, but it is as if the ‘reader’ of the genome instruction manual was occasionally coming across an instruction to turn on a purple pigmentation just before the paragraph that told it to stick with the default yellow, whatever it might read later.
Even today, biologists don’t fully understand transposition, but by 1951 McClintock was able to publish a stab at what was going on inside the chromosome. Parts of the genome would move around in a co-ordinated fashion, she said, vastly increasing its repertoire of products. These ‘controlling elements’ were not mere genes; they were more like managers put in charge of the genes. The rigid recipe book of the genome had turned out to be more than a list of ingredients: it was a complete kitchen, equipped with creative chefs, ingredients and utensils. And the controlling elements would manage the kitchen, determining how all this potential was to be put to work.
To McClintock, the seemingly endless variation in the biological world suddenly made a lot more sense. Biologists had struggled to understand how, when cells in the organism had exactly the same set of genes, some would turn into muscle while others created arteries or produced a filament of hair. McClintock had an answer: through the action of the controlling