any good scientist, Leeuwenhoek realised that he had to compare his observations with a sample taken from a healthy subject. In 1677 he reported his findings in a letter to the president of the Royal Society of London – one of the world’s leading research institutes – in which he gave a detailed description of the animals he’d observed in the healthy sample, and wrote that it was examined ‘immediately after ejaculation, before six pulse strokes had passed’. Afterwards, he was keen to emphasise that the sample was, of course, not obtained in any sinful way, but ‘made available’ to him ‘quite naturally, through marital activity’. (It couldn’t have been easy to be his wife.) At the end of the letter, Leeuwenhoek requested strongly that the president keep their correspondence to himself, should he feel that the observations risked causing disgust among the scholars. A scandal was the last thing he wanted.
Leeuwenhoek was convinced that semen played a decisive role in the beginning of life. This was no clear, empty fluid – it was packed with swarms of microscopic creatures! Could this be the place that miniature humans existed? Surely he needed only a good enough microscope to reveal it. For years Leeuwenhoek worked persistently, but despite continually improving his lenses, he found nothing. He even attempted to remove the membrane around the sperm cell’s head with a tiny brush in the hope of finding something hidden inside. Despite the absence of proof, Leeuwenhoek was certain that the sperm cell contained a great secret – albeit one so small that humankind would never be able to see it.
THE RECIPE FOR A HUMAN
THE FIRST FEW hours. The race is over, and your very first cell floats calmly down the fallopian tube. So much has already been decided. Even though the cell is tinier than the full stop at the end of this sentence, it is large enough to contain all the instructions needed to build you, from the organs that will keep you alive to your eye colour and the shape of your nose.
Leeuwenhoek never managed to discover it, but the cell’s great secret was not a miniature person. It was a molecule. And the story of this molecule begins with pus.
In 1869 a Swiss chemist named Friedrich Miescher contacted a surgical clinic near his laboratory. ‘Would you let me have some used bandages from your patients?’ he asked. ‘Preferably covered in as much pus as possible.’ Miescher was looking for white blood cells, something there were masses of in the whitish-yellow gloop that oozes from sores. White blood cells are the remnants of a battlefield – they work for the immune defences – and many of them give their lives in the war against bacteria that is fought in wounds. Miescher collected the pus, filtered out the cells and conducted a thorough chemical analysis to examine the types of proteins they contained. During one experiment he noticed a sticky, milk-white substance that separated from the rest of the mixture when he added acid. Upon further examination, Miescher realised that it couldn’t be a protein. He called the new substance nuclein, since it sat at the cell’s core, the nucleus. Miescher found that there was an unusually large amount of nuclein in sperm cells, and realised that this substance must play a crucial role at the start of life.
Back then, genetics was still a mystery, driven by invisible forces that no one understood. The idea that hereditary material was actually a specific molecule – one that could be weighed and measured – was inconceivable for most people. But Miescher himself was an avid proponent of this theory. He suggested that the information contained in hereditary material was stored as a chemical code. It took a very long time for anyone to appreciate just how close Miescher was to the truth.
In the years that followed, many scientists studied the mysterious nuclein substance more closely. They discovered that it contained a type of sugar called deoxyribose, and that it was acidic. It was accordingly given the more precise name deoxyribonucleic acid, which was shortened to DNA. For many years DNA was looked upon as nothing more than a supporting material, holding things in place at the cell’s core. The researchers realised that the genes must be in the chromosomes, but even then, the DNA molecule still failed to get the recognition it deserved.
Chromosomes consist of both DNA and proteins, and it seemed far more likely to the researchers that it was the proteins that controlled the genetics. From a chemical perspective, the proteins were more interesting – they existed in an infinite number of forms, with many different chemical and physical properties. DNA, on the other hand, looked to be the same everywhere. But when scientists at the Rockefeller University in New York experimented with bacteria in the 1940s, the results showed what everyone had thought was impossible: genes were made of DNA. How could this simple substance create the multitude of characteristics found in nature? White and pink blossom, curled and straight fur, pointed and rounded noses – were all these contained in the same molecule?
Only when James Watson and Francis Crick presented their model of the DNA molecule’s structure in 1953 did the pieces fall into place. DNA was no aimless, disorganised lump. It was a chemical code. The molecule consists of long chains of four different bases: adenine, thymine, cytosine and guanine – A, T, C and G– which are connected to sugar and phosphate. Two chains connect themselves to a spiral staircase of base pairs. The sugar and phosphate provide supporting stringers: the base pairs form the steps going up. When the bases connect like this, they follow strict rules: an A will always connect to a T while a C will always connect to a G. Which means you will know exactly how one side of the ladder will look if you know the other. The cell can open the molecule in the middle and read these bases, letter by letter, like a book. By attaching matching letters on each side, it can create two identical copies of the recipe that can be passed on. Cell by cell, generation after generation.
A, T, C, G. These four letters are all that is needed. They can code eyes, fingernails, dimples, oak trees, jellyfish, sea grass, elephants, butterflies. Chemically, there’s very little difference between the recipe for a human and that for an oak tree: the building blocks are exactly the same. It all depends on the order in which they are placed.
When that very first cell of yours floats down the fallopian tube, there are forty-six chromosomes sitting safely at its core. Twenty-three of them come from your mother and twenty-three from your father. Each one consists of a long DNA strand, tightly coiled around beads of proteins; which altogether adds up to over two metres of DNA in your one cell. The recipe was fixed when the sperm and the egg cell merged. Now it is time to use it.
THE INVASION
IT IS ONE day after your conception, and the small hairs in the fallopian tube are nudging the tiny round cell down the canal. Slowly. Carefully. On the outside, everything looks perfectly calm. Deep within the cell’s interior, however, a sophisticated mechanism is working tirelessly, creating precise copies of your DNA molecules. Soon, each chromosome becomes an X-shape, formed by two identical DNA molecules, attached at the centre. The chromosomes assemble at the centre of the round cell, row upon row. At the same time, the cell spins a web of protein from either side, and long thin strands reach towards the centre taking hold of the chromosomes. The cell then stretches and becomes elongated, while the strands pull the DNA copies towards each pole. It’s beautiful to observe under the microscope, like a tiny firework. Finally, after about a day, it pinches in the middle and splits into two cells.
And it continues like that. The cells copy and divide, copy and divide. Some creatures, like bacteria or amoebas, are happy to consist of only one cell. They’re still able to eat, move and multiply, and they have never wanted anything more. The male roundworm Caenorhabditis elegans consists of exactly 1,031 cells; we know this because biologists have taken the trouble to count them. But you? You consist of roughly thirty-seven thousand billion cells. ‘Roughly’, because there’s such a crazy number of them that no one would ever sit down and count them one by one. Instead, scientists have calculated roughly how many there should be, knowing what we know about the body and
