Human DNA, the molecule of inheritance, carries the entire set of instructions for making a complete person from a single fertilised egg. So once James Watson and Francis Crick revealed its structure, the stage was set for 50 years of world-changing discoveries. In honour of that tight-knit package of 23 pairs of chromosomes, Steve Connor highlights 23 of the most significant breakthroughs
1. The shape of things to come
Two intertwining strands of DNA wrapped around a central axis to form a gently curving double helix: this instantly recognisable structure is one of the 20th century's most powerful icons. Watson and Crick themselves felt that the double helix was so simple, so beautiful (it's sometimes described as the "Mona Lisa" of science), that it had to be right, while its enigmatic symmetry made a powerful prediction about how genetic information is stored and transmitted from one generation to the next.
2. The gene explained
For centuries philosophers pondered inheritance and how one person can be an amalgam of two parents. Cracking the structure of DNA explained how the fundamental unit of inheritance – the gene – works, and how it is replicated from one cell to the next and one generation to another. Now that they knew the molecular basis of the gene, scientists could understand how it can be damaged, and why such "mutations" can lead to harmful diseases or, occasionally, beneficial traits that can form the basis of a new trend in evolution.
3. Understanding inherited diseases
One of the first inherited diseases to be unravelled at the level of DNA was sickle-cell anaemia, a blood disorder that affects mostly Africans and natives of the Mediterranean region – areas badly affected by malaria. A single mutation in the gene for the blood protein haemoglobin can affect its ability to transport oxygen around the body. People who inherit two copies of the same mutation, one from each parent, have severe symptoms. However, those who inherit just one copy of the mutation do not suffer too badly; in fact, they are resistant to malaria. The discovery told geneticists much about evolution and how harmful mutations can increase to relatively high levels within a population provided they confer some advantage.
4. Curing inherited diseases
By understanding how a gene can go wrong, scientists can work out ways of putting it right – hopefully. In April 2002, Rhys Evans became the first child in Britain to be declared cured of an inherited disorder as a result of gene therapy, which involves "repairing" a defective gene by augmenting it with a healthy version. Rhys had inherited a defective Gamma C gene from his mother; this meant that his immune system did not function, making even the most innocuous infection life-threatening. He had spent his life inside a sterile "bubble". Then a blood transfusion containing his own, genetically modified cells corrected the defect and enabled the 18-month-old to play outside with his friends for the first time in his life.
5. Predicting inherited disease
Some disorders are caused by several genetic defects acting in unison. Cancer is the classic example: it is caused by a cascade of cellular changes triggered by a series of mutations. But sometimes cancer can result from a defect in just one gene that runs in a family, such as the breast-cancer genes BRAC1 and BRAC2. Understanding this has led to the development of breast-cancer tests for women whose families carry the gene.
6. Upholding justice
In 1988, Colin Pitchfork was sentenced to life for the killing of a schoolgirl, Dawn Ashworth, after he became the first murderer to have his DNA matched to that of a tissue sample at the scene of a crime. What is less well known is that DNA fingerprinting, as it is known, was also used on another suspect who had already confessed to the same murder. The test proved that the confession was false. DNA fingerprints have revolutionised criminal investigations and have helped to protect the innocent as well as to convict the guilty.
7. Looking for daddy
Where would Elizabeth Hurley ( left) be without DNA testing? DNA tests on a child can establish paternity beyond reasonable doubt, even in a case where the putative fathers are both brothers (although this is not the case with identical twins, who share the same DNA sequence).
8. A dog's life
DNA tests are used to confirm the pedigrees of pet dogs and cats, racehorses and livestock. They can establish the identity of illegally collected bird's eggs and the hides of protected species.
9. Where we came from 1
DNA analysis supports the view that early humans moved out of Africa less than 100,000 years ago to colonise the world. DNA has shown that the first farmers migrated west from the Middle East across Europe. It has helped to establish when the first humans moved into Australia, reached the Pacific islands on boats and crossed the ice-locked Bering Strait into the uninhabited North American continent.
10. Where we came from 2
DNA tests on ancient bones have shown that Neanderthal man and modern humans are not closely related. Scientists now believe that Neanderthals, who lived alongside modern humans for thousands of years, never interbred with their close cousins. DNA tests on living people have revealed other ethnic and ancestral origins that have been lost over time. The ancestors of many Icelandic women came from Ireland rather than Norway. Many British Afro-Caribbean men inherited their male Y chromosomes from white ancestors who were alive at the time of the slave trade.
11. Egyptian mummies and Russian tsars
DNA tests on members of the British Royal Family helped to confirm that human remains buried in a pit in Russia were those of Tsar Nicholas II and his family, executed in 1918 by Soviet officials in the basement of a house in Ekaterinburg. Archaeologists use DNA tests to establish the family relationships of other long-dead people, from Egyptian pharaohs ( below) to the mummified remains of Mayan and Incan nobility.
12. The myth of race
Analysis of DNA has given racial stereotypes a battering – once you get to a molecular level, even the biological concept of race begins to look meaningless. When scientists studied the DNA of Israeli Jews and Arabs – two groups culturally separated by religion and ethnicity – they discovered a genetic affinity that could only be explained by a close relationship in the relatively recent past. In fact, human DNA shows surprisingly small variation between the races. The similarity of DNA sequences across the globe indicates that the human species has passed through a "genetic bottleneck", the result of the relatively small number of people – perhaps numbering no more than a few thousand – who initially migrated out of Africa.
13. Clean medicines
The protein Factor 8 is one of a number of clotting agents, naturally occurring in a healthy person's blood, that stops them bleeding to death if they cut or bruise themselves. Twenty years ago haemophiliacs – who lack the protein because of a defective gene – relied on transfusions of Factor 8 from blood donors. As a result, many contracted viral infections such as hepatitis and HIV from contaminated donors. Now it is possible to make virus-free Factor 8 artificially, using genetically engineered microbes. Other genetically engineered medicines, such as insulin and human growth hormone, have also liberated patients from the spectre of contamination.
In the 1980s, a genetically modified mouse with a cancer-causing gene became the first living creature to be caught up in the bitter row about the rights and wrongs of animal patenting. Oncomouse was important to science because it was genetically predetermined to develop cancer – and anything that delayed or prevented this destiny might be useful for anti-cancer treatments. The debate it caused continues to this day, as more laboratory animals are genetically engineered for biomedical research.
15. Feeding the world – or not
The first experiments with genetically modified crops took place in the 1980s. Since then, DNA manipulation has enabled scientists to insert genes into plants that confer resistance to pests and herbicides, as well as improving the nutritional content of staple crops – such as rice enriched with beta-carotene. America and China are both planting thousands of square miles of land with GM crops, but Europe is still questioning whether this is an agricultural revolution too far. Like it or not, however, GM food is changing the world.
16. Manufacturing death
An extreme example of genetic engineering is the deliberate manipulation of a deadly microbe to make it even more infectious or toxic to humans. Last year scientists made a complete polio virus from scratch using standard laboratory equipment and DNA reagents ordered over the internet. The virus caused paralysis and death when injected into mice. And there are real fears that terrorists, or a rogue state, could manipulate the genes of more deadly agents such as anthrax, or even smallpox, to make the threat more potent.
17. Delaying ageing
Understanding the nature of DNA has shed light on the ageing process. Certain genes are known to prolong the longevity of fruit flies and in 1984 scientists discovered that the enzyme telomerase, which rebuilds the tips of chromosomes, can extend the life of individual cells. But we're still some way away from discovering the secret of eternal youth.
18. The mega-jab
A radical approach to immunisation is to inject raw DNA directly into patients. Tests are already under way to induce immunity to a range of infections. DNA vaccines could immunise people against a range of infections simultaneously with just one jab.
19. The end of free will?
In 1994, the American murderer Stephen Mobley was sentenced to death, but his lawyers argued for leniency on the grounds that he came from a criminal family and had evidently inherited a genetic predisposition to violence. Last year scientists showed that physically abused boys are more likely to grow up into antisocial and violent men if they have also inherited a certain version of a gene on the X chromosome. DNA has more than re-opened the age-old debate of nature versus nurture; it has virtually eliminated the notion that we are all born as blank slates.
20. An ethical minefield
Discovering the nature of inheritance has presented society with a totally novel set of ethical dilemmas. Should young people be told they have inherited the gene for Huntington's disease, which will strike them in middle age with a lethal and debilitating mental disorder that has no cure? Do insurance companies have the right to access such information? Should parents be able to choose the sex of their babies based on a DNA test? Why shouldn't families be able to alter the genes of eggs or sperm to rid their children of genetic disorders? If this is acceptable, why can't parents genetically engineer their children to improve "cosmetic" traits such as intelligence, height or sexual attractiveness?
21. The human genome
Writing down the entire sequence of genetic letters that makes up the human genome would fill a space equivalent to 800 copies of the Bible. The complete sequence, to be published later this year, is the ultimate book of life. In effect it is the recipe for making a human being: yet, encoded in the form of DNA, it can be packed into the nucleus of a microscopic cell.
22. Movie mania
Hollywood has had a lot of fun (and made a lot of money) with the possibilities inherent in DNA. Jurassic Park addressed the possibility of bringing dinosaurs back to life by retrieving their preserved DNA from fossilised mosquitoes; a highly unlikely scenario. Gattaca envisaged a future of deliberately designed, genetically modified "valids" and the less perfect "in-valids", whose DNA had been left to the vagaries of fortune. Somehow this seems a more likely outcome.
23. Validating Darwin
All animals and plants share the same DNA code. Knowing the structure of DNA and how it encodes genetic information demonstrated that life on Earth has a common origin. In effect, it proved that Charles Darwin was right when he suggested that species are descended from a common ancestor. Even if extraterrestrial life exists, it is highly unlikely to use exactly the same sort of DNA, which is, in all probability, unique to Earth.Reuse content