"We have solved the secret of life!" How the DNA double helix was discovered

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"We have solved the secret of life!" How the DNA double helix was discovered
"We have solved the secret of life!" How the DNA double helix was discovered
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By the 1950s, scientists had no doubt that the traits of living organisms are mostly predetermined before birth and inherited. The child has eyes, because his parents had eyes, and the color of the eyes is not accidental, as is the tendency to myopia. What the researchers couldn't figure out was where all this information is stored. For a long time it was believed that the carrier is proteins with their complex structure, which provide all the diversity of life. But by the mid-1940s, DNA became the main suspect, a huge - in humans its length is about two meters - a molecule found in almost all types of cells.

DNA was discovered back in 1869 by the Swiss Johann Friedrich Mischer, but he did not attach much importance to the find: he was interested in the structure of white blood cells.

Who will solve first

HOW TO SEE YOUR DNA

Finding DNA is not a tricky business, and anyone can do it, you don't have to be a scientist. You need to gently scrape the inside of your cheek with a toothpick, rinse your mouth with water or saline to wash off the epithelial cells, and spit it into a test tube. On top you need to add a little soapy water, and then alcohol. Soon, white threads will appear in the test tube - these are the DNA molecules that have flowed out of the cells with dissolved membranes.

When Watson began working with Francis Crick in an office at Cambridge University in October 1951, DNA was known to consist of four repeating base bricks with sugar and a phosphoric acid residue, with as much adenine as thymine, and guanine - as cytosine. But how these components are connected, scientists had no idea.

It was only assumed that DNA resembled a spiral, more precisely, a screw, but whether double, triple or some other, how the bases were located in it, how this structure could store and reproduce hereditary information, if at all, - all this remained a mystery. Having met, Watson and Crick quickly realized that they wanted to solve it together.

In addition to Watson and Crick, two more groups of scientists tried to figure out the structure of DNA. In London, Maurice Wilkins and Rosalind Franklin, constantly cursing, peered at X-rays of crystallized molecules, and at the California Institute of Technology, the famous chemist Linus Pauling fought over the mystery of life, who before that was the first to determine the structure of protein components.

For his research on chemical bonds in 1954, he was awarded the Nobel Prize. Against his background, Crick and Watson looked like casual passers-by: the first was a physicist by education and only four years earlier switched to biology, and the second was only 23 years old. True, by that time Watson already had a doctorate.

The first DNA model developed by Watson and Crick consisted of three chains with phosphate backbones in the middle. When the model was shown to Franklin, she scoffed at her colleagues: she was sure that the phosphoric acid residues should be located on the outside of the molecule, and not in the center. Watson and Crick's boss, Laurence Bragg, became so angry about this failure that he forbade them to continue studying DNA.

It's not all over yet

However, a year later, Bragg changed his mind. In his laboratory, Linus Pauling's son worked, who said that his father created his own model of DNA. In Bragg, pride leaped.

He and Pauling were the world's largest experts in their field, but the American was the first to determine the structure of both large inorganic molecules and the protein alpha-helix. Bragg was - and still is - the youngest winner of the Nobel Prize in physics, which he and his father were awarded back in 1915. But since the late 1920s, he has always been behind Pauling.

A month later, a yet unpublished article by Pauling describing the model was obtained in Cambridge. To everyone's surprise, the DNA in it appeared to be a triple helix with phosphate backbones in the center, as Crick and Watson had suggested a year earlier. In his autobiography, Watson recalled: "While Francis marveled at Pauling's innovative approach to chemistry, I began to breathe more calmly. By this point, I knew we were still in the game."

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Maurice Wilkins

According to Watson, he came to London to discuss Pauling's article with Franklin, but she did not share his enthusiasm and said that the DNA molecule cannot be coiled. Perhaps Watson was lying: in the laboratory journal Franklin there were earlier records that one of the two forms of DNA could be exactly a spiral. According to Watson, this incident was the last straw for Maurice Wilkins, who worked with Franklin. She was so tired of her stubbornness that in his hearts he took an X-ray of DNA from a box and showed it to Watson. His jaw dropped.

The square plate, just a few centimeters in size, went down in history as "Photo 51". To take this shot, Franklin placed a stretched and crystallized sample of human DNA in a special camera, where X-rays bounced from it onto the film for more than 60 hours, forming an image - a striped cross. For Watson, this cross was clear evidence that DNA is made up of two twisted strands. Franklin didn't see it.

Beauty is in simplicity

Now scientists were confident in the spiral shape of the molecule. But they still had to explain how DNA bricks from two different strands are connected - the black spots in Photo 51. To do this, Watson rearranged the structural formulas of these bricks in different ways, but there was no result. And then the American chemist Jerry Donoghue showed him a fresh article that described slightly different formulas for the building blocks of DNA.

For several days, Watson and Crick pondered a new model, and on February 21, 1953, Watson guessed that adenine from one chain combines only with thymine from the other, and cytosine - with guanine. In this case, the DNA molecule resembles a uniformly twisted staircase with edges made of sugar, phosphoric acid residue and parallel steps of the same length. These combinations explained why any DNA molecule contains the same amount of adenine with thymine and cytosine with guanine. Finally, if each brick has only one pair, then the molecule can split in half and form two copies with the same genetic information. Scientists were amazed at how simple and beautiful the explanation turned out to be.

“We've solved the mystery of life!” Francis Crick said in his favorite bar in Cambridge, where he and Watson were celebrating the opening. However, it was still far from universal recognition.

A new mystery of life

The first thing they did was show Wilkins and Franklin. Those two days compared them with X-rays and found no contradictions. In March, a draft of the article describing the model was sent to Pauling. He praised his colleagues, but did not understand why they rejected the triple helix hypothesis. It was only when Pauling arrived in Cambridge and saw photographs of Franklin that things fell into place for Pauling.

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Francis Crick, James Watson and Maurice Wilkins

In April, Crick and Watson's paper was published in Nature, along with writings by Wilkins and Franklin. In 1962, Watson, Crick and Wilkins were awarded the Nobel Prize. Franklin died in 1958 and was left without a reward. In the decades that followed, other scientists created 3D computer models, decoded the DNA of humans and other species, and in recent years learned how to edit the genes recorded in the DNA. A new mystery has arisen - what will become of life if now a person programs it.

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