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DNA is the molecule that is the hereditary material in all living cells.

Genes are made of DNA, and so is the genome itself. A gene consists of enough DNA to code for one protein, and a genome is simply the sum total of an organism's DNA.

DNA is long and skinny, capable of contorting like a circus performer when it winds into chromosomes. It's skinny as a whip and smart as one too, containing all the information necessary to build a living organism. In a very real sense, DNA is information.

What is DNA made of?

DNA is a very large molecule, made up of smaller units called nucleotides that are strung together in a row, making a DNA molecule thousands of times longer than it is wide.

Each nucleotide has three parts: a sugar molecule, a phosphate molecule, and a structure called a nitrogenous base. The nitrogenous base is the part of the nucleotide that carries genetic information, so the words "nucleotide" and "base" are often used interchangeably. The bases found in DNA come in four varieties: adenine, cytosine, guanine, and thymine—often abbreviated as A, C, G, and T, the letters of the genetic alphabet.

How did people find out that DNA is the hereditary material?

DNA was largely ignored for decades after a German chemist, Friedrich Miescher, first isolated the white, slightly acidic substance from the nucleus of cells in 1869. No one knew what DNA's function was—in fact, some doubted that it had a function at all—so they pretty much left the stuff alone.

Very few people thought that DNA could be the hereditary material. Early studies of DNA suggested, erroneously, that the molecule was made up of the same sequence of four bases repeated over and over—ACGTACGTACGT… for example. No one could imagine how such a monotonously simple molecule could contain the information necessary to build a living organism.

But during the 1930s and 1940s, new experiments began to suggest that DNA might, in fact, be important. It turned out that different strains of bacteria can exchange DNA and that when they do certain traits, such as the ability to cause disease in humans, can be passed from one strain of bacteria to another. Scientists also learned that when a virus infects a cell it injects its DNA into the cell, which then produces many copies of the virus, suggesting that DNA contains instructions for building viruses. And they found that different species of organisms have different proportions of bases in their DNA—one species might have DNA that is 30 percent A, 20 percent C, 20 percent G, and 30 percent T, while another might have 20 percent A, 30 percent C, 30 percent G, and 20 percent T. People began to think that genetic information might be written in the differences between the DNA bases of different species.

What does DNA look like?

A DNA molecule is a double helix, a structure that looks much like a ladder twisted into a spiral. The sides of the ladder are made of alternating sugar and phosphate molecules, the sugar of one nucleotide linked to the phosphate of the next. DNA is often said to have a sugar and phosphate "backbone."

Each rung of the ladder is made of two nitrogenous bases linked together in the middle. The length of a DNA molecule is often measured in "base pairs," or bp—that is, the number of rungs in the ladder. Sometimes, this unit of measurement is shortened simply to "bases."

The structure of DNA was worked out in 1953 by James D. Watson and Francis Crick, who worked together in the Cavendish laboratory in Cambridge, England. By the time they began their work in the early 1950s, it was clear that DNA is the hereditary material, and scientists were racing to find out more about the long-ignored molecule, picking apart the implications of each new detail. Everyone knew they couldn't really understand how DNA works until they understood how its nucleotide building blocks are put together.

When Watson and Crick joined the race, they were supposed to be investigating the structure of proteins. But they were both convinced that DNA was a more important molecule, and they shared a passionate interest in finding out its structure. Like kids hiding comic books inside a copy of Moby Dick, they snuck away from their protein work to think about DNA whenever they could.

While many other discoveries about DNA had emerged from laboratory experiments, Watson and Crick relied mainly on abstract thinking. They synthesized all the information that had been gathered about DNA, throwing out what was contradictory and trying to imagine a structure that would be consistent with as many pieces of known information as possible.

Watson and Crick also liked to play with toys. Specifically, they played with ball-and-stick molecular models to gain an understanding of how nucleotides might fit together in three dimensions. They put models together and took them apart, drew molecular diagrams on paper and scratched them out. Eventually, when they hit on the idea of the double helix, everything else they knew about DNA seemed to fall into place.

For example, they realized that if sugar and phosphate molecules formed the sides of the ladder, then any sequence of bases could form the rungs of the ladder, and genetic information could be encoded in the order of the bases. They also realized that the ladder would only fit together if the rungs were formed by specific pairs of bases. Specifically, A must always pair with T, and C with G. Any other combination and the sides of the ladder would be too far apart or too close together. This helped explain why, although the amount of each base can vary from species to species, the amounts of A and T are always equal, as are the amounts of C and G.

In other words, the order of bases on one DNA strand, or side of the ladder, determines the bases on the other side of the ladder. Thus, DNA sequences are often written as if DNA were only single-stranded: AGTCTGGAT…. Scientists need sequence only one DNA strand in order to know the sequence of both strands.

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