121 DNA  (continued)

The Components and Structure of DNA

You might think that knowing genes were made of DNA would have satisfied scientists, but that was not the case at all. Instead, they wondered how DNA, or any molecule for that matter, could do the three critical things that genes were known to do: First, genes had to carry information from one generation to the next; second, they had to put that information to work by determining the heritable characteristics of organisms; and third, genes had to be easily copied, because all of a cell's genetic information is replicated every time a cell divides. For DNA to do all of that, it would have to be a very special molecule indeed.

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DNA is a long molecule made up of units called nucleotides. As the figure below shows, each nucleotide is made up of three basic components: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous (nitrogen-containing) base. There are four kinds of nitrogenous bases in DNA. Two of the nitrogenous bases, adenine (AD-uh-neen) and guanine (GWAH-neen), belong to a group of compounds known as purines. The remaining two bases, cytosine (SY-tuh-zeen) and thymine (THY-meen), are known as pyrimidines. Purines have two rings in their structures, whereas pyrimidines have one ring.

DNA Nucleotides DNA is made up of nucleotides. Each nucleotide has three parts: a deoxyribose molecule, a phosphate group, and a nitrogenous base. There are four different bases in DNA: adenine, guanine, cytosine, and thymine.

The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide. The nitrogenous bases stick out sideways from the chain. The nucleotides can be joined together in any order, meaning that any sequence of bases is possible.

If you don't see much in the figure above that could explain the remarkable properties of the gene, don't be surprised. In the 1940s and early 1950s, the leading biologists in the world thought of DNA as little more than a string of nucleotides. They were baffled, too. The four different nucleotides, like the 26 letters of the alphabet, could be strung together in many different ways, so it was possible they could carry coded genetic information. However, so could many other molecules, at least in principle. Was there something more to the structure of DNA?

Chargaff's Rules One of the puzzling facts about DNA was a curious relationship between its nucleotides. Years earlier, Erwin Chargaff, an American biochemist, had discovered that the percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA. The same thing is true for the other two nucleotides, adenine [A] and thymine [T], as shown in the table below. The observation that [A] = [T] and [G] = [C] became known as Chargaff's rules. Despite the fact that DNA samples from organisms as different as bacteria and humans obeyed this rule, neither Chargaff nor anyone else had the faintest idea why.

Chargaff's Rules Erwin Chargaff showed that the percentages of guanine and cytosine in DNA are almost equal. The same is true for adenine and thymine.

X-Ray Evidence In the early 1950s, a British scientist named Rosalind Franklin began to study DNA. She used a technique called X-ray diffraction to get information about the structure of the DNA molecule. Aiming a powerful X-ray beam at concentrated DNA samples, she recorded the scattering pattern of the X-rays on film. Franklin worked hard to make better and better patterns from DNA until the patterns became clear.

By itself, Franklin's X-ray pattern does not reveal the structure of DNA, but it does carry some very important clues. The X-shaped pattern in the photograph in the image below shows that the strands in DNA are twisted around each other like the coils of a spring, a shape known as a helix. The angle of the X suggests that there are two strands in the structure. Other clues suggest that the nitrogenous bases are near the center of the molecule.

X-Ray Diffraction Image of DNA  X-ray diffraction is the method that Rosalind Franklin used to study DNA.

The Double Helix At the same time that Franklin was continuing her research, Francis Crick, a British physicist, and James Watson, an American biologist, were trying to understand the structure of DNA by building three-dimensional models of the molecule. Their models were made of cardboard and wire. They twisted and stretched the models in various ways, but their best efforts did nothing to explain DNA's properties.

Then, early in 1953, Watson was shown a copy of Franklin's remarkable X-ray pattern. The effect was immediate. In his book The Double Helix, Watson wrote: “The instant I saw the picture my mouth fell open and my pulse began to race.” Using clues from Franklin's pattern, within weeks Watson and Crick had built a structural model that explained the puzzle of how DNA could carry information, and how it could be copied. They published their results in a historic one-page paper in April of 1953.   Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other.

Discovering the Role of DNA

A double helix looks like a twisted ladder or a spiral staircase. When Watson and Crick evaluated their DNA model, they realized that the double helix accounted for many of the features in Franklin's X-ray pattern but did not explain what forces held the two strands together. They then discovered that hydrogen bonds could form between certain nitrogenous bases and provide just enough force to hold the two strands together. As the figure below shows, hydrogen bonds can form only between certain base pairs—adenine and thymine, and guanine and cytosine. Once they saw this, they realized that this principle, called base pairing, explained Chargaff's rules. Now there was a reason that [A] = [T] and [G] = [C]. For every adenine in a double-stranded DNA molecule, there had to be exactly one thymine molecule; for each cytosine molecule, there was one guanine molecule.

DNA Structure   DNA is a double helix in which two strands are wound around each other. Each strand is made up of a chain of nucleotides. The two strands are held together by hydrogen bonds between adenine and thymine and between guanine and cytosine.