![]() Despite what he got wrong, Levene's contributions to our understanding of the DNA molecule were substantial. This strongly argued against Levene's hypothesis that DNA was simply a circular tetranucleotide, and scientists began to propose other possible structures of the DNA molecule. However, much later, in the 1940s, Austrian-American scientist Erwin Chargaff reported that DNA from various species of life forms had different amounts of the four nucleotides ( Vischer and Chargaff, 1948). Figure 1: Phoebus Levene incorrectly hypothesized that DNA was made of circular "tetranucleotides." image ©īecause he thought DNA was a simple circular structure, Levene rejected the notion that it could be the genetic material and sided firmly with those who believed that proteins contained the genetic code of organisms. Levene was correct in identifying the three parts of a nucleotide, and determining that nucleotides were linked together to make DNA however, in 1928, he also incorrectly proposed that one of each of the four nucleotides was linked together in a small circular molecule and that these "tetranucleotides" were the basis of DNA ( Levene and London,1928) (Figure 1). Thus, Levene correctly deduced that the DNA molecule was made of smaller molecules linked together, and these smaller molecules, which he named nucleotides, were made of three parts – a five-carbon sugar, a phosphate group (PO 4), and one of four possible nitrogen bases – adenine, cytosine, guanine, or thymine (often abbreviated A, C, G, and T). When Levene analyzed the chemical properties of nucleic acid, he discovered that DNA was abundant in three things: five-carbon sugars (pentoses), phosphate (as Miescher had previously found), and nitrogen bases. Although Levene was not the first scientist to successfully purify DNA, he was uniquely qualified to correctly determine its composition – he had extensive expertise in the area of carbohydrate and sugar chemistry. In the early 1900s, the Lithuanian-American biochemist Phoebus Levene probed deeper into the chemical composition of nucleic acid and was able to further purify the material. We now know that Miescher's "nuclein" (later renamed nucleic acid, for its acidic chemical properties) contained DNA. He called this substance "nuclein" because it was found in the nucleus of the cells. He found that the nucleus of these cells was rich in a then-unknown substance that contained several elements, among them phosphorous and nitrogen. Because white blood cells are the principal component of pus, Miescher would go to the nearby hospital and collect pus from used bandages. Friedrich Miescher, a Swiss chemist working in Germany, was studying white blood cells (leukocytes). Scientists first began to investigate the unique chemical properties of DNA long before the structure of the molecule was understood, and even before DNA was discovered to be the genetic material. As we all know, living things do eventually age and deteriorate, much like the old house and rusty car, but by making copies of our DNA and passing it to our offspring, life continues. And it is the unique chemical properties of DNA that allow it to generate copies of itself. To do this, they must first copy their genetic material, their DNA (see our DNA I module for more information). This is because living things have a fascinating and somewhat unique ability to reproduce and make "copies" of themselves. Your children are no weaker or more likely to fall to pieces than you are. ![]() Yet, life on Earth continues to flourish. Most objects you are familiar with will eventually fall into ruin if not constantly maintained: a car will eventually rust and fall to pieces a house will spring leaks in the roof and fall to the ground even mountain ranges are eroded by wind and rain.
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