Today we understand much more detail about the structure of DNA. It turns out that Watson, Crick, Wilkins, Franklin, and the many others involved gave essentially the correct answer about the DNA molecule and how it is structured.
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In previous episodes we learned about the discovery of DNA and how its structure was determined. Today we understand much more detail about the structure of DNA. It turns out that Watson, Crick, Wilkins, Franklin and the many others involved gave essentially the correct answer about the DNA molecule and how it is structured.
DNA has two such intertwining backbones that spiral around each other. These are made up of form of sugar called deoxyribose and phosphate groups. We are all accustomed to the word sugar, something we sweeten coffee with, but to chemists a sugar means a molecule having certain properties. So to chemists there are a great many different sugar molecule structures. Deoxyribose is a particular type of sugar. A phosphate is simply a phosphorus atom bonded to several oxygen atoms. The backbones in DNA lie on the outside surface of the linear DNA molecule and is composed of sugar-phosphate-sugar units linked together.
There is no conceivable limit on how many such links can be chained together. Hundreds of millions of units can be linked together. Some DNA molecules, if stretched out, would be several inches long. In fact, taking all the DNA in a human cell and stretching it all out would reach about 6 feet long.
Inside the two intertwining backbones of DNA are the nucleotides. They are called adenine, cytosine, thymine, and guanine. For short we can call them A, C, T, and G. As Chargaff discovered, As only pair with Ts, and Cs only pair with Gs. And so the DNA backbones support paired nucleotides on the inner portion of the DNA molecule.
We call DNA in this structure double-stranded DNA. If we were to separate the paired nucleotides, then we would have two single strands of DNA. The sequence of A, C, T, and G on a strand can be anything. That’s what makes it able to form a code. For instance, we could have a strand with AAGCTTTA. The strand that would pair with it, called the complementary strand, would be TTCGAAAT.
Now there is a naming convention for the backbone of DNA. Chemists refer to the terminal end of a phosphate backbone the 5 prime end. This can be thought of as the head. The tail at the other end of the molecule is called the 3 prime end.
The two strands that make up a DNA molecule go in opposite directions. In other words one strand goes 5 prime to 3 prime and the strand complementary to it goes 3 prime to 5 prime.
By the way, the chemical bonds between As and Ts, and Cs and Gs, are relatively weak. And so under the right conditions, a DNA molecule can be unzipped as if being a spiral zipper. That information will have more relevance in future episodes.
And so now you know the basic structure of the DNA molecule. Today it seems simple and elegant, but before the structure was determined through piecing together empirical evidence, it was a mystery and the structure was anyone’s guess.
In future episodes I will talk in more detail about many aspects of DNA. One of the remarkable things about DNA is how such long molecules can be packed inside a cell’s nucleus in an orderly fashion, and organized in a way that the cell’s molecular machinery can read the sequence of As, Cs, Ts, and Gs and decode the meaning of the sequence. It is an amazing thing, and has great ramifications for how DNA organization can be involved in health and disease.