The Double Helix

The race to figure out the structure of DNA ended when a young American biochemist named James Watson and a British physicist named Francis Crick published a short paper in 1953. The structure they proposed was both simple and elegant, and complex enough to show how this deceptively monotonous molecule was capable of carrying so much information.

Because of the three-dimensional shape of the structure they discovered, DNA has come to be described as a "double helix." A helix is a spiral shape, like a slinky or the hand rail of a spiral staircase. Remember that the What Is A Gene essay informed you that DNA is composed of a long chain of nucleotides. Well, that's only half of the truth. It's actually composed of two of those chains. These two strands wrap around each other, creating that double helical shape which has become so well known.

This shape is more than just a beautiful geometric form. The double stranded nature of DNA turns out to be a vital functional part of its structure--it couldn't do what it does if it weren't double stranded.

To understand this importance, it's necessary to take a bit closer look at the structures out of which a DNA molecule is made: nucleotides.

Nucleotides are made out of three small molecules bonded together: a sugar, a phosphate group and a nitrogenous base. In DNA, all of the nucleotides contains the sugar deoxyribose (thus the name, "deoxyribonucleic acid"). The nucleotides in RNA contain a slightly different sugar called ribose" (thus "ribonucleic acid).

The nucleotides that make up a DNA molecule come in four types. All of these deoxyribodnucleotides are identical in their sugar and phosphate portions. But there are four different kinds of nitrogenous bases, called adenine, guanine, cytosine and thymine. (RNA's ribonucleotides also come in adenine, guanine, and cytosine versions. RNA doesn't have any thymine in it; instead, it has a very similar base called uracil.) It is very common among biologists and chemists to simply refer to these four deoxyribonucleotides as A, G, C and T. Here are the molecular structures of these bases. (Note: you won't be asked to remember these structures.)

So now we get to the information aspect of DNA. Recall that one of the requirements for the chemical which makes up genes is that it be able to carry information, and that the information content be highly variable. Think of 500 to 1000 nucleotides strung together in a long chain. The attachment between nucleotides involves only the sugars and the phosphates, so your long, long chain has a backbone composed of alternating deoxyribose and phosphate group molecules--generally referred to as the sugar-phosphate backbone of the DNA. Check out the diagram to the right.

The significant thing about this observation is that the four nitrogenous bases aren't involved in the backbone at all. What this means is that they can occur in any order. Any order. And as the length of a gene can vary, this freedom of order means that it is possible to create literally an infinite number of different base sequences. And this means that the information content of a DNA molecule is infinitely expandable.

This brings us to the really magical aspect of DNA structure. When scientists first began investigating the nature of the genetic material, one of the expectations they had was that, whatever the molecule was, and whatever its structure, it must somehow be able to copy itself perfectly. After all, it was clear that genetic information was passed from generation to generation--it had to be able to copy itself.

And that leads us to The Secret to Life: complementary base pairing. Recall that DNA isn't simply a single string of nucleotides--it's two strands, side by side and twisted around each other. It turns out that the two strands are held together by interactions between the bases on one strand and the bases on the other--they pair up, holding the two sides together. And the rules about pairing are very strict--Adenine will pair only with Thymine; Guanine will pair only with Cytosine. So Adenine and Thymine are complements of each other, and Guanine and Cytosine are complements of each other. (To be complementary means to "fit together.") This base pairing is strictly maintained in all DNA. So once the base sequence of one side is set, the sequence of the other side is automatic. Check out the diagram to the left to see how this works.

A final aspect of the structure of DNA is that the way the two sides pair together forces a twist in each of the backbones, creating the famous and beautiful three dimensional shape of a double helix--two helices (coils) wrapped around each other.

Looking at the structure they'd proposed, Watson and Crick immediately realized the potential for self-replication intrinsic to complementary base pairing. Each side of this molecule imposes the order of the other side. So if you separate the two sides ("unzip" the molecule), and assemble new partners out of free (single) nucleotides, the bases of the old chains will select the "correct" partners by complementary base pairing, and you will end up with two new molecules, identical to each other and to the original molecular. Though the specifics of DNA replication are much more complex than this simple model, in fact what happens to the molecule is very much exactly what Watson and Crick expected.

So complementary base pairing explains this vital ability of the genetic material--self-replication. But that's not all DNA has to do. The task of a gene is to provide the instructions for the construction of a specific protein. At the most fundamental level, that's what a gene is--a recipe for a protein. At first thought, this seems a bit unlikely, as DNA is "written" in on kind of biological "language" (nucleotides) and protein is written in a completely different kind of biological "language" (amino acids). It turns out that complementary base pairing also explains how this task is accomplished.

The production of protein takes place in two major steps, involving several different components. One component is, of course, the DNA, which is the archive of the information needed to make all of the proteins the cell is able to make. The DNA in a cell is a bit like --a giant cookbook containing all of the recipes needed for supplying the cell. A second component is a group of RNA's called messenger RNA's. Messenger RNA's are the intermediaries between the information archive--the DNA--and the protein synthesis machines--the ribosomes. So the ribosomes are also an important component.

One more component is a very interesting group of small RNA's called transfer RNA's. The transfer RNA's are the adapters which allow the nucleotide language of DNA and RNA to be converted to the amino acid language of proteins.

The two processes involved in making protein are called transcription and translation.

The word "transcription" means "copy." In the transcription process, a segment of DNA (typically, though not always, a single gene) is used to create a messenger RNA which contains a copy of the information in the gene. Remember that the structure of DNA and the structure of RNA are very similar. This messenger RNA is created by complementary base pairing between ribonucleotides (to make RNA) and one side of the DNA. Uracil behaves just like Thymine in base pairing. If DNA is our cookbook, messenger RNA is a recipe card. DNA contains the entire library of the cell's genetic information; messenger RNA typically contains the information for only one protein. And messenger RNA is easily replaceable--if it gets damaged or degraded, all the cell has to do is transcribe the gene again.

"Translation" means "convert from one language to a different language." In the translation process, a ribosome reads the information coded into a messenger RNA and constructs a string of amino acids according to those instructions. The information in the messenger RNA is interpreted by the ribosome in three-base "words" called codons. Each codon is translated into a particular amino acid. And complementary base pairing between messenger RNA and transfer RNA's is the key to this process.

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Updated 25 September 2004