By Robert Hazen, George Mason University
The key in genetics is the passing of information from one generation to another. How does it happen? How does the DNA carry biological information? Who was the first to understand the double-helix configuration of the DNA structure? Let’s find out more.
Rosalind Franklin’s Work
In the early 1950s, British crystallographer and chemist, Rosalind Franklin, was only 38 when she obtained the first X-ray photographs of DNA. Getting the actual X-ray diffraction measurements off a crystal of DNA, was the key to understanding its structure.
Franklin found that there were two different forms of DNA. She also suggested that the structure had some kind of helical aspect, like a spiral staircase, but she wasn’t sure of the exact details of that.
It was in 1952, that building on the Rosalind Franklin studies, James Watson and Francis Crick fully solved the astonishingly beautiful DNA structure. Watson and Crick’s brilliant deduction was arrived at as much by inspired guesswork as any sort of systematic method. They tinkered with models, built structures in their spare time and saw how they fit together. They eventually came up with an astonishing structure, something like a twisted ladder. It had two side supports, and those side supports were made of alternating sugar and phosphate: sugar-phosphate, sugar-phosphate, going up and up and up on both sides of this ladder.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Helix, a Double-polymer Structure
Then there were rungs of this ladder, and these rungs were made of base pairs; and this was the key discovery. It turns out that they had pairs of bases that fit exactly together. A fits with T; C fits with G. But A never fits with C or G; T never fits with C or G, and so forth. There were only two possible pairs, A-T or C-G, and those form the rungs of the ladder.
That’s why it always has a 1:1 ratio of A and T, and a 1:1 ratio of C and G. The whole structure is twisted into a helical shape: the double helix, because it’s a two-sided polymer. Indeed, it’s called, sometimes, a double polymer. This DNA strand can be of virtually any length; it can be up to billions of nucleotides long. A long, continuous ladder; a double-polymer structure, twisted on itself into a helix.
Storehouses of Information
Watson and Crick immediately recognized the power of this elegant DNA structure; it lies in its ability to store and to copy vast amounts of information. The four bases, A, C, G, and T, were a four-letter alphabet; the genetic alphabet. They were capable of conveying any amount of information, depending only on the length of the molecule that one puts together.
This double-helix configuration of the DNA structure, with complementary bases opposite each other, also gives a way of duplicating information, because each side of the structure contains all of the molecule’s genetic information.
Cell Copying its DNA
The cell DNA carries biological information and can copy itself. To do this, the cell needs several things. First, it needs to have an enzyme—that is, a protein—that unzips the DNA double helix. This is a specially designed protein that has to be in every single cell. Just before the cell divides, the DNA has to duplicate it. In this process, the DNA is essentially unzipped, just like a zipper, by this protein, exposing the two halves of the double helix, with its two single strands.
The cell also has to have vast numbers of unattached nucleotides—that is, the sugar-phosphate base combinations—to fit in. If there’s an exposed A, a T comes in to fill its spot; if there’s an exposed C, a G comes in and fills in. Where, before, we had a single double helix, now we have two double helices; we’ve made a copy of the original, just by unzipping it and exposing the two sides of the original double helix. We, thus, end up with two identical chromosomes, where before we had only one.
In their landmark paper in Nature, in 1953, Crick wrote, in the final sentence, “It has not escaped our notice that the pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
A wonderful understatement, if there ever was one.
A Nobel Prize
For their discovery, Watson and Crick won the Nobel Prize, in 1962. Harvard biologist E. O. Wilson has commented on the significance of this discovery. He said, “It is hard to imagine the impact the discovery of the structure of DNA had on our perception on how the world works. Reaching beyond the transformation of genetics, it injected into all of biology a new faith in reductionism. The most complex processes, the discovery implied, might be simpler than we had thought.”
Indeed, the power of the double helix lays in its simplicity. It’s simple ladder-like structure, with variable rungs that could at once store and duplicate vast quantities of information. In its discovery, scientists had, in a sense, found the book of life. And yet, a tremendous challenge still lay ahead-they had yet to learn to read that genetic language.
Common Questions about the Double-helix Structure of DNA
Building on the Rosalind Franklin studies, James Watson and Francis Crick fully solved the DNA structure. Watson and Crick’s deduction was arrived at as much by inspired guesswork as any sort of systematic method.
The double-helix configuration of the DNA structure, with complementary bases opposite each other, also gives a way of duplicating information, because each side of the structure contains all of the molecule’s genetic information.
Just before the cell divides, the DNA has to duplicate it. In this process, the DNA is essentially unzipped, just like a zipper, by this protein, exposing the two halves of the double helix, with its two single strands.