Technology is a broad term for the application of science to achieve objectives. There are many different technological techniques and also many different technological projects. Many of these techniques are used by researchers from different scientific fields. For instance, Crick as a physicist and Franklin as a biochemist both used x-ray crystallography. It is important not to overlook the tools of a scientist and to recognize the implications of using these tools to increase understanding.
Since x-ray crystallography was integral in the discovery of the double helix, this unit will briefly explore this technique. X-ray crystallography is a method where an x-ray beam passes through a crystal of a particular substance. This causes the atoms of the crystal to deflect the x-rays in an orderly array. These diffracted x-rays can expose photographic film to produce a pattern of spots. These spots can be interpreted by crystallographers who use complicated mathematical equations to determine structural information. These spots can reveal information about the position of atoms in three-dimensional space.
To better understand this technique and to better understand Rosalind Franklin’s x-ray photograph one has to have an understanding of light and its properties. Light has been considered as both a wave and as a particle, or photon. Before this seminar I actually thought I knew what light was, and even now it is very challenging to attempt to adequately describe it.
Electromagnetic energy, or radiation, is a form of energy released both naturally (sun), or it can be simulated (x-ray generator, light bulb, laser). This energy travels as a wave as does a pebble thrown in a puddle of water. These waves, as opposed to the water waves, are disturbances of electric and magnetic fields over a fixed position or time. Light induces a change in an electrical field. The energy contained in a photon is related to its wavelength. There is an inverse relationship, the shorter the wavelength, the greater the energy of each photon of that light. The order of light waves from longest wavelength to shortest is: Radiowaves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays and cosmic rays. Therefore, the least energy is found in radiowaves and the most in cosmic rays.
Atoms of a molecule are arranged in a specific order, which is what distinguishes different molecules from one another. This order of atoms can be detected with a technique such as x-ray crystallography. As stated, light induces a change in an electrical field. Electrons are the particles of an atom that are negatively charged are those particles that will be affected by light.
Let’s now adopt an analogy, to assist our understanding. Most individuals have a CD player, and many discs for it to play. Each CD contains a spiral track that holds the audio information. This spiral track is detected by a laser beam in your CD player. Each disc has a unique spiral track grating pattern that consists of different length elevated areas (pits) separated by flat areas (land). Ultimately, the laser (Light Amplification by the Stimulated Emission of Radiation) will pass over this pattern, making sound due to the pattern of the distances between the pits and land.
The wavelength of the laser is important. In order for the CD player to read this pattern, it must be able to detect the distances between the pits and land. The distance between two objects is best resolved with light that has a wavelength close to the length of the distance between two objects.
A CD is an example of an artificial or commercially made grating, but nature produces some, although they don’t play music. Nature’s gratings are the arrays of regularly spaced atoms that exist in a crystalline substance. A crystal of DNA would contain atoms that are separated from each other by distances that are much smaller than the wavelengths of lasers. Where as the laser could read the distance between the pits and land of a CD, the distance between the atoms (“pits”) and the space in between adjacent atoms (“land”) must be read with light that has a much smaller wavelength. X-rays are the source of light that must be used to detect these distances since their wavelength ( approximately 0.5 x 10-10m) is closest to the distance between adjacent atoms.
When an x-ray beam is passed through a crystal of DNA, a diffraction pattern results. The angle of diffraction will tell us the distance between the repeating units. The x-rays are bombarding the molecule, but it is the electrons of the atoms and the electrons that are emitted by a heated filament in an x-ray tube that are interacting.
The planes of the molecule that have a lot of electron density are those that produce the distinct scattering pattern when bombarded by x-rays. This complicated arrangement of diffracted spots detected in a photograph is due to the three-dimensional structure of the molecule. Looking at Franklin’s x-ray diffraction photograph of a DNA molecule, one can see an “X” with dark bands at its top and bottom. The two arms of the “X” are not solid, rather they are spaced bands which indicates a repeated pattern. These are produced because of areas of electron density. The vertical axis of a strand of DNA does not touch as many points on the molecule as would an axis tilted to the right or left of the vertical axis. The dark areas at the top and bottom of the “X” result from the scattering of the electrons from the nucleotide base pairs.
Mathematical formulas are used to make calculations from the raw data. An x-ray crystallographer will be able to determine the distance between adjacent atoms using these formulas and measurements of the angles shown in the x-ray photographs. Thereafter, they can begin to generate a model for structure which is based on the actual raw data.
This is just one of many different techniques that uses light to determine composition and/or structure of molecules. There are also many other techniques and protocols for manipulating DNA. Recombinant DNA technology has swept the planet and so many biotechnological industries are profiting from the work of bacteria! The Human Genome Project has been in effect for quite some time, and has taken on the task of sequencing all estimated 3 billion nucleotide pairs of human DNA. Any and all data is being logged in with a gene bank, and credit and patents are granted when appropriate. Soon we will know the identity of every gene in our genome, and this will lead to all new technologies for genetic enhancement and engineering. Then will come the quest to determine what the function is of all these genes. Genetic screening has been available for years, yet now there will be much more to screen for.
