Those ladders of fuzzy black dashes used as evidence in court are just a way of making DNA visible to jurors and other ardent scholars of biochemical science certain things that are too small to see, even with a microscope. They’re the end result of a number of laboratory manipulations that never get explained in the courtroom. But before we describe them, will the real DNA please stand up?
DNA is the most intricate and awe-inspiring substance on Earth, but it is not too hard to understand if we stay away from the big words and stop just this side of more-than-you-want-to-know.
Suppose that you are Mother Nature, and you want to set up a general scheme of life that will work for all living things, both plant and animal. The biggest problem you face is how to get from one generation to another. After all, manufacturing one exquisite rose, cockroach, or horse, no matter how difficult that may be, isn’t going to get you very far unless you give it the power to make more roses, cockroaches, and horses. How, then, can a rose beget a rose? How can a horse inform its offspring that it should be a horse, rather than a blade of grass or a cockroach, having four legs instead of six, no chlorophyll or antennae, and so on, and on and on?
There are an enormous number of explicit specifications that must be noted and carried out to ensure that each succeeding generation follows the same pattern. How has Mother Nature arranged to record and play back, time after time, without benefit of pencil and paper, videotape, or CD ROM, the immense amount of complex information that, taken all together, says “horse”?
Answer: She writes it all down on strips of a remarkable substance called DNA, as if on strips of recording tape.
DNA is a merciful abbreviation for deoxyribonucleic acid. This substance is made up of certain specific clusters of atoms, lined up into long ribbons that are twisted into spirals and then coiled up into compact little packages and tucked into the nuclei of virtually every cell of every part of every life-form on Earth, from six-ton elephants to one-celled bacteria and lawyers.
The information on the DNA ribbons is written in a code. The code consists of the exact sequences in which those clusters of atoms are arranged along the ribbon. If you think of the atom clusters as words, their sequences are sentences. Specific sequences of atom clusters convey specific pieces of information, just as specific sequences of words do in a sentence.
Scientists refer to the atom-cluster “words” as nucleotides and the “sentences” as genes. Each gene “sentence” states an essential bit of information about what the baby horse, or cockroach or human being, shall or shall not be. Genes even distinguish each individual baby from all others. In a single human DNA ribbon, there are so many “words” (a few million, perhaps), combined into so many gene “sentences” (maybe a hundred thousand), that except for identical twins, no two individuals among the five billion people now on Earth, or among all those who have gone before should have exactly the same combination.
Just imagine the odds. If you had a basket with a few million words in it and you reached in blindfolded and picked out enough words, one by one, to make a (rather long) book of a hundred thousand sentences, what do you think your chances are of repeating the process and getting exactly the same collection of words in the same sequence, that is, of getting exactly the same book?
In the case of humans, the odds are even more extreme because of historical and geographical isolation: the odds of exactly duplicating a certain black African baby in a Swedish maternity ward are even slimmer than simple mathematics would indicate.
Aha! Then if every human being on Earth has a unique set of genes on his or her DNA ribbons, can we tell what characteristics an individual has by examining his or her DNA? In principle, yes, except that we haven’t yet worked out the entire sequence of genes on anyone’s DNA. But if DNA is found in every cell in the body, from skin to blood, hair, fingernails, and semen, couldn’t we identify the perpetrator of a crime, for example, by matching a suspect’s DNA with the DNA from cells found at the scene? Definitely. And that’s what forensic DNA analysis is all about.
How do they do it? They extract the DNA from the cell samples and treat it with enzymes that “grow” the DNA make repeated identical copies of it, until there is enough to work with. Other enzymes then cut up the ribbons into various manageable-sized fragments, like cutting up a book into various pages, paragraphs, sentences, and phrases.
Then the technicians spread out all the cut-up fragments according to their sizes (I’ll tell you how) and compare exactly which arrangements of words show up in both of the samples that they want to compare. Same fragments means same DNA and same person. Think about it. If you can cut two books into hundreds of pieces and wind up with even half a dozen identical pages or assortments of paragraphs in exacdy the same order, then by golly you’ve got two copies of the same book. (Or one heck of a case of plagiarism.)
Now, about the infamous black smudges. Those ladders of thick, black lines are made by the fragments of DNA, which have been spread out along a kind of racetrack according to their sizes in an electrical apparatus.
Technicians give the fragments a negative electric charge and allow them to drift slowly along a surface toward a positive electric pole. The smallest, lightest fragments drift fastest and travel farthest, winding up at the top of the ladder when the race ends; heavier fragments lag behind to various degrees. Thus, they are spread out according to their sizes.
The invisibly small groups of separated DNA fragments are made radioactive, so that their radiations will expose spots on a sheet of photographic film, thus visually revealing their final locations on the racetrack. That developed sheet of film, bearing black exposure marks wherever the fragments wound up at the end of the race, is what the scientists compare, thereby comparing the DNA structures of the two samples.
The same finish positions at the end of the race indicate the same DNA and therefore the same individual, with odds that can be as high as hundreds of trillions to one. Of course, there is always a slim chance that the murderer was a horse.