By Robert Hazen, George Mason University
Every organism stores genetic information in the four-letter DNA alphabet, and every organism uses RNA: messenger RNA to carry that gene message, transfer RNA to link up genetic information to amino acids, and finally, ribosomal RNA to assemble the proteins. The exact same genetic code is used by every organism, so human genes can be inserted into any other life form.
In the past half century, molecular biologists have learned to read the genetic code, and they have come to recognize that every individual has a unique genome. The human genome is an incomprehensibly complex array of some 80,000 genes, distributed on 23 pairs of chromosomes. The power of molecular genetics lies in its potential: we can affect everyone’s life by understanding this unique genetic code that we each carry with us. Scientists now have the ability to differentiate the unique genetic pattern of almost every human individual, often on the basis of DNA contained in just a few cells.
Genetic fingerprinting makes it possible to identify an individual from the genetic material in just a single cell. Now, single techniques have begun to tell us a person’s genetic predispositions as well, perhaps months before they’re even born.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Differences in Genetic Code
All humans have the same set of genes; they’re arranged in the same way on our 23 pairs of chromosomes. But among these three billion base pairs, millions of individual differences exist between any two of us.
Typically, once in every few hundred, maybe once in every thousand base pairs, there will be a difference, and that’s what makes us unique. Some of these differences are insignificant; they’re hardly noticed because they’re just noise in the background of genetic information. But other differences are responsible for things like inherited diseases, things that can change our life, things that can lead to premature death or debilitating disease. We are all different, none of us is alike, and that is because of these differences in the genetic code.
DNA fingerprinting is now used routinely to set free the innocent and convict the guilty. Our ability to manipulate and analyze tiny amounts of DNA rests on a simple and elegant technique. It’s called the polymerase chain reaction, or PCR for short. This technique also plays a very important role in understanding how a gene works; it documents the protein structure for which it codes.
The first step is to find the location and the sequence on a strand of DNA. In some cases, the DNA sequence exactly matches the protein, that is, every triplet of bases matches one amino acid in the protein. In such a case, determining the protein structure is straightforward.
But remember that in many cases, the structure of a gene is very complex. We may have breaks in the sequence; we may have things called introns, so we have to have a variety of editing instructions in the original chromosome to get the messenger RNA and then to make the protein.
Just reading the sequence of letters in DNA isn’t enough; we have to know the editing instructions. Cellular mechanisms have to remove these intervening sequences, and they have to splice the complete gene together before we can make a correct messenger RNA. That’s something the cell does very efficiently, but it’s something that we don’t know how to do yet. Since we don’t know how to do it, we have to trick the cell into doing it in a different way.
We actually can take an RNA, a messenger RNA strand, which is all set to make the protein, and use that and reverse transcriptase to go back and make the DNA; then we can use the polymerase chain reaction to duplicate that.
So, we let the cell make our messenger RNA for us, because the cell knows how to do it, even if we don’t. Then we can take the messenger RNA, and we can apply this special protein, reverse transcriptase, which converts messenger RNA back into DNA—something that viruses are very good at—and now we have a complete strand of DNA that exactly matches the gene. It doesn’t have any of that interfering intron material in it. We multiply that with polymerase chain reaction, and then we can sequence the DNA, and determine the sequence of amino acids. That can then tell us something about the structure and the function of the protein.
Here we must consider one of the greatest ethical challenges spawned by our growing understanding of the human genome. This newfound ability to read an individual’s unique genetic heritage, and our growing understanding of the specific functions of various genes, leads inevitably to ethical questions about how we control, manipulate, and store this information—who gets to see it, and so forth.
We can certainly admit that each person’s interests and abilities and behavior arise from a very complex intermix of environment, background, and also genetic attributes. It’s undeniable that genetic factors play a role in who we are and what we do. Some of these factors are obvious. For example, there are some genetic diseases that are invariably fatal by a certain age. But there are other traits, such as height, musical ability, sex drive, which are much more subtle; and how much those are environmental concerns and our background and experience when you we are growing up, as opposed to specific genetic controls, is very, very hard to say.
Common Questions about DNA Fingerprinting
Genetic fingerprinting makes it possible to identify an individual from the genetic material in just a single cell. Scientists now have the ability to differentiate the unique genetic pattern of almost every human individual, often on the basis of DNA contained in just a few cells.
PCR is short for polymerase chain reaction. It is a technique that lets scientists manipulate and analyze tiny amounts of DNA. It also plays a very important role in understanding how a gene works; it documents the protein structure for which it codes.
This newfound ability to read an individual’s unique genetic heritage, and our growing understanding of the specific functions of various genes, leads inevitably to ethical questions about how we control, manipulate, and store this information—who gets to see it, and so forth.