By Robert Hazen, George Mason University
Of all the scientific discoveries of the 20th century, none has had a more profound impact on our understanding of the physical universe, and our place in it, than the deciphering of the genetic code, our understanding of how DNA and RNA make proteins. We now understand that all living things pass information from one generation to the next with DNA.
The Genetic Code
DNA is the molecule that contains the genetic code, and all living things use RNA to manufacture the proteins of life. As molecular biologists grow ever more fluent in genetic language, they have learned to edit the genetic code, thus altering life itself.
Genetic engineering—which involves the alteration of a genome: to add, or to subtract, or to alter genes in other ways—exemplifies the promise and the peril of genetic technologies.
Chromosomes and Genes
The behaviors of chromosomes transform the idea of genes from an abstraction to real physical structures inside the cells. We can stain chromosomes, count them, watch them divide during the processes of mitosis and meiosis, separate them using a centrifuge, and study what those chemicals are made of. We can crystallize them and can actually study their composition in their crystal structure—that structure is the double-helix structure of DNA.
Each chromosome, we now know, carries thousands of genes, thousands of individual sets of instructions. Each of those instruction codes for one protein, so the genetic code is really nothing more than a chemical procedure for controlling the productions of proteins in every living thing.
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DNA
Biological information is stored in deoxyribonucleic acid, or DNA, which makes chromosomes, and also the genes, which are the segments of chromosomes where the action really is.
DNA is a ladder-like structure; it’s a structure that carries information in its sequence of four different letters: A, C, G, T. These letters, which form pairs, make the rungs of the ladder. The pairing of bases—this combination of A and T, or C and G—means that the double-helix strand can separate, and then duplicate itself during cell division. Hence, one DNA double-helix strand becomes two double-helix strands in this process.
Every single cell in our body carries the same genetic information. Every cell can be traced, by cell division, back to that fertilized egg, with 23 chromosomes from our mother and 23 chromosomes from our father.
RNA
The genetic code converts the four-letter alphabet of DNA—that is, A, C, G and T—into a correct sequence of amino acids, and it uses ribonucleic acid, or RNA, to do this.
RNA’s structure is closely related to DNA; they’re both made of nucleic acids, but RNA is a single-stranded molecule, as opposed to DNA, which is the double-stranded molecule.
Single-stranded messenger RNA copies the base sequence of DNA. It copies a single gene segment, letter by letter, and then it carries that message to other parts of the cell. Groups of three bases on messenger RNA correspond to one amino acid, so that a DNA sequence corresponds directly to a sequence of amino acids in a protein.
What DNA and RNA Do
Transfer RNA is the molecule that makes this physical link. At one end of the transfer RNA, we have three base pairs, corresponding to three bases on the messenger RNA; on the other end, we have an amino acid. Then the ribosome provides the template for manufacturing the protein from messenger RNA.
As the ribosome spins, and as the messenger RNA feeds through it, one transfer RNA after another alights on the surface, fits into special places, and the amino acids snap together like beads in a string, forming the long chain which folds up into the protein, which functions in our body.
Every living thing, every cell in every living thing, uses the exact same genetic information in the exact same way. This understanding of the identical role of DNA and RNA in every organism has led to new technologies, at least new approaches for diagnosis, and new kinds of medical treatment.
Practical Applications
One key technology is polymerase chain reaction, or PCR, by which tiny amounts of DNA can be amplified. Relying on DNA’s ability to duplicate itself, PCR targets a specific short segment of DNA and copies it over and over and over again.
PCR is used in DNA fingerprinting, which is a technique that allows the identification of an individual from the DNA in a single cell. PCR is also used to target specific DNA sequences associated with various genetic diseases, for specific viruses, and other genetic regions of interest in various sorts of research activities.
The Human Genome Project provided a detailed map of the distribution of genes on all of our 23 chromosomes, as well as the distribution of genes on many other organisms. The mapping was accomplished by identifying the target sequences that are similar on every human genome. These sequence tags, as many as 100,000 of them, are like mile markers that define specific chromosome locations, and can be used to target these specific regions. Sequencing is now an automated process, and it’s one that allows us to learn more and more about the genomes of individuals of different species and eventually many different humans.
Common Questions about DNA and RNA
Deoxyribonucleic acid, or DNA, is the molecule that contains the genetic code. Biological information is stored in the DNA, which makes chromosomes and also the genes, which are the segments of chromosomes where the action really is.
The genetic code converts the four-letter alphabet of DNA—that is, A, C, G and T—into a correct sequence of amino acids, and it uses ribonucleic acid, or RNA, to do this. All living things use RNA to manufacture the proteins of life.
Polymerase chain reaction, or PCR, is used in DNA fingerprinting, which is a technique that allows the identification of an individual from the DNA in a single cell. PCR is also used to target specific DNA sequences associated with various genetic diseases, for specific viruses, and other genetic regions of interest in various sorts of research activities.
