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From DNA to People: A Molecular Primer

By Brian W. Simpson



People—like mollusks, ibex, mosquitoes or petunias for that matter—aren't magic. They don't just appear. They're made and remade according to regimented recipes stored in the deoxyribonucleic acid (DNA) contained in their cells. The celebrated double helix not only houses key information about you but also has the power to replicate and translate itself into action. As one expert advises, think of DNA as a blueprint that builds itself into a house and then maintains itself.

A, C, T, G

Each rung of DNA's twisted ladder is made up of a pair of chemical bases—adenine, cytosine, thymine and guanine (known by their letters, A, C, T and G). The bases are a choosy bunch: A pairs only with T, and C pairs only with G. The ladder's vertical struts are long strands of phosphate and sugar molecules. A base and its attached strut comprise a nucleotide.


Long lines of chemical bases on the DNA molecule that code for particular proteins, genes are the basic hereditary units that dictate everything from your hair and eye color to how your body processes sugar and how tall you are. Genes are homebodies. They stay in the nucleus. When one is "expressed," it is transcribed into a molecule called messenger RNA, which hustles out of the nucleus to ribosomes—protein-manufacturing plants in the cell's cytoplasm. The messenger RNA runs through the ribosomes, and the protein molecule is created. Genes also have a great influence on our susceptibility to disease. But keep things in perspective: Genes exist primarily to create you and maintain your health. As science writer Matt Ridley emphasizes in his book Genome, "Genes are not there to cause diseases."


There's a reason researchers dedicate their lives to discovering the ins and outs of particular proteins. Proteins = Life. The 100,000-plus kinds of proteins in humans make virtually everything happen in the body. They comprise muscles and tendons, transport nutrients to the cells, carry oxygen molecules to the body's hinterlands, shuttle messages between cells and act as catalysts for the body's many chemical reactions.


All human life hangs by a thread—46 of them to be exact. Chromosomes, tiny strings of DNA populate the nuclei of cells in the body. They are actually 23 pairs of threads (each pair has contributions from the mother and father). Most chromosomes hold thousands of genes.


The sum total of a living thing's DNA is called its genome. Humans are now believed to have about 25,000 to 30,000 genes. Just don't get cocky about being an oh-so-complex organism. Chimps, mice and even zebrafish have roughly the same number of genes. Even Arabidopsis thaliana, commonly known as mustard grass, has 25,000 genes. The number of genes does not determine your place on evolution's pecking order; it's what's done with them that really matters. Take that, zebrafish!


Now that you've mastered Genomics 101, you think that your genes are your destiny. Case closed, right? Wrong. An emerging field called "epigenetics" (which literally means "on genes") is discovering why the same set of genes in different cells of the body may lead to completely different results. The reason is chemical modifications to a gene have a great influence on whether a gene is expressed or not, or how strongly it is expressed. If, for example, specific molecules known as methyl groups attach themselves to the DNA like pearls on a string, or if the DNA is wound very tightly around globular proteins called histones, then it's difficult for the gene to be expressed. The gene is there, but it's essentially been silenced. This could be a good thing if it's a cancer-causing gene, or a bad thing if the gene helps the immune response.


Researchers spend a lot of time looking for small variations in DNA between people, called single nucleotide polymorphisms or SNPs. When a specific DNA sequence like AAGGTTA is altered to ATGGTTA in a significant portion of the population, it's called a SNP. Researchers often look for such genetic variations between a population affected by a disease and one that is not, using SNPs as guides to help them locate genes that may be related to susceptibility of disease.


An energetic metropolis, each cell in your body contains an all-powerful city hall surrounded by chemical factories, archives, communications centers and power plants. With a very few exceptions, each cell nucleus contains the complete DNA for an organism. But only certain genes are active, depending on the type of cell and its role. Liver cells, for example, activate the genes needed to perform functions such as filtering toxins from the blood. Skin cells, meanwhile, activate a largely different set of genes necessary to create a human's tough hide.

Cell Signaling Pathways

Your body's most important communications are not the thoughts that bounce about in your head, but the messages between and within your cells. Cell signaling pathways—the means of this communications—allow cells to respond to their community. Think of it as hundreds of different circuits all wired to receive specific messages and generate specific responses by the cell. Such pathways are essential to health—and, when they go awry, a major cause of disease.


DNA is serious about organization. After the mystical union of sperm and egg, it directs the countless rounds of cell division and specialization. Cells organize themselves into four types of tissues, which in turn form the body's 10 organ systems—the liver, lungs, brain and so on. And, more or less presto, a human body.


Ironically, one of the best ways for scientists to know what is happening in an individual's body is by studying entire populations. By examining genes, SNPs, proteins, signaling pathways, environmental interactions, pathogens and other factors in many people, researchers are discovering more about the causes of disease.

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