By Vera Shively and Katherine Kim, Lurie Children's Division of Genetics, Birth Defects and Metabolism
April 25th is DNA Day. In April 2003 the United States Congress approved the first National DNA Day. It celebrates the 50th anniversary of the discovery of the structure of DNA and the 2003 completion of the Human Genome Project. Today, DNA Day is recognized around the world. It is a great opportunity for everyone to learn more about DNA.
Deoxyribonucleic acid (DNA) is a very large molecule that carries the instructions for an organism. Organisms include plants, animals, fungi, microorganisms and people! Molecules are groups of atoms held together by chemical bonds. The information in DNA is stored as a chemical code comprised of four nucleotide bases: adenine (A), cytosine (C), guanine (G), and thymine (T). We can think of DNA's nucleotide bases as a four letter alphabet. A gene is a long string of DNA that has instructions for making a specific product that an organism needs to grow and function. Parents pass down genes (the DNA code) to their child.
Scientists discovered DNA's three-dimensional structure to be a double helix, resembling a twisted ladder. Once scientists understood the structure of DNA, we were able to start unlocking the mysteries of genes. Each rung of the DNA ladder is made of a base pair. Recall the four nucleotide bases mentioned above, abbreviated by A, C, G and T. These bases pair (bind chemically) in a predictable way. An A always pairs with T, and C always pairs with G.
Since DNA is the basic unit of heredity, it has to be copied from one generation to the next. Understanding DNA structure revealed how this is possible. Notably, the base pairs that form the rungs of the DNA ladder can separate. Each half then acts as a template that allows a full copy of itself to be made. This simple mechanism is how genetic material is copied and passed to the next generation. It also provides a way to access and copy the instructions used to make products organisms need to grow and function.
The Human Genome Project was an international effort to sequence and map the human genome. It took about 12 ½ years to complete. The genome of an organism is the complete set of its DNA instructions. The instructions are coded in a specific order of DNA’s four nucleotides (A’s, T’s, C’s and G’s). The order is called its sequence. DNA sequences are really long. The human genome contains about three billion base pairs and an estimated 30,000 genes.
The Human Genome Project also produced maps to show the locations of genes on chromosomes. Genes are tightly packaged into structures called chromosomes. Mapping where genes are on chromosomes helps scientists track inherited traits over generations. Besides things like eye color and dimples, inherited traits include those associated with genetic disorders. For example, genetic mapping was used to find the gene responsible for disorders such a cystic fibrosis and Duchenne muscular dystrophy. Identifying the gene or genes linked to a disorder furthers the understanding of the disorder and the development of new treatments.
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