WHAT IS DNA?
Our DNA is the instruction manual for guiding the process by which we develop from a single cell into a complicated being with many different cell types. We each start as a single cell, created by the fusion of a single sperm and egg. From that single cell, we grow into a mature human being comprised of 30 trillion cells. There are several hundred different cell types (estimates range from 200–300 major categories), each with a different function. For example, our intestinal cells help us absorb nutrients from food, our skin cells protect us from the environment, our retinal cells process light so that we can see, and the neurons in our brain help us to think and communicate. The information in DNA helps guide development and function of all the cells that make a human being.
DNA is a polymer made up of four constituent parts—four nucleotide bases (Figure 1). The nucleotide bases are adenine (A), cytosine (C), guanine (G), and thymine (T). The instructions encoded by DNA are determined by the order in which those bases appear (just as the information in this book is determined by the order in which the words and letters appear). DNA is double-stranded and twisted into a spiral or helical shape, hence the common appellation, “the double helix.” The sequence of bases on one strand of DNA is complementary to the sequence of bases on the other strand. This arrangement arises because the bases on the two strands are paired: A (adenine) pairs with T (thymine), and C (cytosine) pairs with G (guanine).
Figure 1. Our DNA is made of four “letters” or (top) that are arranged in a specific order (middle). The bases are paired (middle), and the 6 billion bases are packaged into 46 chromosomes (bottom), 23 from our father and 23 from our mother. Bottom image attribution: By National Cancer Institute [Public domain], via Wikimedia Commons.