The Structure of DNA
Now that we know where the genes are, let’s look more closely at what genes aremade of, at DNA it self. What does DNA look like? A molecule of deoxy ribonucleic acid, or DNA, consists of two strands that, when put together, resemble a spiraling ladder with two “sides” and a series of regularly spaced “rungs” (Figure 1). Because DNA consists of two strands twisted into a spiral or helix, it is often described as a double helix. Let’s examine a single strand of DNA, and then consider how the two strands ﬁt together.
Each DNA strand has a backbone (or a “side” of the ladder) that is made up of alternating molecules of deoxyribose sugar and phosphate. Attached to this backbone is a series of nitrogenous bases (each represents one half of a “rung”). Only four different nitrogenous bases are used in DNA—adenine (A), guanine (G), cytosine (C), and thymine (T). A single nitrogenous base bound to a single molecule of sugar and a single phosphate is called a nucleotide. So, a strand of DNA can also be described as a string of nucleotides.
Now let’s put the two strands of DNA together. Each base binds with a base on the opposite strand using hydrogen bonds. The binding occurs in a very speciﬁc way—adenine always pairs with thymine (A–T), and guanine always pairs with cytosine (G–C).