Double-Helix Structure of DNA
Image Attribution: 3D helix structure of DNA (public domain)
DNA has a double-helix structure (see images above and below). The sugar and phosphate lie on the outside of the helix, forming the backbone of the DNA. The nitrogenous bases form stacks in the interior, like the steps of a staircase, in pairs.
Hydrogen bonds join the pairs to each other. Every base pair in the double helix has a separation of 0.34 nm from the next base pair. The two strands of the helix run in opposite directions, meaning that the 5′ carbon end of one strand will face the 3′ carbon end of its matching strand. (We refer to this as antiparallel orientation and it is important to DNA replication and in many nucleic acid interactions.)
Native DNA is an antiparallel double helix. The phosphate backbone (indicated by the curvy lines) is on the outside, and the bases are on the inside. Each base from one strand interacts via hydrogen bonding with a base from the opposing strand. Image Attribution: Jerome Walker/Dennis Myts
Base Complementary Rule
Only certain types of base pairing are allowed. For example, a certain purine can only pair with a certain pyrimidine. This means A can pair with T, and G can pair with C, as shown in the image below. This is known as the base complementary rule. In other words, the DNA strands are complementary to each other. If the sequence of one strand is AATTGGCC, the complementary strand would have the sequence TTAACCGG. During DNA replication, each strand is copied, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesized strand.
In a double stranded DNA molecule, the two strands run antiparallel to one another so that one strand runs 5′ to 3′ and the other 3′ to 5′. The phosphate backbone is on the outside, and the bases are in the middle. Adenine forms hydrogen bonds (or base pairs) with thymine, and guanine base pairs with cytosine. Image Attribution: OpenStax Biology