Describe the structure and complementary base pairing of DNA.

DNA, or deoxyribonucleic acid, is a molecule that carries genetic information in living organisms. The structure of DNA is a double helix, which resembles a twisted ladder. This shape is due to the fact that it is made up of two strands that run in opposite directions (anti-parallel) and are connected by complementary base pairs.

DNA is made up of small subunits called nucleotides. Each nucleotide consists of three parts: a sugar molecule (deoxyribose), a phosphate group, and a nitrogen-containing base. There are four different nitrogen bases in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T).

The double helix structure of DNA is formed by the interaction between the sugar-phosphate backbone and the nitrogen bases. The sugar (deoxyribose) and phosphate groups form the backbone or sides of the twisted ladder, while the nitrogen bases form the rungs. The phosphate group of one nucleotide is linked to the sugar molecule of the next nucleotide in a single strand through a covalent bond. This creates a long chain running in one direction, also called the 5' to 3' direction.

In the DNA double helix, the two strands are connected by hydrogen bonds between the nitrogen bases. This connection follows specific rules, called complementary base-pairing rules: adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). The A-T and C-G base pairs form two hydrogen bonds and three hydrogen bonds, respectively. This specific base-pairing ensures that the DNA structure is stable and maintains its shape. To summarize, the DNA structure is a double helix formed from two strands connected by complementary base pairs. These strands are made up of nucleotides, which include a sugar, phosphate, and nitrogen base. The complementary base-pairing rules govern the formation of the DNA double helix and ensure its stability.