Deoxyribonucleic acid, commonly known as DNA, serves as the fundamental instruction manual for all known forms of life. It carries the genetic information that guides the development, functioning, growth, and reproduction of living organisms. Understanding its composition is important for comprehending how biological information is stored and transmitted across generations. Nearly every cell in an organism contains DNA, which dictates unique characteristics and biological processes.
The Fundamental Unit of DNA
DNA is a large biological molecule, or polymer, built from many smaller, repeating units called nucleotides. Each nucleotide serves as a monomer, a single molecular unit that can link with others to create the extensive DNA polymer.
The Three Molecular Pieces
Each nucleotide is composed of three distinct chemical components: a phosphate group, a five-carbon sugar called deoxyribose, and a nitrogenous base. The phosphate group connects to the sugar, which links to the nitrogenous base. The nitrogenous base is the variable part, carrying specific genetic information.
The Genetic Alphabet
The informational content of DNA resides in its four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). These bases function as the “letters” of the genetic code, with their specific sequence determining genetic instructions. They are categorized into two groups based on chemical structure: Adenine and Guanine are purines (double-ring), while Cytosine and Thymine are pyrimidines (single-ring).
Building a Single DNA Strand
Individual nucleotides connect to form a single DNA strand. This connection creates a “sugar-phosphate backbone,” an alternating chain of deoxyribose sugars and phosphate groups that forms the strand’s structural support. The nitrogenous bases extend inward from this backbone. Nucleotides are linked by strong covalent phosphodiester bonds, which form between the phosphate group of one nucleotide and the sugar of the next. This linkage creates directionality, referred to by its 5′ (five-prime) and 3′ (three-prime) ends.
Forming the Double Helix
The full DNA molecule consists of two single strands coiled around each other, forming the double helix structure. This structure resembles a twisted ladder, with sugar-phosphate backbones as uprights and nitrogenous bases as rungs. The two strands are held together by specific pairing rules: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). These pairings, known as complementary base pairing, are stabilized by hydrogen bonds between the bases. The two strands also run in opposite directions, a characteristic known as antiparallel orientation.