Deoxyribonucleic acid, commonly known as DNA, serves as the genetic material in all living organisms and many viruses. This molecule holds the blueprints for life, guiding development, growth, and reproduction. It transmits genetic information from one generation to the next.
The Basic Building Blocks of DNA
The repeating unit of DNA is called a nucleotide. Each nucleotide is composed of three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. The sugar in DNA is deoxyribose, which differs from the sugar found in RNA by lacking a hydroxyl group at its 2′ carbon position.
The phosphate group attaches to the 5′ carbon of the deoxyribose sugar. The nitrogenous base, a nitrogen-containing ring structure, extends from the 1′ carbon. These bases are categorized into two types: purines and pyrimidines.
DNA contains four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). Adenine and Guanine are purines, characterized by their double-ring structure. Cytosine and Thymine are pyrimidines, which have a single-ring structure. The specific base determines the name of the individual nucleotide.
Assembling the Double Helix
Nucleotides link to form a single DNA strand through phosphodiester bonds. These covalent bonds form between the phosphate group of one nucleotide and the deoxyribose sugar of the next, creating an alternating sugar-phosphate backbone. This backbone provides structural support for the DNA strand, with the nitrogenous bases extending inward.
A double-stranded DNA molecule consists of two polynucleotide strands twisted into a double helix. Hydrogen bonds hold the two strands together, forming between specific pairs of nitrogenous bases on opposing strands. This complementary base pairing dictates that Adenine (A) always pairs with Thymine (T) via two hydrogen bonds, and Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.
The two strands of the DNA double helix run in opposite directions, known as antiparallel. One strand is oriented 5′ to 3′, while the complementary strand runs 3′ to 5′. This antiparallel arrangement supports the stability of the double helix and allows for proper base pairing and biological processes.
How DNA’s Structure Relates to Function
DNA’s double helix structure relates to its functions of storing and transmitting genetic information. The sequence of the four nitrogenous bases along the DNA strand forms the genetic code, which contains instructions for building proteins. Proteins carry out most body functions and determine an organism’s traits.
Complementary base pairing is essential for accurate DNA replication. During replication, the two strands separate, each serving as a template for a new complementary strand. This ensures genetic information is copied and passed to new cells. This structural organization also allows genetic information to be compactly stored within a cell’s nucleus.