DNA

Deoxyribonucleic acid (DNA) is a biological macromolecule composed of two long polynucleotide chains that coil around each other to form the characteristic double helix. As the primary carrier of genetic information in nearly all known organisms and many viruses, DNA underpins the development, functioning, growth, and reproduction of living systems. Together with RNA, proteins, lipids, and complex carbohydrates, DNA is one of the essential classes of biological macromolecules.

Nucleotide Composition and Double-Helical Structure

Each strand of DNA is built from repeating nucleotide units. A nucleotide consists of three components: a nitrogenous base, a pentose sugar known as deoxyribose, and a phosphate group. The bases fall into two families: the purines—adenine (A) and guanine (G)—and the pyrimidines—cytosine (C) and thymine (T). These bases attach to the sugar–phosphate backbone, which is formed through phosphodiester bonds linking the 5′ carbon of one sugar molecule to the 3′ carbon of the next.
The two polynucleotide strands align in an antiparallel orientation, meaning one strand runs in the 5′ to 3′ direction and the complementary strand runs 3′ to 5′. Base pairing between the strands follows strict rules: adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine via three hydrogen bonds. These interactions stabilise the double helix alongside base-stacking forces generated by aromatic nucleobases.
DNA adopts multiple conformations in nature, but the most common form is B-DNA, a right-handed helix featuring major and minor grooves. These grooves differ in width, with the major groove providing greater accessibility for protein binding. Transcription factors and other DNA-binding proteins typically recognise specific sequences by interacting with the exposed edges of bases within the major groove.

Genetic Information and Its Expression

The sequence of nucleobases along the DNA backbone encodes genetic information. During replication, the two strands of DNA separate, and each serves as a template for synthesising a complementary strand. This semiconservative mechanism ensures faithful inheritance of genetic information during cell division.
Only a small proportion of DNA in complex organisms encodes proteins; in humans, less than 2% of the genome is protein-coding. The remainder consists of regulatory sequences, noncoding RNAs, structural elements, and regions with unknown or evolving functions.
Gene expression involves two key processes. In transcription, RNA polymerase uses one DNA strand as a template to produce RNA, substituting uracil (U) for thymine. The RNA strand then guides protein synthesis through translation, during which ribosomes decode the sequence into specific amino acids, forming polypeptides according to the genetic code.

Chromosomal Organisation in Cells

In eukaryotes—including animals, plants, fungi, and protists—DNA is arranged into linear chromosomes located within the nucleus. Before cell division, each chromosome is duplicated to ensure accurate distribution to daughter cells. Eukaryotic cells also contain DNA in their mitochondria and, in plants and algae, in chloroplasts.
Within the nucleus, DNA associates with histone proteins to form chromatin. Chromatin organisation plays an essential role in regulating gene accessibility and governing interactions between DNA and gene-regulatory proteins.
By contrast, prokaryotic organisms such as bacteria and archaea store their DNA in the cytoplasm, typically as circular chromosomes without surrounding nuclear membranes.

Physical Properties and Molecular Dynamics

DNA is a long, flexible polymer whose structure can vary depending on environmental conditions. The double helix is stabilised by hydrogen bonding and base-stacking interactions. Each complete turn of B-DNA spans approximately 3.4 nanometres, and the helix has a diameter of about 2 nanometres. DNA’s density, typically around 1.7 g/cm³, allows it to sediment predictably under suitable laboratory conditions.
The molecule is dynamic and capable of adopting alternative shapes, such as loops and supercoils, that are essential for packaging within cells. The differential exposure of bases in the major and minor grooves creates specific binding sites for regulatory proteins and enzymes involved in replication, repair, and transcription.

Chemical Variation and Modified Bases

In addition to the standard nucleobases, DNA may contain noncanonical bases, many of which arise from enzymatic modification of canonical bases. These modified bases influence gene regulation, genetic stability, and defence against viral infection.
Examples include:

• 5-methylcytosine, a key epigenetic marker associated with transcriptional regulation
• N6-methyladenine, found in certain bacteria and involved in restriction–modification systems
• Several modified guanine, cytosine, and thymine derivatives observed in bacteriophages and specialised organisms

Uracil, a pyrimidine normally present in RNA, also appears in modified forms within DNA in certain biological contexts.
These chemical modifications are central to epigenetics, contributing to the control of gene expression and the establishment of heritable regulatory states.

Structural Features: Grooves and Binding Sites

The spatial arrangement of the two DNA strands generates major and minor grooves along the helix. These grooves follow the twist of the molecule and vary in size due to the asymmetrical positioning of the sugar–phosphate backbones. The major groove is wider, providing better access to the hydrogen-bonding edges of bases and enabling highly specific interactions with DNA-binding proteins.
Although unusual DNA conformations can alter groove dimensions, the terms “major” and “minor” remain consistent, referring to the relative sizes that would be observed in B-DNA.

Role of DNA in Biological Systems

DNA serves as the universal repository of hereditary information, ensuring the continuity of life across generations. Its double-helical structure, precise base-pairing mechanisms, capacity for error correction, and ability to undergo controlled modifications make it a robust and adaptable molecule suited to both stability and change.
Through its interactions with proteins, RNA, and cellular structures, DNA directs the synthesis of molecules that form the structural, enzymatic, and regulatory foundations of life. Its properties illustrate the intricate relationship between molecular form and biological function, highlighting DNA’s central place in genetics, biochemistry, and evolutionary biology.