By Sukhsatej Batra, Ph.D. | Updated March 24, 2022
DNA is composed of nucleotides—adenine (A), cytosine (C), guanine (G), and thymine (T). Each nucleotide contains a phosphate group, a deoxyribose sugar, and a nitrogenous base. Purines (adenine and guanine) have a double-ring structure, while pyrimidines (cytosine and thymine) possess a single-ring structure.
The backbone of the DNA double helix is formed by alternating sugar and phosphate groups, creating a ladder-like structure. Complementary base pairing—adenine with thymine via two hydrogen bonds, cytosine with guanine via three hydrogen bonds—holds the two strands together, ensuring accurate replication and transcription. The sugar‑phosphate backbone imparts the characteristic helical twist.
The Human Genome Project mapped roughly 3 billion base pairs, encoding about 20 000 protein‑coding genes across 23 chromosome pairs. This comprehensive sequence enables clinicians to diagnose genetic disorders, researchers to develop targeted therapies, and forensic scientists to identify individuals.
If all the DNA in a single human body were stretched end-to-end, it would span approximately 70 times the distance from Earth to the Sun—an astonishing demonstration of molecular scale versus cosmic distances.
