DNA, or deoxyribonucleic acid, is the universal genetic material that stores and transmits the hereditary information essential for life. It is the blueprint for every protein your body produces and the foundation of all biological diversity.
DNA is a long polymer of nucleotides. Each nucleotide contains a five‑carbon sugar (deoxyribose), a phosphate group, and a nitrogenous base. The four bases—adenine (A), cytosine (C), guanine (G), and thymine (T)—are divided into purines (A, G) with two rings and pyrimidines (C, T) with one ring.
In nature, DNA exists as two complementary strands that pair via hydrogen bonds to form the classic double helix. The helix’s two strands run antiparallel, meaning one strand runs 5’→3’ while the other runs 3’→5’.
Adenine pairs exclusively with thymine (A–T) and cytosine pairs exclusively with guanine (C–G). This one‑to‑one pairing ensures that if one strand’s sequence is known, the complementary sequence can be deduced.
In prokaryotes, DNA is free in the cytoplasm; in eukaryotes, it resides within the nucleus, organized into 46 chromosomes (23 from each parent). Chromosomes are visible under a microscope and are numbered 1–22, plus sex chromosomes X and Y. Corresponding chromosomes from each parent are homologous.
Both mitochondria and chloroplasts contain their own DNA, reflecting their bacterial ancestry. Mitochondrial DNA (mtDNA) is inherited exclusively from the mother, as it is packaged in the egg cell.
Before cell division, each DNA molecule is replicated in a semiconservative manner: the double helix unwinds, and each strand serves as a template for a new complementary strand. The result is two identical DNA molecules, each containing one original and one newly synthesized strand.
Genes are composed of exons (coding sequences) and introns (non‑coding sequences). During transcription, a strand of messenger RNA (mRNA) is synthesized from the DNA template, replacing thymine with uracil.
Before translation, the pre‑mRNA undergoes splicing: introns are removed and exons are joined to produce a mature, coding mRNA that can be translated into a protein.
Transcription takes place in the nucleus, where RNA polymerase binds to a promoter region and synthesizes a complementary RNA strand. The mature mRNA exits the nucleus and binds to a ribosome, where translation converts the nucleotide sequence into a polypeptide chain.
The iconic double‑helix model was proposed by James Watson and Francis Crick in 1953, building on key data from Erwin Chargaff (base‑pair ratios), Rosalind Franklin (X‑ray diffraction images), and numerous other researchers. Their synthesis of experimental evidence and theoretical modeling illuminated the structure that underlies heredity.
Understanding DNA’s structure and function remains a cornerstone of genetics, biotechnology, and medicine.
