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DNA (deoxyribonucleic acid) is the molecule that carries the genetic information of all living organisms. It is a double-stranded helix made of nucleotides — each consisting of deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (A, T, G, C). The two strands are held by complementary base pairing: A:T (2 H-bonds) and G:C (3 H-bonds). DNA encodes every protein and regulatory element in the cell. The central dogma describes the flow: DNA → RNA (transcription) → Protein (translation).

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DNA Double Helix Structure

Watson and Crick described the double helix in 1953: two antiparallel strands wound around a common axis; sugar-phosphate backbone outside; bases paired inward. The helix turns every 10.5 bp (3.57 nm). Major groove (transcription factor binding) and minor groove provide regulatory access.

Base Pairing (Chargaff's Rules)

  • A pairs with T via 2 H-bonds; G pairs with C via 3 H-bonds
  • [A]=[T] and [G]=[C] in any dsDNA; GC-rich regions have higher melting temperature
  • Complementarity is the basis for PCR, hybridization, replication, and transcription

DNA Replication

Semiconservative: each new double helix contains one original and one new strand. Enzymes: helicase (unwinds); primase (RNA primer); DNA polymerase III (synthesizes 5'→3'); ligase (seals nicks). Leading strand synthesized continuously; lagging strand in Okazaki fragments (5'→3').

Central Dogma

DNA → RNA (transcription by RNA polymerase, template read 3'→5') → Protein (translation at ribosome). mRNA is processed (5' cap, poly-A tail, intron splicing) before export. Reverse transcription (RNA → DNA) occurs in retroviruses.

Glossary

DNA (Deoxyribonucleic Acid)
A double-stranded helix of nucleotides encoding genetic information; bases pair A:T and G:C; the template for RNA synthesis (transcription) and self-replication.
Complementary Base Pairing
A:T (2 H-bonds) and G:C (3 H-bonds) in DNA; the molecular basis for replication, transcription, PCR, and all hybridization-based molecular biology techniques.
Central Dogma
The flow of genetic information: DNA → RNA (transcription) → Protein (translation); DNA also replicates itself; reverse transcription (RNA → DNA) occurs in retroviruses.

Frequently Asked Questions

DNA is a double-stranded helix of two antiparallel polynucleotide chains. Each chain has a sugar-phosphate backbone with bases extending inward. Bases pair specifically: A:T (2 H-bonds) and G:C (3 H-bonds). The strands run antiparallel — one 5'→3', the other 3'→5'. The helix has a major and minor groove where regulatory proteins bind. The human genome contains ~3 × 10⁹ base pairs packaged in 23 chromosome pairs.

DNA replication is semiconservative — each new double helix contains one original and one new strand. Steps: (1) Helicase unwinds the helix at replication forks. (2) Primase synthesizes short RNA primers. (3) DNA polymerase III extends from primers, adding nucleotides 5'→3'. (4) The leading strand is synthesized continuously; the lagging strand in Okazaki fragments. (5) DNA polymerase I replaces RNA primers with DNA. (6) DNA ligase seals nicks. The entire human genome replicates in ~8 hours via thousands of simultaneous origins.

The central dogma describes the flow of genetic information: DNA → RNA (transcription) → Protein (translation). DNA also replicates itself. Transcription: RNA polymerase reads the DNA template strand 3'→5' and synthesizes mRNA 5'→3'. mRNA is processed (5' cap, poly-A tail, splicing) before export to the cytoplasm. Translation: ribosomes read mRNA codons (triplets) and assemble the corresponding amino acid chain using tRNA adaptors. The genetic code is universal — the same codons specify the same amino acids in virtually all organisms.

Core DNA techniques: PCR — amplifies specific sequences using primers and thermocycling; used in diagnostics, genotyping, cloning. Gel electrophoresis — separates DNA fragments by size through agarose. Restriction digestion — restriction enzymes cut DNA at specific sequences for cloning and mapping. Sanger sequencing — determines exact nucleotide sequence. NGS (next-generation sequencing) — massively parallel sequencing of millions of fragments for whole-genome sequencing, RNA-seq, and ChIP-seq. Southern blotting — hybridization-based detection of specific sequences in genomic DNA.