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DNA (deoxyribonucleic acid) is the molecule that carries the genetic code in living organisms. It is found in most cells (such as skin, muscle, and nerve cells) and contains instructions that cells use to make proteins, many of which are enzymes. DNA is inherited from parents, so children share traits with them; a person’s DNA is a combination of DNA from both parents. Structurally, DNA is a double helix made of two polynucleotide strands, with “rungs” formed by nucleotide base pairs. In DNA, adenine (A) pairs only with thymine (T), and cytosine (C) pairs only with guanine (G), held together by hydrogen bonds. This base-pairing creates a code written in the order of bases. Genes are sections of DNA that contain instructions for making proteins. Because DNA stays in the nucleus, cells copy DNA information into RNA (such as mRNA), which ribosomes read using codons (triplets of bases) to assemble amino acids into proteins; tRNA helps match amino acids to codons. DNA can be copied through DNA replication, and errors during copying can cause mutations (such as deletions, insertions, substitutions, or duplications), which may be harmful, neutral, or beneficial and can drive evolution. Cells also use multiple repair mechanisms to fix DNA damage.
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DNA is made of two polynucleotide strands twisted into a double helix, often compared to a ladder spiraled around its length. The sides of the ladder consist of alternating deoxyribose sugars and phosphate groups, while the rungs are pairs of nitrogenous bases. The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Each nucleotide contains a sugar, a phosphate group, and one nitrogenous base. The bases pair according to fixed complementary rules: adenine pairs only with thymine (A–T), forming two hydrogen bonds, while cytosine pairs only with guanine (C–G), forming three hydrogen bonds. Pairing can be written in either direction, such as A–T or T–A. These complementary rules allow DNA to store genetic information in the sequence of its bases and enable accurate replication, because each original strand serves as a template for building a matching new strand.
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