Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life

This section presents an introductory and therefore incomplete overview of DNA.

  • Genes can be loosely viewed as the organism's "cookbook" or "blueprint";

  • A strand of DNA contains genes, areas that regulate genes, and areas that either have no function, or a function we do not (yet) know (also see last bullet point in this section for the difference between DNA and RNA);

  • DNA is organized as two complementary strands, head-to-toe, with bonds between them that can be "unzipped" like a zipper, separating the strands;

  • DNA is a chain of chemical "building blocks", called "bases", of which there are four types: these can be abbreviated A, T, C, and G. Each base can only "pair up" with one single predetermined other base: A+T, T+A, C+G and G+C are the only possible combinations; that is, an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand;

    • N.B.: U occasionally replaces T, notably in PBS1 phage DNA; you can thus substitute "U" for "T" throughout this section.

  • Because each strand of DNA has a directionality, the sequence order does matter: A+T is not the same as T+A, just as C+G is not the same as G+C;

  • For each given base, there is just one possible complementary base, so naming the bases on the conventionally chosen side of the strand is enough to describe the entire double-strand sequence;

  • The genetic information contained in a strand of DNA is determined by the sequence of bases along its length;

  • The cell begins DNA replication by forcibly unzipping the DNA double strand down the middle, and then recreates the "other half" of each new single strand by drowning each half in a "soup" made of the four bases. An enzyme makes a new strand by finding the correct "base" in the soup and pairing it with the original strand. In this way, the base on the old strand dictates which base will be on the new strand, and the cell ends up with an extra copy of its DNA.

  • Mutations are simply chemical imperfections in this process: a base is accidentally skipped, inserted, or incorrectly copied, or the chain is trimmed, or added to; many basic mutations can be described as combinations of these accidental "operations". Mutations can also occur through chemical damage (through mutagens), light (UV damage), or through other more complicated gene swapping events.

  • DNA (for DeoxyriboNucleic Acid) differs from RNA (for RiboNucleic Acid) by having the sugar 2-deoxyribose instead of ribose in its backbone (ribose contains one extra oxygen atom compared to deoxyribose -- in other words, DNA contains deoxygenated ribose, whereas RNA contains "plain" ribose.) This is the basic chemical distinction between RNA and DNA.

    DNA is a long polymer of nucleotides and encodes the sequence of the amino acid residues in proteins using the genetic code, a triplet code of nucleotides.

    In complex cells (eukaryotes), such as those from plants, animals, fungi and protists, most of the DNA is located in the cell nucleus. By contrast, in simpler cells called prokaryotes (the eubacteria and archaea), DNA is not separated from the cytoplasm by a nuclear envelope. The cellular organelles known as chloroplasts and mitochondria also carry DNA.

    DNA is often referred to as the molecule of heredity as it is responsible for the genetic propagation of most inherited traits. These traits can range from hair colour to disease susceptibility. During cell division, DNA is replicated and can be transmitted to offspring during reproduction. Lineage studies can be done based on the facts that the DNA in mitochondria (mitochondrial DNA) only comes from the mother, and the male "Y" chromosome only comes from the father.

    Every person's DNA, their genome, is inherited from both parents. The mother's mitochondrial DNA together with twenty-three chromosomes from each parent combine to form the genome of a fertilized egg. As a result, with certain exceptions such as red blood cells, most human cells contain 23 pairs of chromosomes, together with mitochondrial DNA inherited from the mother.