DNA was found in 1869 and ignored as the stuff of heredity for decades
Friedrich Miescher pulled a strange substance out of cell nuclei at Tübingen in 1869 and called it nuclein. Seventy-five years passed before an experiment showed it carried genetic information. For most of that time, biologists treated it as a chemical curiosity while they looked elsewhere for the secret of inheritance.
The chemistry came first. In the early 1880s Albrecht Kossel purified the material further, noticed it was strongly acidic and later identified its bases. Richard Altmann coined the name nucleic acid in 1889, reflecting where it was found and the phosphate groups that make it acidic; nobody yet distinguished DNA from RNA. Phoebus Levene worked out the basic building plan. In 1938 William Astbury and Florence Bell produced the first X-ray diffraction image of DNA, and in 1944 the Avery–MacLeod–McCarty experiment finally tied it to heredity. The double helix of Watson and Crick followed in 1953.
The building blocks are modest. Each nucleotide is a five-carbon sugar, a phosphate and a nitrogen-containing base. The sugar decides the family: deoxyribose makes DNA, while ribose, which carries one extra hydroxyl group, makes RNA. Adenine, cytosine and guanine appear in both; thymine is found only in DNA and uracil only in RNA. Sugars and phosphates alternate along the backbone, giving each strand a direction, and in a double helix the two strands run opposite ways.
Sizes span a staggering range. Small interfering RNA molecules are just 21 nucleotides long, whereas human chromosome 1 is one continuous molecule of about 247 million base pairs, and DNA molecules are probably the largest single molecules known. Shapes vary too. Bacterial chromosomes and the DNA in mitochondria and chloroplasts are usually closed loops, while the chromosomes in a nucleus are linear.
The textbook rule that DNA is double-stranded and RNA single-stranded has exceptions. Some viruses carry double-stranded RNA and others single-stranded DNA, and under certain conditions three or four strands can wind together. Nearly every living cell holds both kinds, mature red blood cells being a notable exception, while a virus usually carries only one. Today laboratories around the world read hundreds of millions of nucleotides every day, and chemists have built artificial versions with altered backbones, such as peptide and locked nucleic acids.
Source: Nucleic acid
