There are two different nucleic acids found in the living world, DNA and RNA, each of them used as the genetic material in some types of organism.
The bases in DNA, in sets of three, code for amino acids in a protein, and this code is common to all forms of life, suggesting it evolved very early.
When DNA is copied, the replication is semi-conservative, with one strand of the original DNA gaining a new second strand to make two new double strands.
In most organisms, DNA is used as the genetic material. It is transcribed to RNA and then this becomes the pattern for a protein, following the genetic code.
Ribonucleic acid (RNA) occurs in our cells in a number of forms: it can be found in eukaryotic cells as messenger RNA, ribosomal RNA and transfer RNA.
A given triplet of bases in DNA always codes for the same amino acid. Other triplets can code for the same amino acid. Some triplets have other functions.
The order of the bases in a strand of DNA can be determined in a variety of ways, usually after PCR has been used to increase the amount of DNA available.
The base sequences of DNA can be determined, but modern DNA sequencing would be impossible without the DNA microarray, YACs and BACs, and bioinformatics.
In 1869, Johann Friedrich Miescher had proposed that all cells' nuclei must have a specific chemistry. This substance came to be known as nucleic acid.
In 1871, Miescher isolated a substance which he called nuclein from the nuclei of white blood cells that was soluble in alkalis but not in acids.
In 1876, Oskar Hertwig and Hermann Fol showed that fertilized eggs have both male and female nuclei, that is the nuclei of both parents make a contribution.
In 1884, Eduard Strasburger suggested, based on studies of fertilization, that the nucleus of the cell must be the carrier of the genetic material.
In 1928, Frederick Griffith discovered that some unknown 'transforming principle' had changed the harmless R strain of Diplococcus to the virulent S strain.
In his 1945 book, 'What Is Life?', Erwin Schrödinger proposed that the genetic material, when discovered, would turn out to be some sort of aperiodic crystal.
Schrödinger's gentle little probes and cryptic questions like "Why are atoms so small?" set the scene for the new generation of molecular biologists.
About the time Erwin Schrödinger decided that the gene had to be an aperiodic crystal, Oswald Avery's team had found what the aperiodic crystal must be made of.
In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty reported an experiment that arose out of Griffith's 1928 work, and pointed to DNA as the key material.
Avery, MacLeod and McCarty used two enzymes, one to break down DNA, and one to break down protein: transformation only happened when the DNA was left intact.
The answer basically is that you can't build something as complex as us out of just a few components. So atoms have to be small, from our point of view.
The other half of Schrödinger's question is "Why are we so much larger than the atoms that we are made of?", and that is probably the more interesting question.
In 1950, Erwin Chargaff discovered a one-to-one ratio of adenine to thymine and guanine to cytosine in DNA samples from a variety of organisms.
The key to understanding DNA structure lies in Chargaff's rules from 1948 about the linkages between adenine and thymine and between cytosine and guanine.
Chargaff first showed that the number of guanine units equals the number of cytosine units and the number of adenine units equals the number of thymine units.
In 1951, Lederberg and Zinder showed that bacteria can exchange genes indirectly by transduction, when a virus carries genes into the next cell it infects.
In 1952, Rosalind Franklin used X-ray diffraction to study the structure of DNA and suggested that its sugar-phosphate backbone was on its outside.
In 1952, Alfred Hershey and Martha Chase used bacteriophages in their 'blender experiments' to establish with certainty that DNA was the genetic material.
Hershey and Chase used protein that was labelled with sulfur 35 and DNA labelled with phosphorus 32 for their final proof that DNA carried the information.
In 1954, George Gamow proposed that the genetic code must be made of triplets of nucleotides, based on the argument that 2 was not enough, 4 would be too much.
In 1956, Arthur Kornberg discovered DNA polymerase, and used this enzyme to show that DNA is always constructed in a single direction, the 5' to 3' direction.
In 1957, Francis Crick and George Gamow worked out the 'central dogma' of genetics, explaining how they considered that DNA must function to make protein.
Crick and Gamow proposed their 'sequence hypothesis', which said in effect that the DNA sequence specifies the amino acid sequence in a protein.
In 1958, Matthew Meselson and Franklin Stahl demonstrated semiconservative replication in DNA using 15N and ultracentrifugation in a density gradient.
In 1959, François Jacob and Jacques Monod proposed the role of RNA in transmitting information to the sites of protein synthesis, the repressor-operon model.
They also suggested that genetic information flows only in one direction, from DNA to messenger RNA to protein, the central concept of the central dogma.
In 1961, Marshall Nirenberg built a strand of RNA, composed entirely of uracil, and determines that the codon, the genetic code for phenylalanine was UUU.
In 1965, small supernumerary chromosomes called plasmids, were seen to carry genetic material between bacteria, including genes for antibiotic resistance.
In 1966, Marshall Nirenberg and H. Gobind Khorana led teams that cracked the genetic code, finding what base combinations code for which amino acids.
In 1968, Fred Sanger used radioactive phosphorus as a tracer to decipher a 120 base long RNA sequence, using a complicated piece of chromatography.
In 1970, Howard Temin and David Baltimore independently discovered reverse transcriptase enzymes that produce DNA from RNA, going against the usual pattern.
In 1973, Annie Chang and Stanley Cohen show that recombinant DNA molecules can be maintained and replicated in E. coli: the first recombinant DNA organism.
In 1974, Manfred Eigen and Manfred Sumper showed that mixtures of nucleotide monomers and RNA-replicase gives RNA molecules which replicate, mutate, and evolve.
In 1977, Fred Sanger and his team sequenced the entire phage X174 virus, base by base, all 5386 bases of it, in a single circular strand of DNA.
In 1983, the complete 48,502 base pair sequence of the linear double-stranded DNA of a virus, the temperate E. coli bacteriophage lambda, was published.
In 1985, Kary B. Mullis published a paper describing the polymerase chain reaction (PCR), the most sensitive assay for DNA which has yet been devised.
In 1989, Alec Jeffreys coined the term 'DNA fingerprinting' and was the first to use DNA polymorphisms in paternity, immigration, and murder cases.
In 1990, Mary Claire King reported the discovery of the gene linked to breast cancer in families with a high degree of incidence before age 45.
In 1990, Michael Fromm reported the stable transformation of corn using a high-speed gene gun to introduce new and desirable genes into the nucleus.
In 1997, Dolly the sheep was the first higher mammal cloned from a single adult cell when a prepared nucleus from an adult cell was added to an enucleated ovum.
In 1997, The first-ever completed genome was published in Nature, the genome of the yeast, Saccharomyces cerevisiae. It was published as a separate supplement.