The macromolecule we now call DNA was first discovered my Friedrich Miescher while working in the laboratory of Felix Hoppe-Seyler in about 1868.

After a long line of search, and much investigation, it was eventually found that this new, mysterious, material contained sugars, phosphate and nitrogen containing bases. However, very little was known for a long time concerning how these components were put together - or even what this substance did in the cells!

By 1934, however, Torbjorn Casperson had reported the astonishing fact that complexes of nucleic acids were very long molecules indeed. Evidence was also coming to light that there were some strange regularities in the amounts of the nucleotide bases. What did it all mean?

The full three dimensional structure of the DNA molecule was finally revealed in 1953 by James Watson and Francis Crick, and shown to be double helix. This structure explained a lot about the potential mechanisms of DNA replication, but nothing about how it carried the codes for life on earth.

But DNA does not lie unprotected in the body of a cell. In eukaryotes the DNA is packaged into structures so large they can be seen with a light microscope. These are the chromosomes, whose dynamic structure changes throughout the cell cycle and is slowly being discovered.

Clever experiments carried out by growing bacteria in heavy isotopes of atoms used by the DNA molecule showed that, broadly, the mechanism of DNA replication involved a pattern of semiconservative replication. This set the sceen for more research into the complete mechanism where by genetic information is copied.

Replicating chromosomes and their DNA is a complicated process involving many steps and stages, all of which are now recognized, if not completely understood.