Friedrich Miescher, Albrecht Kossel, Phoebus Levene, Frederick Griffith, Oswald Avery, Hershey, Erwin Chargaff, Watson and Crick are scientists who have made great advances in exploration of DNA and genetic material. DNA double helix opens door of molecular biology.

Biography of Friedrich Miescher. He extracted "nuclein" from pus cells via HCl, Na₂SO₄ and ether. This was the earliest crude nucleic acid extract and he was the first man identified DNA in history. He also got protamine from salmon sperm.

Albrecht Kossel discovered that nucleic acids are composed of phosphates, carbohydrates, and the nitrogenous bases—adenine, guanine, thymine, cytosine, and uracil.

Phoebus Levene found a pentose, a phosphate and a base make up a nucleotide which are the basic units of nucleic acids. The ❌ false tetranucleotide hypothesis misled the exploration of nucleic acids for about 40 years, so it is considered a scientific catastrophe.

The classic transformation experiment done by British biologist Frederick Griffith. He discovered that transforming principle will make mild R-type bacteria into virulent, pathogenic S-type bacteria, even if S bacteria was dead.

Oswald Avery, Colin MacLeod, Maclyn McCarty removed polysaccharide, lipid, protein and RNA to get pure transforming principle. Avery experiment proved transforming principle (hereditray material) was DNA.

Chargaff used paper chromatography to accurately measure base ratio in DNA. Chargaff's rule of base pairing: A=T, C=G, A≠G. Base ratio or nucleotide ratio is the same in tissue of same species, but varies in different species.

Hershey-Chase experiment is also called Blender Experiment. It used phages labeled with radioactive sulfur and phosphorus to infect E. coli, ultimately ending debate over whether DNA is the genetic material.