Gregor Mendel discovered that hereditary is transmitted in units. His experiments on peas demonstrated that heredity is transmitted in discrete units. The understanding that genes remain distinct entities even if the characteristics of parents appear to blend in their children helps to explain how natural selection works and provides support for Darwin's Theory of Evolution.
Friedrich Mischer was the first scientist to to identify DNA as a distinct molecule, while investigating the nuclei of leukocytes found in pus. At the time scientists believed that cells were made largely of proteins. He showed that this substance was derived from the nucleus of the cell alone and therefore named it "nuclein". He started the theory that nuclein was the basis of heredity.
Thomas Hunt Morgan discovered the chromosome theory by studying fruit fly chromosomes, specifically the characteristic of eye colour in their offspring. He observed the transmission of genetic traits through successive generations and was able to definitively link the inheritance of a specific trait with a particular chromosome.
Oswald Avery discovered that DNA carries a cell’s genetic material and can be altered through transformation. When isolated from one strain of bacteria, DNA was able to transform another strain and confer characteristics onto that second strain. His research led to the understanding of the genetic code.
McClintock discovered transposition and used this information to demonstrate that genes are responsible for the different physical characteristics found within families. Using a maize plant experiment, she worked on developing theories to show the suppression and expression of genetic information from one generation to another.
Erwin Chargaff discovered rules to prove that DNA had a double helical structure, known today as 'Chargaff's rules'. The 1st rule stated that DNA had an equal percentage of adenine to thymine and an equal percentage of guanine to cytosine. The 2nd rule said that adenine and thymine, and guanine and cytosine, are always paired together and cannot be cross-paired.
Franklin & Wilkins took photos of the structure of DNA using x-ray crystallography, that showed the helical form of a DNA molecule. Their findings were later used by Watson and Crick to form their 3D model of DNA.
Watson & Crick used x-ray crystallography images of DNA to outline it's structure. They determined that the structure of DNA was a double-helix polymer, or a spiral of two DNA strands, with each containing a long chain of monomer nucleotides wound around each other. They built the first 3D model of DNA, outlining the individual chemical components of the four bases and other nucleotide subunits. There was later some controversy around this discovery, as the images produced by the x-ray crystallography were used without the permission of the original photographer Rosalind Franklin.
Frederick Sanger found that DNA contains thousands of small chemical units called nucleotides and developed techniques for the sequencing of nucleotides. He laid the foundation for sequencing DNA in 1955, and by completing the sequencing of the amino acids in insulin. This provided the first conclusive evidence that proteins were chemical entities with a specific molecular pattern rather than a random mixture of material suspended in fluid. By 1977 he sequenced approximately 5000 nucleotides along one strand of DNA from a bacterial virus.
Marshall Nirenberg discovered the first "triplet" - a sequence of three bases of DNA that codes for one of twenty amino acids, that serve as the building blocks of proteins.