1948-1949
Linus Pauling suggests sickle cell anemia is caused by a defect in the molecular structure of protein, and later describes the shape of certain proteins as an alpha helix.

1950
Erwin Chargaff shows there are equal amounts of the nucleotides adenine (A) and thymine (T) as well as equal amounts of guanine (G) and cytosine (C), i.e. there is an A for every T and a G for every C.

1952
Alfred Hershey and Martha Chase show that viral DNA and not viral protein direct the replication of new viruses, confirming that DNA is the molecule that mediates heredity.

Maurice Wilkins and Rosalind Franklin take X-ray images of DNA crystals which will steer Watson and Crick toward their famous conclusions.

1953
James Watson and Frances Crick describe the three-dimensional structure of DNA as a double helix: two spiraling strands held together by complimentary base pairs.

1959
Jerome Lejeune determines that Down’s syndrome results from an extra chromosome -- a total of three copies of chromosome 21