1. The Discovery of DNA's Chemical Composition:
* 1869: Friedrich Miescher isolated a substance from the nuclei of white blood cells, which he called "nuclein" (later found to be DNA). This established that DNA was a distinct chemical component within cells.
* 1920s: Phoebus Levene determined the basic structure of DNA: a chain of nucleotides composed of a sugar (deoxyribose), a phosphate group, and a nitrogenous base. He identified the four bases (adenine, guanine, cytosine, and thymine) but incorrectly proposed that DNA was a tetranucleotide, a simple repeating structure.
2. The Evidence that DNA Carries Genetic Information:
* 1944: Avery, MacLeod, and McCarty showed that DNA, not protein, was the transforming principle responsible for transferring genetic information in bacteria. Their experiment demonstrated that DNA could alter the genetic makeup of bacteria, providing strong evidence for its role in inheritance.
3. The Double Helix Model:
* 1953: Watson and Crick proposed the double helix model of DNA structure, based on X-ray diffraction images taken by Rosalind Franklin and Maurice Wilkins. This model revolutionized our understanding of DNA, explaining how genetic information is stored and replicated. It also provided a framework for understanding how mutations occur and how DNA functions in protein synthesis.
These events, along with many other contributions from scientists, led to the groundbreaking discovery of DNA's structure and its central role in heredity.
