**Core Concept**
The concept being tested is the contribution of a scientist to the field of molecular genetics, specifically regarding the discovery of a crucial gene or mechanism. This involves understanding the historical development of genetic research and its impact on our understanding of heredity and disease.

**Why the Correct Answer is Right**
James Watson and Francis Crick are credited with the discovery of the double helix structure of DNA, but the question is likely referring to another scientist. A correct answer could be Rosalind Franklin, who contributed significantly to the understanding of DNA structure. Her X-ray crystallography images of DNA, particularly "Photograph 51," provided crucial evidence for the double helix model. Franklin's work laid the foundation for Watson and Crick's model, and her contributions to the field of molecular genetics are essential to understanding the structure and function of DNA.

**Why Each Wrong Option is Incorrect**
**Option A:** James Watson and Francis Crick's discovery of the double helix structure is well-known, but they did not contribute to the understanding of DNA structure alone. Their work built upon the discoveries of others, including Franklin's.

**Option B:** Gregor Mendel is known as the father of genetics, but his work focused on the principles of inheritance rather than the molecular structure of DNA.

**Option C:** Louis Pasteur's contributions to vaccination and germ theory are significant, but he is not associated with the discovery of the DNA structure.

**Option D:** Alexander Fleming's discovery of penicillin revolutionized the field of antibiotics, but he did not contribute to the understanding of DNA structure.

**Clinical Pearl / High-Yield Fact**
Rosalind Franklin's work on DNA structure highlights the importance of interdisciplinary research and collaboration in advancing our understanding of molecular genetics. Her contributions to the field demonstrate that scientists from diverse backgrounds can make significant contributions to our understanding of the structure and function of DNA.

**Correct Answer:** D.