Molecular Biology

Step 1: Franklin Stahl.
He worked along with Matthew Meselson in the Meselson–Stahl experiment demonstrating the semi-conservative replication of DNA. They used isotopes of nitrogen, showing that duplex DNA strands can be entirely heavy or light. Hence, Franklin Stahl → IV.
Step 2: Maurice Wilkins.
Wilkins, with Rosalind Franklin, worked on X-ray diffraction studies of DNA. He estimated the absolute amount of bases and contributed to crystallographic imaging. Hence, Maurice Wilkins → II.
Step 3: Erwin Chargaff.
He is known for Chargaff’s rules which state that in DNA, the amount of adenine equals thymine, and cytosine equals guanine. This laid the base-pairing rule and helped deduce DNA’s structure. His work supports two polynucleotide chains with complementary bases. Hence, Erwin Chargaff → III.
Step 4: Watson and Crick.
They proposed the double-helix model of DNA, with the B-form being the most stable under physiological conditions. Hence, Watson and Crick → I.
Final Matching:
A - IV, B - II, C - III, D - I

Step 1: Understanding the structure of DNA.
DNA exists in the form of a double helix where nucleotides are stacked one above the other in a very specific manner. Each nucleotide consists of a base, sugar, and phosphate group. The spatial arrangement gives a uniform helical structure.
Step 2: Distance between nucleotides.
The correct distance between two successive nucleotides along the helix axis is 0.34 nanometers (nm), which is equivalent to 3.4 Å. This measurement has been confirmed by X-ray crystallography studies (notably by Rosalind Franklin).
Step 3: Eliminating incorrect options.
- Option (1) 0.15 nm: Too short to represent the actual stacking distance.
- Option (3) 3.0 Å: Slightly less than the accurate known distance.
- Option (4) 3.1 Å: Also incorrect, as the true value is closer to 3.4 Å.
Thus, the most precise answer is 0.34 nm.
Final Answer: