DNA Structural Determination

The structure of DNA was determined through a series of scientific discoveries involving X-ray crystallography, chemical analysis, and model building. Understanding DNA structure helped reveal how genetic information is stored, replicated, and passed on.

Key Discoveries in DNA Structural Determination

1. Friedrich Miescher (1869)

• First isolated nuclein (later known as DNA) from white blood cells.
• Identified it as a new biological substance rich in phosphorus.

2. Phoebus Levene (1919-1930s)

• Discovered that DNA consists of nucleotides, each made of:
  A sugar (deoxyribose)
  A phosphate group
  A nitrogenous base (Adenine, Thymine, Cytosine, Guanine)
• Proposed the tetranucleotide hypothesis (incorrectly suggested that nucleotides were arranged in equal amounts).

3. Erwin Chargaff’s Rules (1949-1951)

• Analyzed DNA composition in different species.
• Found that:
  A = T (Adenine pairs with Thymine)
  C = G (Cytosine pairs with Guanine)
• Indicated base pairing and disproved Levene’s tetranucleotide hypothesis.

4. Rosalind Franklin and Maurice Wilkins (1951-1952)

• Used X-ray diffraction to study DNA structure.
• Franklin’s famous Photo 51 revealed DNA’s helical shape.
• Suggested that DNA is a double helix with bases stacked inside.

5. James Watson and Francis Crick (1953)

• Used Franklin’s X-ray data and Chargaff’s rules to build a double-helix model.
• Proposed:
  DNA is a right-handed double helix.
  Sugar-phosphate backbone is on the outside.
  Base pairs (A-T and C-G) are inside, forming hydrogen bonds.
  Strands run antiparallel (5’ → 3’ and 3’ → 5’).

Key Features of DNA Structure

Double helix: Two strands twisted around each other.
Antiparallel strands: One runs 5’ → 3’, the other 3’ → 5’.
Complementary base pairing: A-T (2 hydrogen bonds), C-G (3 hydrogen bonds).
Major and minor grooves: Important for protein binding.

Experimental Techniques for DNA Structural Determination

1. X-ray Crystallography

• Used by Rosalind Franklin and Maurice Wilkins.
• Helped determine helical shape and dimensions of DNA.

2. UV Absorption Spectroscopy

• DNA absorbs UV light at 260 nm, helping in structural analysis.

3. Atomic Force Microscopy (AFM)

• Provides direct visualization of DNA at the nanometer scale.

Significance of DNA Structure Discovery

Explained DNA replication (each strand serves as a template).
Provided a basis for genetic inheritance.

Led to advances in genetic engineering and biotechnology.