Question

Explain the DNA Double Helix structure as proposed by Watson and Crick.

Hint:

Watson–Crick memory keys:

• Double helix, antiparallel strands
• A=T (2 bonds), G$\equiv$C (3 bonds)
• 10 base pairs per turn

DNA = Blueprint of life.

Answer 1

Concept: The double helix model of DNA was proposed by James Watson and Francis Crick in 1953. It explains the molecular structure of DNA and the basis of heredity.
Key Features of the Double Helix Structure:
1. Double-Stranded Helix:

• DNA consists of two long polynucleotide chains.
• The strands coil around each other forming a right-handed double helix.

2. Antiparallel Strands:

• The two strands run in opposite directions.
• One strand runs 5' $\rightarrow$ 3' and the other 3' $\rightarrow$ 5'.

3. Sugar-Phosphate Backbone:

• The outer part of each strand is made of alternating deoxyribose sugar and phosphate groups.
• Forms the structural framework of DNA.

4. Nitrogenous Bases Inside:

• Bases project inward and pair with bases of the opposite strand.
• Four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

5. Complementary Base Pairing:

• Adenine pairs with Thymine (A=T) via two hydrogen bonds.
• Guanine pairs with Cytosine (G$\equiv$C) via three hydrogen bonds.

This ensures Chargaff’s rule: \[ A = T, \quad G = C \]
6. Helical Dimensions:

• Distance between two base pairs: 0.34 nm
• One complete turn: 3.4 nm
• About 10 base pairs per turn

7. Stability of DNA:

• Hydrogen bonds between bases
• Hydrophobic stacking of bases
• Strong covalent backbone

Significance:

• Explains DNA replication mechanism
• Basis of genetic inheritance
• Foundation of molecular biology and biotechnology