Question

Double-helix structure of DNA as proposed by Watson and Crick.

Hint:

{Watson--Crick Model Key Points:} Double helix, antiparallel strands, complementary base pairing (A--T, G--C).

Answer 1

Double-Helix Structure of DNA as Proposed by Watson and Crick:

The double-helix structure of DNA was proposed in 1953 by James Watson and Francis Crick. Their model explained the molecular structure of DNA (Deoxyribonucleic Acid) and how genetic information is stored and transmitted from one generation to the next.

Main Features of the Double-Helix Model:

1. Two Polypeptide Strands:
DNA consists of two long strands of nucleotides that run in opposite directions (antiparallel). One strand runs from 5′ to 3′ direction, while the other runs from 3′ to 5′ direction.

2. Helical Shape:
The two strands are twisted around each other forming a right-handed spiral called a double helix.

3. Sugar-Phosphate Backbone:
Each strand has a backbone made of alternating deoxyribose sugar and phosphate groups. This backbone forms the outer part of the helix.

4. Nitrogenous Bases Inside:
The nitrogenous bases project inward and pair specifically with complementary bases on the opposite strand.

5. Complementary Base Pairing:
– Adenine (A) pairs with Thymine (T) by two hydrogen bonds.
– Guanine (G) pairs with Cytosine (C) by three hydrogen bonds.
This complementary pairing maintains a uniform diameter of the DNA molecule.

6. Constant Diameter:
The diameter of the DNA helix is approximately 2 nanometers (nm).

7. Distance Between Base Pairs:
The distance between two consecutive base pairs is about 0.34 nm, and one complete turn of the helix contains about 10 base pairs.

Significance of the Model:
The Watson and Crick model explained how DNA replicates accurately. Because of complementary base pairing, each strand can serve as a template for the formation of a new strand during cell division.

Conclusion:
The double-helix model of DNA proposed by Watson and Crick describes DNA as two antiparallel strands twisted into a helix with complementary base pairing. This structure is fundamental to understanding heredity, genetic information storage, and biological inheritance.