The Double Helix: Unraveling DNA's Structure
Understanding the molecular basis of inheritance is crucial for mastering cell biology and genetics, especially for competitive exams like NEET. This module focuses on the groundbreaking Watson-Crick model of DNA structure, the blueprint for life itself.
The Building Blocks: Nucleotides
DNA, or deoxyribonucleic acid, is a polymer made up of repeating units called nucleotides. Each nucleotide consists of three components: a deoxyribose sugar, a phosphate group, and a nitrogenous base. There are four types of nitrogenous bases in DNA: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
A deoxyribose sugar, a phosphate group, and a nitrogenous base.
The Watson-Crick Model: A Double Helix
In 1953, James Watson and Francis Crick, building on the work of Rosalind Franklin and Maurice Wilkins, proposed the now-famous double helix model for DNA. This model describes DNA as two polynucleotide strands wound around each other in a helical structure.
DNA is a double helix with antiparallel strands held together by specific base pairing.
The DNA molecule resembles a twisted ladder. The sides of the ladder are formed by alternating sugar and phosphate groups, while the rungs are made of paired nitrogenous bases.
The two strands of the DNA double helix run in opposite directions, a property known as antiparallel. One strand runs in the 5' to 3' direction, and the other runs in the 3' to 5' direction. The nitrogenous bases are located on the inside of the helix, and they pair specifically: Adenine (A) always pairs with Thymine (T) via two hydrogen bonds, and Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds. This specific base pairing is known as complementary base pairing.
The Watson-Crick model depicts DNA as a right-handed double helix. The backbone of each strand is a sugar-phosphate chain. The bases are attached to the sugar and project inwards. Adenine (A) pairs with Thymine (T) via two hydrogen bonds, forming a purine-pyrimidine pair. Guanine (G) pairs with Cytosine (C) via three hydrogen bonds, also a purine-pyrimidine pair. This consistent pairing maintains the uniform width of the DNA helix. The antiparallel nature of the strands is indicated by the directionality of the phosphodiester bonds connecting the sugar molecules.
Text-based content
Library pages focus on text content
Key Features of the Double Helix
| Feature | Description |
|---|---|
| Structure | Double helix |
| Strand Directionality | Antiparallel (5' to 3' and 3' to 5') |
| Backbone Composition | Deoxyribose sugar and phosphate groups |
| Base Pairing | Adenine (A) with Thymine (T) (2 H-bonds); Guanine (G) with Cytosine (C) (3 H-bonds) |
| Base Pairing Rule | Complementary base pairing (Purine with Pyrimidine) |
| Helix Type | Right-handed helix |
The specific base pairing (A-T and G-C) is fundamental to DNA replication and transcription, ensuring accurate copying of genetic information.
Significance for Inheritance
The elegant structure of the DNA double helix directly explains how genetic information is stored and passed on. The sequence of bases along the DNA molecule constitutes the genetic code. During cell division, the two strands separate, and each serves as a template for the synthesis of a new complementary strand, a process known as DNA replication, ensuring that daughter cells receive an identical copy of the genetic material.
DNA replication.
Learning Resources
A comprehensive video explaining the structure of DNA, including the double helix, base pairing, and the process of replication.
Official summary of the 1962 Nobel Prize in Physiology or Medicine awarded to Watson, Crick, and Wilkins for their discoveries concerning the molecular structure of nucleic acids.
An overview of DNA structure, focusing on the double helix model and its significance in genetics.
A visual explanation of the Watson-Crick model, highlighting the key features and discoveries.
A lesson on the structure of DNA, covering nucleotides, the double helix, and base pairing rules.
Information about Rosalind Franklin's crucial contributions, particularly her X-ray diffraction images that were key to determining DNA's structure.
An excerpt from a foundational molecular biology textbook detailing the structure and properties of DNA.
An interactive visualization of the DNA double helix, allowing exploration of its components and bonds.
A historical account of the discovery of DNA's structure, including the roles of key scientists.
An official glossary entry from the National Human Genome Research Institute explaining DNA structure and its function.