DNA Structure

Introduction to DNA Structure

Introduction

In 1953, James Watson and Francis Crick made a momentous discovery—the DNA double helix. This crucial breakthrough in molecular biology revolutionised the understanding of genetic inheritance.

• Watson's Contribution: Emphasised base pairing specificity.
• Crick's Contribution: Described the double helix's structural configuration.

Prior to this, models like the triple helix could not accurately reflect key features such as DNA stability and precise base pairing.

Significance in Molecular Biology

The discovery had a profound impact on genetic research and substantially advanced our understanding of heredity and genetic information storage. Published in Nature, it was heralded as transformative, leading to significant developments such as:

• Human Genome Mapping
• Progress in Genetic Engineering

Double Helix Structure

Double Helix: Two intertwined helices effectively store genetic information and ensure accurate replication.

• Core Components:
  • Sugar-Phosphate Backbone: Comprises alternating sugar and phosphate groups, forming a robust structure essential for DNA's integrity.
  • Nitrogenous Bases: Adenine (A), thymine (T), cytosine (C), guanine (G). Base pairing involves A-T and C-G pairs.

Sugar-Phosphate Backbone

• Consists of an alternating sugar-phosphate sequence, crucial for DNA's structural robustness.
• Provides a strong framework for maintaining DNA integrity.

Nitrogenous Base Pairs

• Hydrogen Bonding: Ensures accurate base pairing and stabilises the DNA helix. The specific pairing (A-T, C-G) is vital for precise replication.

Antiparallel Strands

Antiparallel Strands: Two strands running in opposite directions, each with a 5' end and a 3' end.

• Essential for directional replication.

Significance in Heredity

DNA's structure provides an effective mechanism for genetic data storage and ensures replicative precision.

• Genetic Stability: Achieved through specific base pairing and structural steadfastness.

Understanding Nucleotides

Components of a Nucleotide

• Phosphate Group: Constitutes the DNA backbone.
• Deoxyribose Sugar: Supports the integrity of the DNA chain.
• Nitrogenous Base: Comprises adenine (A), guanine (G), cytosine (C), and thymine (T).

Purines vs. Pyrimidines

• Purines:

  • Double-ring structures
  • Include adenine (A) and guanine (G)

• Pyrimidines:

  • Single-ring structures
  • Include cytosine (C) and thymine (T)

Base Pairing and Bonding

Complementary Base Pairing

Complementary Base Pairing: Pairs A with T and C with G.

• A-T Pairs: Linked by two hydrogen bonds.
• C-G Pairs: Linked by three hydrogen bonds.

Chargaff's Rules

Diagrams and Visual Aids

Overview of the Semi-Conservative Model

Semi-Conservative Replication

Semi-Conservative Replication: Each strand serves as a template, ensuring genetic accuracy.

Steps in Semi-Conservative Replication

Initiation

• Helicase: Unwinds DNA.

Primer Synthesis

• Primase: Synthesises RNA primers.

Elongation

• Leading Strand: Synthesised continuously.
• Lagging Strand: Forms Okazaki fragments.

Termination

• Ligase: Connects fragments.

Key Concepts

• Directional Activity: Enzymes operate in a 5' to 3' direction.

Role of DNA Polymerase in Editing and Proofreading

• DNA Polymerase: Integral enzyme for adding nucleotides and proofreading for precision.

Mechanisms for Correcting Replication Errors

• Mismatch Repair: A secondary error-correcting layer involving enzymes such as MutS, MutL, and MutH.

Examples of Mutation from Replication Errors

• Types of Mutations: Transitions and Transversions.

Overall, robust mechanisms of DNA proofreading and error correction are essential for maintaining life's genetic blueprint across generations.