UNIT 1: DNA Structure and Replication

Exam Focus: The structure of DNA (Watson & Crick model) and the mechanism of its replication, particularly the **Semi-conservative model** and the enzymatic differences between Prokaryotic and Eukaryotic replication, are core exam topics. Understand the roles of different DNA polymerases and helicase.

Table of Contents

1. DNA Structure
   • DNA as genetic material
   • Structure of DNA
   • Types of DNA
2. DNA Replication
   • Semi-conservative nature of DNA replication
   • Replication of DNA in prokaryotes
   • Replication of DNA in eukaryotes
   • Bi-directional replication
   • DNA polymerases
   • Rolling circle replication
   • Unique aspects of eukaryotic chromosome replication

1. DNA Structure

DNA as genetic material

The conclusive proof that **DNA (Deoxyribonucleic Acid)** is the genetic material, carrying hereditary instructions, was established through landmark experiments by Avery, MacLeod, and McCarty (demonstrating the transforming principle) and Hershey and Chase (using bacteriophages).

Structure of DNA

The standard model is the **B-DNA** double helix, proposed by Watson and Crick.

• It consists of two polynucleotide strands wound around each other in a **right-handed helix**.
• The strands are **antiparallel** (one running 5'-to-3' and the other 3'-to-5').
• The sugar-phosphate backbone is on the outside, and the nitrogenous bases are stacked in the interior.
• The bases pair specifically: **Adenine (A) pairs with Thymine (T)** via two hydrogen bonds, and **Guanine (G) pairs with Cytosine (C)** via three hydrogen bonds (Chargaff's rules).

[Image of DNA double helix structure with antiparallel strands and base pairing]

Types of DNA

DNA can exist in different structural forms based on hydration and salt concentration:

• **B-DNA:** The most common form in living cells; right-handed helix.
• **A-DNA:** Right-handed helix, less hydrated than B-DNA, typically found in desiccated conditions.
• **Z-DNA:** A left-handed helix; its formation may be important in gene regulation.

2. DNA Replication

Semi-conservative nature of DNA replication

DNA replication is **semi-conservative**, meaning each new DNA molecule consists of one original (parental) strand and one newly synthesized (daughter) strand.

This model was experimentally proven by the Meselson and Stahl experiment.

Replication of DNA in prokaryotes

• **Origin:** Replication begins at a single origin of replication (OriC).
• **Replication Fork:** Two replication forks form, moving away from the origin.
• **Key Enzyme:** **DNA Polymerase III** is the primary enzyme responsible for synthesis.
• **Termination:** Replication terminates when the two forks meet at a specific termination site.

Replication of DNA in eukaryotes

• **Origin:** Replication occurs at **multiple origins of replication** along the linear chromosomes.
• **Replication Bubble:** Multiple replication bubbles form, leading to two replication forks at each bubble.
• **Key Enzymes:** Multiple DNA polymerases are involved (e.g., Pol α initiates synthesis, Pol δ and Pol \varepsilon perform elongation).

Bi-directional replication

In both prokaryotes and eukaryotes, the replication process moves in **both directions** away from the origin of replication, creating two replication forks that proceed simultaneously. [Image of a replication bubble demonstrating bi-directional replication]

DNA polymerases

These enzymes catalyze the formation of the phosphodiester bonds to link nucleotides, creating the new DNA strand. They have an intrinsic **proofreading activity** (3'-to-5' exonuclease) to correct errors.

Rolling circle replication

A type of unidirectional DNA replication used by some viruses (phages) and plasmids, particularly in conjugation.

1. One strand of the circular DNA is nicked.
2. The exposed 3'-OH end serves as a primer.
3. DNA polymerase adds nucleotides, displacing the nicked strand, which "rolls off."
4. The displaced strand is then synthesized using the rolling strand as a template.

Unique aspects of eukaryotic chromosome replication

Eukaryotic replication involves specific features not found in prokaryotes:

• **Multiple Origins:** Necessary to replicate the large genome size rapidly.
• **Nucleosome Assembly:** DNA synthesis is tightly coupled with the assembly of histones and formation of chromatin.
• **Telomere Replication:** Due to the linear nature of chromosomes, special mechanisms (using the enzyme **telomerase**) are needed to replicate the ends (telomeres) completely, preventing shortening.