DNA Replication and Repair: Mechanisms and Key Concepts

Chapter 6: DNA Replication and Repair

Overview

This chapter covers the fundamental processes by which cells duplicate their genetic material and maintain genome integrity. The two main topics are DNA replication and DNA repair, both essential for cell division and survival.

DNA Replication

Purpose and Process

Before a cell divides, it must accurately copy its genome. DNA replication is the process by which a cell makes an exact copy of its DNA, ensuring that each daughter cell receives a complete set of genetic instructions.

• Template Mechanism: Each strand of the DNA double helix serves as a template for the synthesis of a new, complementary strand.

• Complementary Base Pairing: New nucleotides are added according to the AT/GC rule (A pairs with T, G pairs with C).

• Result: Two identical DNA double helices are produced, each containing one parental and one newly synthesized strand.

Example: If the template strand sequence is 5'-ATCG-3', the new strand will be 3'-TAGC-5'.

Semiconservative Replication

DNA replication is described as semiconservative because each daughter DNA molecule consists of one old (parental) strand and one newly synthesized strand.

• Mechanism: During replication, the two strands of the parental DNA separate, and each serves as a template for a new strand.

• Experimental Evidence: The Meselson-Stahl experiment demonstrated that DNA replication results in molecules that are "half-old, half-new."

Diagram: (See image: Each round of replication produces DNA molecules with one parental and one daughter strand.)

Base Pairing and Fidelity

Accurate DNA replication depends on strict base pairing rules and proofreading mechanisms.

• AT/GC Rule: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• Proofreading: DNA polymerases have proofreading activity to correct errors during synthesis.

Equation: The fidelity of DNA replication can be expressed as the error rate per nucleotide incorporated, typically $1 \text{ error per } 10^7 \text{ nucleotides}$.

Replication Fork and Directionality

DNA replication occurs at structures called replication forks, where the double helix is unwound and new strands are synthesized.

• Bidirectional Replication: Replication proceeds in both directions from the origin.

• Leading and Lagging Strands: The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments (Okazaki fragments).

Diagram: (See image: Replication fork showing template and newly synthesized strands.)

DNA Repair

Importance of DNA Repair

DNA is constantly exposed to damage from internal and external sources. DNA repair mechanisms are essential to correct errors and maintain genetic stability.

• Types of Damage: Includes mismatches, base modifications, strand breaks, and crosslinks.

• Repair Pathways: Cells use multiple pathways, such as mismatch repair, base excision repair, and nucleotide excision repair.

Example: UV light can cause thymine dimers, which are repaired by nucleotide excision repair.

Summary Table: Key Features of DNA Replication

Feature	Description
Template	Each parental strand serves as a template for a new strand
Semiconservative	Each daughter DNA has one old and one new strand
Base Pairing	AT/GC rule ensures accurate copying
Replication Fork	Site where DNA is unwound and new strands are synthesized
Proofreading	DNA polymerase corrects errors during synthesis

Additional info:

• DNA replication is essential for cell division and inheritance.

• Errors in replication or repair can lead to mutations and disease.