Lecture 9 (ch 11)

DNA Replication and Recombination

Overview and Requirements of DNA Replication

DNA replication is a fundamental process in genetics, ensuring the accurate duplication of genetic material prior to cell division. This process is highly regulated and requires several key components and steps.

• Double-stranded DNA template: The original DNA molecule serves as the template for synthesis.

• Origin of replication: Specific sequences where replication begins.

• Enzymes: DNA polymerases, helicases, primases, ligases, and topoisomerases are essential for unwinding, synthesis, and joining DNA fragments.

• Primers: Short RNA sequences synthesized by primase provide a free 3'-OH group for DNA polymerase to initiate synthesis.

• dNTPs: Deoxyribonucleotide triphosphates are the building blocks for new DNA strands.

Direction of DNA Replication

DNA synthesis always proceeds in the 5' to 3' direction, meaning new nucleotides are added to the 3' end of the growing strand.

• Leading strand: Synthesized continuously toward the replication fork.

• Lagging strand: Synthesized discontinuously away from the fork in short segments called Okazaki fragments.

Three Hypotheses for Modes of DNA Replication

Early models proposed different mechanisms for how DNA is duplicated:

• Conservative: The parental double helix remains intact, and a completely new double helix is synthesized.

• Semiconservative: Each new DNA molecule consists of one parental (old) strand and one newly synthesized strand.

• Dispersive: Parental and new DNA are interspersed in both strands after replication.

Example: The semiconservative model is supported by experimental evidence and is the accepted mechanism in all cells.

How Does DNA Replicate? The Template Mechanism

DNA replication relies on the complementarity of nitrogenous bases (A-T, G-C) to ensure accurate copying.

• Template strands: Each parental strand serves as a template for the synthesis of a new complementary strand.

• Base pairing: Adenine pairs with thymine, and guanine pairs with cytosine, ensuring fidelity.

Meselson and Stahl’s Experiment: Demonstrating Semiconservative Replication

This classic experiment used 15N-labeled E. coli to distinguish between old and new DNA strands, providing evidence for the semiconservative model.

• Method: Bacteria were grown in heavy nitrogen (15N), then transferred to light nitrogen (14N) medium. DNA was extracted after each generation and analyzed by density gradient centrifugation.

• Results: After one round of replication, DNA was of intermediate density, consistent with semiconservative replication. Subsequent generations showed both light and intermediate DNA bands.

Example: The experiment is often called "the most beautiful experiment in biology" for its elegant demonstration of DNA replication mechanism.

DNA Replication in Prokaryotes

Prokaryotic DNA replication is well-characterized, especially in E. coli. It involves a single origin of replication and proceeds bidirectionally.

• Initiation: Begins at the origin (oriC), where DNA helicase unwinds the helix.

• Elongation: DNA polymerase III synthesizes new DNA; primase synthesizes RNA primers.

• Termination: Occurs when replication forks meet; proteins such as Tus bind to terminator sequences to halt replication.

• Enzymes: DNA polymerases I, II, III (with III being the main replicative enzyme), ligase, helicase, primase, and gyrase (topoisomerase).

Theta and Rolling Circle Replication

Prokaryotes and some viruses use distinct mechanisms for DNA replication.

• Theta replication: Common in circular DNA (e.g., bacterial chromosomes); replication proceeds bidirectionally, forming a structure resembling the Greek letter theta (θ).

• Rolling circle replication: Used by some viruses and plasmids; involves nicking one strand and synthesizing a new strand while displacing the old one.

Example: The F plasmid in bacteria replicates via the rolling circle mechanism.

DNA Replication in Eukaryotes

Eukaryotic DNA replication is more complex due to larger genomes, linear chromosomes, and chromatin structure.

• Multiple origins: Eukaryotic chromosomes have many origins of replication to ensure timely duplication.

• Initiation: Involves assembly of the prereplication complex (pre-RC) at origins, including the origin recognition complex (ORC).

• Enzymes: Multiple DNA polymerases (α, δ, ε for nuclear DNA; γ for mitochondrial DNA), primase, ligase, helicase, and topoisomerase.

• Chromatin: DNA is packaged with histones into nucleosomes, requiring additional remodeling during replication.

Replication at the Ends of Chromosomes: Telomeres and Telomerase

Linear chromosomes present a challenge for complete replication of ends, known as the end-replication problem.

• Telomeres: Repetitive DNA sequences at chromosome ends protect against loss of genetic information.

• Telomerase: A ribonucleoprotein enzyme that extends telomeres using an RNA template and reverse transcriptase activity.

• Cellular aging: Most somatic cells lack active telomerase, leading to gradual telomere shortening and cellular senescence. Cancer cells often maintain telomerase activity, contributing to immortality.

Example: Hutchinson-Gilford progeria syndrome is associated with defects in telomere maintenance.

Homologous Recombination

Homologous recombination is a process of genetic exchange between homologous DNA molecules, crucial for genetic diversity and DNA repair.

• Mechanism: Involves alignment of homologous sequences, strand invasion, formation of a Holliday junction, branch migration, and resolution.

• Enzymes: Nucleases, recombinases, and ligases facilitate the process.

• Biological significance: Essential for meiosis, DNA repair, and proper chromosome segregation.

Properties of Bacterial DNA Polymerases

DNA polymerases in bacteria have distinct roles and enzymatic activities.

Polymerase	Polymerization	3'→5' Exonuclease (Proofreading)	5'→3' Exonuclease	Main Function
Pol I	+	+	+	Primer removal, gap filling
Pol II	+	+	-	DNA repair
Pol III	+	+	-	Main replicative polymerase

Key Enzymes and Their Functions in DNA Replication

Enzyme	Function
Helicase	Unwinds the DNA double helix
Single-stranded binding proteins (SSBPs)	Stabilize unwound DNA
DNA gyrase (topoisomerase)	Relieves supercoiling ahead of the fork
Primase	Synthesizes RNA primers
DNA polymerase III	Elongates new DNA strand
DNA polymerase I	Removes RNA primers and fills gaps
DNA ligase	Joins Okazaki fragments

Formulas and Equations

• DNA Synthesis Direction: $5' \rightarrow 3'$

• Base Pairing: $\text{A} = \text{T}$ $\text{G} = \text{C}$

• Okazaki Fragment Formation: $\text{Lagging strand} = \sum_{i=1}^{n} \text{Okazaki fragments}_i$

Summary

DNA replication is a highly coordinated process involving multiple enzymes and regulatory mechanisms. The semiconservative model, proven by Meselson and Stahl, ensures genetic fidelity. Differences between prokaryotic and eukaryotic replication reflect the complexity of their genomes. Telomere maintenance and homologous recombination are essential for chromosome stability and genetic diversity.