DNA Replication and Repair: Synthesis, Chromosome Ends, and Error Correction

DNA and the Cell Cycle

Overview of DNA Structure and Cell Cycle Phases

DNA is the hereditary material in cells, stored in the nucleus as chromatin. Throughout the cell cycle, DNA undergoes replication and segregation to ensure genetic continuity.

• Double-stranded DNA: DNA consists of two complementary strands forming a double helix.

• Chromatin: DNA is packaged with proteins in the nucleus.

• G1 phase: DNA exists as unreplicated chromosomes, each with one double-stranded DNA molecule.

• S (Synthesis) phase: DNA replication occurs, duplicating each chromosome to form two identical sister chromatids.

• M (Mitosis) phase: Chromatids are separated into daughter cells.

Example: During S phase, a human cell duplicates its 46 chromosomes, resulting in 92 chromatids before mitosis.

Prokaryotic DNA Replication

Initiation and Unwinding of DNA

Prokaryotic DNA replication begins at a single origin and proceeds bidirectionally. Several enzymes coordinate the unwinding and stabilization of the DNA helix.

• Helicase: Breaks hydrogen bonds between bases to unwind the double helix at the replication fork.

• Topoisomerase (DNA gyrase): Relieves torsional (twisting) stress ahead of the fork by cutting and rejoining DNA strands.

• Single-Strand Binding Proteins (SSBPs): Bind to separated DNA strands to prevent re-annealing.

• Semidiscontinuous replication: One strand (leading) is synthesized continuously, while the other (lagging) is synthesized in short fragments.

Example: In Escherichia coli, replication starts at the origin and proceeds in both directions around the circular chromosome.

Origin of Replication and Bidirectional Synthesis

Bacterial chromosomes have a single origin of replication. Replication proceeds in both directions, forming two replication forks.

Feature	Description
Origin of Replication	Single, specific site where DNA synthesis begins
Direction	Bidirectional from the origin
Replication Fork	Y-shaped region where DNA is actively unwound and replicated

Mechanisms of DNA Synthesis

Leading and Lagging Strand Synthesis

DNA polymerases synthesize new DNA by adding nucleotides to the 3' end of a growing strand, using a template strand. Synthesis differs between the leading and lagging strands due to the antiparallel nature of DNA.

• Leading Strand: Synthesized continuously in the 5' to 3' direction toward the replication fork.

• Lagging Strand: Synthesized discontinuously in short segments (Okazaki fragments) away from the replication fork.

• Primase: Synthesizes short RNA primers to provide a free 3'-OH group for DNA polymerase III.

• DNA Polymerase III: Extends the primer, synthesizing new DNA.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA.

• DNA Ligase: Joins Okazaki fragments to form a continuous strand.

Example: The lagging strand in E. coli is synthesized as Okazaki fragments, which are later joined by DNA ligase.

Okazaki Fragments and the Discontinuous Replication Hypothesis

The discontinuous replication hypothesis explains the synthesis of the lagging strand as short DNA fragments, later joined to form a complete strand.

• Okazaki Fragments: Short DNA segments synthesized on the lagging strand.

• Discontinuous Synthesis: DNA polymerase synthesizes fragments away from the replication fork.

• Fragment Joining: DNA ligase seals nicks between fragments.

Example: Okazaki and colleagues experimentally confirmed the existence of these fragments in bacteria.

DNA Replication at Chromosome Ends: Telomeres and Telomerase

Challenges at Eukaryotic Chromosome Ends

Linear eukaryotic chromosomes face a unique problem during replication: the inability to fully replicate the ends (telomeres) of the lagging strand, leading to progressive shortening.

• Telomeres: Repetitive, noncoding DNA sequences at chromosome ends that protect genes from loss.

• Replication Problem: No available 3'-OH group to replace the final RNA primer, resulting in a single-stranded overhang.

• Cellular Aging: Somatic cells lacking telomerase experience gradual chromosome shortening, limiting cell lifespan.

Example: Human somatic cells lose telomeric DNA with each division, contributing to aging.

Role of Telomerase in Telomere Maintenance

Telomerase is an enzyme that extends telomeres, preventing chromosome shortening in certain cell types.

• Telomerase: Uses its own RNA template to add repetitive DNA sequences (e.g., TTAGGG) to the lagging strand's overhang.

• Extension Process: Telomerase binds to the overhang, extends it, primase adds an RNA primer, DNA polymerase fills in the complementary strand, and ligase seals the backbone.

• Cell Types Expressing Telomerase: Germ cells, stem cells, and many cancer cells.

Example: HeLa cells, derived from Henrietta Lacks, are immortalized cancer cells that express telomerase and maintain telomere length.

DNA Error Correction and Repair Mechanisms

Proofreading by DNA Polymerase

DNA polymerases possess proofreading activity to ensure high fidelity during DNA synthesis.

• Proofreading: DNA polymerase checks each new base pair; if incorrect, the enzyme pauses, removes the mismatched base via its 3'→5' exonuclease activity, and replaces it with the correct one.

• Error Rate: DNA replication is highly accurate, with less than one mistake per billion bases.

Example: The exonuclease subunit of DNA polymerase acts like a "backspace key," correcting errors as they occur.

Mismatch Repair

Mismatch repair corrects errors that escape proofreading after DNA synthesis is complete.

• Recognition: Repair enzymes detect mismatched bases.

• Excision: The section of DNA containing the mismatch is removed.

• Replacement: DNA polymerase fills in the correct bases, and ligase seals the strand.

Example: Mismatch repair prevents the propagation of mutations that could lead to genetic diseases or cancer.

DNA Damage Repair Mechanisms

Cells employ specific and nonspecific repair systems to fix DNA damage caused by environmental factors or replication errors.

• Specific Repair: Targets particular types of damage, such as photorepair of thymine dimers by photolyase.

• Nonspecific Repair: Includes excision repair, where damaged DNA is removed and resynthesized using the undamaged strand as a template.

• SOS Repair: Emergency response to extensive DNA damage.

Repair Type	Mechanism	Example
Photorepair	Photolyase uses light energy to break thymine dimers	UV-induced damage
Excision Repair	Damaged section is removed and replaced	Bulky adducts, chemical damage
SOS Repair	Induced under severe damage, error-prone	Extensive DNA lesions

Example: Excision repair is crucial for removing UV-induced thymine dimers and restoring DNA integrity.

DNA Repair Genes and Cancer

Mutations in genes responsible for DNA repair are often associated with cancer development.

• Defective Repair Genes: Increase mutation rates, raising the likelihood of cancer-causing mutations.

• Cell Cycle Regulation: Mutations in cell cycle genes can lead to uncontrolled cell growth and tumor formation.

Example: Inherited mutations in DNA repair genes, such as those causing xeroderma pigmentosum, result in high cancer risk.

Study Guide: Key Concepts

• Compare and contrast leading and lagging strand DNA replication.

• Describe what happens at the ends of eukaryotic chromosomes during DNA replication and how telomerase can change it.

• Explain the three ways that DNA can be corrected for base pairing mistakes: proofreading, mismatch repair, and excision repair.

Key Terms

• Somatic cells

• Primase

• Single-strand binding proteins (SSBPs)

• Exonuclease

• Telomerase

• Mismatch repair enzymes

• Okazaki fragments

• Ligase

Relevant Equations

• Base pairing rule: $\%A + \%T = \%G + \%C$ (in double-stranded DNA)

• Direction of DNA synthesis: $5' \rightarrow 3'$

Additional info: Some context and terminology have been expanded for clarity and completeness, including the role of DNA repair in cancer and the molecular details of replication enzymes.