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DNA Replication, Repair, and Recombination (Chapter 17 Study Notes)

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DNA Replication, Repair, and Recombination

Introduction

Cells must accurately replicate and repair their genetic material to ensure faithful transmission of information during cell division. Errors in these processes can lead to mutations and disease. This chapter explores the mechanisms of DNA replication, the cell cycle, and the importance of repair and recombination in maintaining genome integrity.

The Cell Cycle and DNA Replication

The Eukaryotic Cell Cycle

The cell cycle is the ordered sequence of events that leads to cell division and duplication of its DNA. It consists of interphase (G1, S, and G2 phases) and the mitotic (M) phase.

  • Interphase: The period between cell divisions, where the cell grows and DNA is replicated.

  • G1 phase: The first gap phase after mitosis; cell grows and prepares for DNA synthesis.

  • S phase: DNA synthesis occurs, doubling the amount of nuclear DNA.

  • G2 phase: The second gap phase; cell prepares for mitosis.

  • M phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division) occur.

Example: During S phase, each chromosome is replicated to produce two sister chromatids, which are later separated during mitosis.

Importance of S Phase

  • DNA replication is restricted to the S phase of the cell cycle.

  • Ensures that each daughter cell receives an identical set of chromosomes. '

Mechanisms of DNA Replication

Semiconservative Replication

The Watson and Crick model proposed that DNA replication is semiconservative: each new DNA molecule consists of one parental and one newly synthesized strand.

  • Semiconservative: Each daughter DNA has one old and one new strand.

  • Conservative: Parental molecule remains intact; new molecule is entirely new.

  • Dispersive: Each strand is a mix of old and new DNA segments.

Experimental Evidence: The Meselson-Stahl experiment used isotopic labeling (15N and 14N) and density gradient centrifugation to demonstrate semiconservative replication in bacteria.

Replication Origins and Forks

  • Origin of Replication: Specific DNA sequence where replication begins.

  • Replication Fork: Y-shaped region where the DNA is split into two strands and copied.

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

  • Theta Replication: Seen in circular DNA (e.g., E. coli), forming a structure resembling the Greek letter θ.

Replicons in Eukaryotes

  • Eukaryotic chromosomes have multiple origins of replication, forming replicons.

  • Each replicon is a unit of DNA replicated from a single origin.

  • Replication bubbles form as DNA is unwound at each origin.

Key Proteins and Enzymes in DNA Replication

Initiation Proteins

  • Origin Recognition Complex (ORC): Binds to origins in eukaryotes to initiate replication.

  • DnaA, DnaB, DnaC: Bacterial proteins that recognize and unwind the origin.

  • Helicase: Unwinds the DNA double helix using ATP.

  • Single-Stranded Binding Proteins (SSB): Stabilize unwound DNA.

  • Topoisomerase: Relieves supercoiling ahead of the replication fork.

DNA Polymerases

  • DNA Polymerase I (bacteria): Removes RNA primers and fills in DNA; has 3'→5' exonuclease (proofreading) and 5'→3' exonuclease activity.

  • DNA Polymerase III (bacteria): Main enzyme for DNA synthesis; high processivity.

  • DNA Polymerase α (eukaryotes): Initiates DNA synthesis; forms complex with primase.

  • DNA Polymerase δ and ε (eukaryotes): Major enzymes for lagging and leading strand synthesis, respectively; have proofreading activity.

Other Essential Enzymes

  • Primase: Synthesizes short RNA primers to initiate DNA synthesis.

  • DNA Ligase: Joins Okazaki fragments on the lagging strand.

  • RNase H (eukaryotes): Removes RNA primers.

Summary Table: Key DNA Replication Proteins

Protein/Enzyme

Function

Organism

DNA Polymerase I

Removes RNA primers, fills DNA, proofreading

Bacteria

DNA Polymerase III

Main DNA synthesis

Bacteria

DNA Polymerase α

Initiates DNA synthesis with primase

Eukaryotes

DNA Polymerase δ/ε

Lagging/leading strand synthesis, proofreading

Eukaryotes

Primase

Synthesizes RNA primers

Both

Helicase

Unwinds DNA

Both

SSB

Stabilizes single-stranded DNA

Both

Topoisomerase

Relieves supercoiling

Both

DNA Ligase

Joins DNA fragments

Both

Telomerase

Extends telomeres

Eukaryotes

Mechanics of DNA Synthesis

Directionality and Strand Synthesis

  • DNA polymerases add nucleotides to the 3' end; synthesis proceeds 5'→3'.

  • Leading strand: Synthesized continuously toward the replication fork.

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

  • Okazaki fragments are joined by DNA ligase to form a continuous strand.

Role of RNA Primers

  • DNA polymerase cannot initiate synthesis de novo; requires a primer.

  • Primase synthesizes short RNA primers (~10 nucleotides).

  • On the leading strand, one primer is needed; on the lagging strand, each Okazaki fragment requires a new primer.

  • RNA primers are later removed and replaced with DNA.

Proofreading and Fidelity

  • DNA polymerases have 3'→5' exonuclease activity for proofreading.

  • Incorrect nucleotides are excised and replaced, reducing error rate to a few per billion base pairs.

  • Exonucleases remove nucleotides from the ends; endonucleases make internal cuts.

Unwinding the DNA Double Helix

Helicases, SSB, and Topoisomerases

  • Helicases: Unwind the DNA helix at the replication fork using ATP.

  • Single-Stranded Binding Proteins (SSB): Bind and stabilize unwound DNA.

  • Topoisomerases: Prevent excessive supercoiling by making transient cuts in DNA.

  • Gyrase: A bacterial topoisomerase essential for replication.

The Replisome

  • The replisome is a large protein complex that coordinates DNA synthesis at the replication fork.

  • Includes helicase, primase, DNA polymerases, SSB, and ligase.

  • Powered by nucleoside triphosphate hydrolysis.

Special Features of Eukaryotic DNA Replication

Nucleosome Disassembly and Reassembly

  • During replication, nucleosomes (DNA-histone complexes) are temporarily disassembled and reassembled on newly synthesized DNA.

Telomeres and the End-Replication Problem

  • Linear chromosomes face the end-replication problem: lagging strand synthesis cannot fully replicate the 3' ends, leading to progressive shortening.

  • Telomeres: Repetitive, noncoding DNA sequences at chromosome ends (e.g., TTAGGG in humans) protect coding regions.

  • Telomerase: A ribonucleoprotein enzyme that extends telomeres using an RNA template.

  • Telomerase activity is high in germ cells and cancer cells, low in most somatic cells.

Telomeres, Aging, and Disease

  • Shortening of telomeres is associated with cellular aging and senescence.

  • Telomerase is active in immortalized cell lines and most cancers.

  • Defects in telomere maintenance (e.g., Werner syndrome) can cause premature aging.

Key Terms and Definitions

  • Mitosis: Division of the nucleus.

  • Cytokinesis: Division of the cytoplasm.

  • Sister Chromatids: Identical copies of a chromosome joined at the centromere.

  • Replication Fork: The Y-shaped region where DNA is unwound and replicated.

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

  • Primase: Enzyme that synthesizes RNA primers.

  • Telomeres: Repetitive DNA at chromosome ends.

  • Telomerase: Enzyme that extends telomeres.

Additional info: For a deeper understanding, students should also study DNA repair mechanisms and recombination, which are essential for genome stability but are not detailed in this summary.

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