DNA replication Study Guide

📖 Core Concepts DNA replication – Creation of an exact copy of the genome; semiconservative (each daughter DNA contains one old and one new strand). Origin of replication – Specific DNA sequence where unwinding begins; many origins in eukaryotes, single origin in most bacteria. Helicase – Motor enzyme that separates the two strands, generating two replication forks that move in opposite directions. Primer – Short RNA segment synthesized by primase; provides the 3′‑OH required for DNA polymerase to start synthesis. DNA polymerase – Enzyme that adds deoxyribonucleotides to the 3′ end of a primer; cannot start a strand de novo. Leading strand – Synthesis that proceeds continuously in the same direction as fork movement (5′→3′). Lagging strand – Synthesis that proceeds discontinuously, producing short Okazaki fragments (5′→3′) opposite to fork movement. Okazaki fragment processing – RNA primers removed (polymerase I in bacteria; RNase H/Pol δ in eukaryotes), gaps filled, and fragments ligated by DNA ligase. Replisome – Multiprotein complex (helicase, primase, polymerases, sliding clamp, clamp loader, SSB proteins, topoisomerase, histone chaperones). Telomeres & telomerase – Repetitive ends of linear chromosomes; telomerase adds repeats in germ cells, somatic cells lose repeats each division (Hayflick limit). Cell‑cycle regulation – Eukaryotic S‑phase entry controlled by cyclin‑dependent kinases (CDKs) and inhibitors (e.g., geminin); bacterial initiation controlled by DnaA‑ATP levels and origin methylation. 📌 Must Remember DNA synthesis is 5′→3′ only; polymerase adds nucleotides to the 3′‑OH of the growing strand. Polymerase requires a primer with a free 3′‑OH; primase provides the RNA primer. Intrinsic error rate of most polymerases: <1 mistake per 10⁷ nt; proofreading (3′→5′ exonuclease) further reduces errors to <1 per 10⁸ nt. Post‑replication mismatch repair brings overall fidelity to <1 mistake per 10¹¹ nt. AT‑rich sequences are favored as origins because they unwind more easily (2 H‑bonds vs 3). Eukaryotic restriction: Once an origin fires, it cannot fire again until CDK activity drops in late mitosis (prevents re‑initiation). Bacterial restriction: Origin re‑initiation blocked by hemimethylation and SeqA binding; DnaA must be ATP‑bound. Telomere shortening = cellular aging in somatic cells; telomerase activity = immortalization. Topoisomerases (including DNA gyrase in bacteria) relieve supercoiling ahead of the fork. 🔄 Key Processes Initiation (eukaryotes) ORC binds origin → recruits Cdc6 & Cdt1 → loads MCM helicase (ATP‑dependent). CDK phosphorylation activates helicase, disassembles pre‑replication complex. Helicase unwinds DNA, primase synthesizes first RNA primer. Clamp loader places sliding clamp; DNA polymerase α‑primase starts synthesis. Elongation Leading strand: single primer → continuous synthesis by Pol ε (euk) or Pol III (bacteria). Lagging strand: multiple primers → Pol δ (euk) or Pol III synthesizes Okazaki fragments. Primer removal: Pol I (bacteria) or RNase H/Pol δ (euk) excises RNA, fills gaps. Ligation: DNA ligase seals nicks, completing the continuous strand. Termination Eukaryotes: Replication reaches telomeres; telomerase adds repeats in germ cells; somatic cells lose repeats. Bacteria: Two forks meet in a termination region; replication stops when forks converge. PCR Cycle (in vitro) Denaturation: 94‑98 °C → dsDNA → ssDNA. Annealing: 50‑65 °C → primers bind complementary sites. Extension: 72 °C → Taq polymerase adds nucleotides (5′→3′). Each cycle doubles target copies → exponential amplification. 🔍 Key Comparisons Leading vs. Lagging Strand Leading: one primer, continuous synthesis, same direction as fork. Lagging: many primers, discontinuous synthesis, opposite direction to fork. Eukaryotic vs. Bacterial Initiation Control Eukaryotes: CDK‑driven, origin licensing restricted to S‑phase; geminin blocks Cdt1. Bacteria: DnaA‑ATP level, origin hemimethylation, SeqA binding; not tied to a defined cell‑cycle phase. Polymerase Functions Pol III (bacteria) – bulk chromosomal synthesis. Pol I (bacteria) – RNA primer removal, gap filling. Pol α (euk) – initiates synthesis with primase (RNA primer + short DNA). Pol ε (euk) – leading‑strand synthesis. Pol δ (euk) – lagging‑strand synthesis & repair. Helicase Orientation Eukaryotes: encircles leading‑strand template. Bacteria: encircles lagging‑strand template. 5′→3′ vs. 3′→5′ Synthesis (hypothetical) 5′→3′: energy from incoming dNTP triphosphate; compatible with proofreading. 3′→5′: would require strand‑derived energy; mismatched removal would halt synthesis. ⚠️ Common Misunderstandings Polymerase can start DNA de novo – False: it always needs a primer. Only one origin exists in eukaryotes – False: thousands of origins are scattered throughout the genome. Telomeres are fully replicated by the same polymerase as the rest of the genome – False: telomerase adds repeats in germ cells; conventional polymerases cannot finish the very end. DNA ligase works on the leading strand – Misleading: leading strand is already continuous; ligase is essential for joining Okazaki fragments on the lagging strand. Helicase always moves 5′→3′ on the same strand as polymerase – Incorrect: orientation differs between bacteria and eukaryotes. 🧠 Mental Models / Intuition Replication fork = zipper: helicase pulls the “zipper” apart, polymerases stitch new “fabric” onto each side. Leading strand = highway, lagging strand = construction site: traffic (polymerase) moves smoothly on the highway, but on the construction site it must stop, lay down a segment (Okazaki fragment), then backtrack to start the next segment. Telomeres = protective caps: think of them as the plastic tips on shoelaces that prevent fraying; each replication round trims a tiny bit off unless telomerase adds new material. 🚩 Exceptions & Edge Cases Telomerase activity – present in germ cells, stem cells, many cancer cells; absent in most somatic cells. Overlapping bacterial replication cycles – fast‑growing E. coli start a new round before the previous one ends, producing “future” chromosomes two generations ahead. Topoisomerase action – without gyrase (in bacteria) or topoisomerase II (in eukaryotes), supercoiling would stall forks. Primer removal – in bacteria, Pol I’s 5′→3′ exonuclease removes RNA; in eukaryotes, RNase H + Pol δ’s exonuclease activity perform this step. Replication stress – ribonucleotide incorporation, hairpin structures, transcription‑replication collisions, and scarcity of essential factors all can cause fork stalling. 📍 When to Use Which Choosing a DNA polymerase (in vivo) Bacterial bulk synthesis: DNA polymerase III. RNA primer removal (bacteria): DNA polymerase I. Eukaryotic leading strand: DNA polymerase ε. Eukaryotic lagging strand: DNA polymerase δ (also participates in primer removal). Initiation (eukaryotes): DNA polymerase α‑primase complex. Choosing an enzyme for PCR Standard PCR: Taq DNA polymerase (thermostable, lacks proofreading). High‑fidelity PCR: Pfu or Phusion polymerases (3′→5′ exonuclease activity). When to invoke topoisomerase – any situation where helicase unwinding creates positive supercoils ahead of the fork (normal replication, transcription, or high‑speed polymerization). When telomerase is required – replicating linear chromosome ends in germ cells, stem cells, or immortalized/cancer cell lines. 👀 Patterns to Recognize AT‑rich origin motifs → likely site of helicase loading. Multiple primers on one template strand → indicates lagging‑strand synthesis (look for Okazaki fragments). Presence of single‑strand binding protein footprints → region ahead of the fork; protects exposed ssDNA. Checkpoint activation signals (e.g., CDK phosphorylation of pre‑RC components) → cell is transitioning from G₁ to S phase. Supercoiling ahead of fork → topoisomerase binding sites often co‑localize with helicase. PCR amplification curves – exponential phase appears after 15‑20 cycles if primers are correctly designed and annealing temperature is optimal. 🗂️ Exam Traps “DNA polymerase can synthesize a primer” – confusing polymerase with primase; primer synthesis is a separate primase activity. “Replication occurs during mitosis” – in eukaryotes replication is restricted to S‑phase, not mitosis. “All origins fire once per cell cycle” – true for eukaryotes, but bacterial origins can fire multiple times if overlapping cycles occur. “Telomere shortening is caused by DNA polymerase errors” – the shortening is a consequence of the end‑replication problem, not polymerase fidelity. “Helicase moves 3′→5′ on the leading strand in both domains of life” – orientation differs: eukaryotic helicase encircles the leading‑strand template, bacterial helicase encircles the lagging‑strand template. “DNA ligase is needed for the leading strand” – the leading strand is synthesized continuously; ligase is essential for joining Okazaki fragments on the lagging strand. “Mismatch repair occurs before DNA polymerase proofreading” – proofreading is intrinsic to the polymerase; mismatch repair acts after synthesis is completed. --- Use this guide to scan quickly before the exam—focus on the bolded keywords, remember the step‑by‑step flow of initiation → elongation → termination, and watch out for the common trap statements!