Lernzettel: DNA Replication Fundamentals

DNA Replication Revision Sheet

1. 📌 Essentials

• DNA replication copies the entire DNA molecule to produce two identical strands.
• Occurs in the nucleus of eukaryotic cells; cytoplasm of prokaryotes.
• Key enzymes: helicase, primase, DNA polymerase, ligase.
• Replication is semi-conservative: each new DNA contains one original.
• Directionality of synthesis: 5' to 3'.
• Leading strand: continuous synthesis; lagging strand:azaki fragments.
• Base pairing rules: A=T, C≡G.
• Replication begins at specific origins.
• Forks are Y-shaped structures where DNA unwinding occurs.
• Proofreading by DNA polymerase ensures fidelity.

2. 🧩 Key Structures & Components

• DNA molecule — double helix with antiparallel strands.
• Nucleotides — sugar (deoxyribose), phosphate, nitrogenous base.
• Base pairs — A=T (2 H-bonds), C≡G (3 H-bonds).
• Replication fork — site of active DNA unwinding.
• Origin of replication — specific sequence initiating replication.
• Enzymes:
  • Helicase — unwinds DNA
  • Primase — synthesizes RNA primer
  • DNA polymerase — adds nucleotides
  • Ligase — joins Okazaki fragments
• Okazaki fragments — short DNA segments on lagging strand.
• Single-strand binding proteins — stabilize unwound DNA.

3. 🔬 Functions, Mechanisms & Relationships

• Helicase unwinds DNA at the replication fork, creating two single strands.
• Primase synthesizes RNA primers complementary to the DNA template.
• DNA polymerase extends the new strand in the 5' to 3' direction.
• Leading strand is synthesized continuously towards the replication fork.
• Lagging strand is synthesized discontinuously in Okazaki fragments away from the fork.
• Ligase joins Okazaki fragments to form a continuous strand.
• Proofreading activity of DNA polymerase corrects errors during synthesis.
• Replication proceeds bidirectionally from the origin.
• The replication fork moves as helicase unwinds DNA.

4. Comparative Table

Item	Key Features	Notes / Differences
Leading strand	Continuous synthesis	3' to 5' template, 5' to 3' new strand
Lagging strand	Discontinuous, Okazaki fragments	Synthesized away from fork
DNA polymerase	Synthesizes DNA, proofreads	Adds nucleotides in 5' to 3' direction
Helicase	Unwinds DNA	Creates replication fork
Ligase	Joins Okazaki fragments	Seals nicks in sugar-phosphate backbone

5. 🗂️ Hierarchical Diagram (ASCII)

DNA Replication
 ├─ Initiation
 │    ├─ Helicase unwinds DNA
 │    └─ Origin recognition
 ├─ Elongation
 │    ├─ Primase adds RNA primer
 │    ├─ DNA polymerase extends leading strand
 │    ├─ DNA polymerase extends lagging strand in fragments
 │    └─ Ligase joins Okazaki fragments
 └─ Termination
      ├─ Replication completes at termination sites
      └─ DNA is proofread and repaired

6. ⚠️ High-Yield Pitfalls & Confusions

• Confusing leading vs. lagging strand synthesis.
• Assuming DNA polymerase synthesizes DNA in the 3' to 5' direction.
• Overlooking the role of primers in initiation.
• Misidentifying the function of ligase (joins fragments, not synthesizes).
• Forgetting that DNA replication is semi-conservative.
• Confusing the roles of different enzymes (helicase vs. primase).
• Assuming replication occurs only in one direction; it is bidirectional.
• Overlooking the importance of proofreading in fidelity.

7. ✅ Final Exam Checklist

• Know the structure of DNA and base pairing rules.
• Identify the key enzymes involved in replication.
• Understand the roles of leading and lagging strands.
• Describe the process of unwinding, primer synthesis, elongation, and joining.
• Recognize the significance of replication origin and forks.
• Explain the concept of semi-conservative replication.
• Be able to draw and interpret the replication fork.
• Understand the importance of proofreading for fidelity.
• Know where DNA replication occurs in eukaryotic vs. prokaryotic cells.
• Be familiar with Okazaki fragments and their joining.
• Understand the directionality of DNA synthesis.
• Recognize the sequence of enzymatic actions during replication.
• Be aware of common errors and pitfalls in understanding the process.
• Connect replication to cell division and genetic inheritance.

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