Edexcel A Level (IAL) Biology -2.10 DNA Replication- Study Notes- New Syllabus

DNA Replication & Meselson-Stahl Experiment

🌱 (i) Process of DNA Replication

Introduction

DNA replication ensures each new cell receives an exact copy of genetic material. It occurs before cell division (during S-phase of interphase). The process is semi-conservative – each new DNA molecule has one old strand and one new strand.

Steps in DNA Replication

1. Unwinding the DNA: DNA helicase breaks hydrogen bonds between complementary bases (A-T, G-C). The double helix unzips, forming two template strands.
2. Binding of Free Nucleotides: Free DNA nucleotides align next to complementary bases on template strands: A ↔ T, G ↔ C.
3. Formation of New Strands: DNA polymerase catalyses phosphodiester bond formation between adjacent nucleotides. → Forms sugar-phosphate backbone.
   Leading strand = made continuously
   Lagging strand = made in short fragments (Okazaki fragments) DNA ligase joins these fragments.
4. Result: Two identical DNA molecules are produced, each having: one parental strand, one newly synthesized strand.

✳️ Why It’s Called Semi-Conservative

Each daughter DNA molecule conserves one old strand from the parent and one new strand → “semi” = half preserved.

🌿 (ii) Meselson and Stahl Experiment

Aim: To determine how DNA replicates – whether conservative, semi-conservative, or dispersive.

Experimental Design:

• Bacteria (E. coli) grown in heavy nitrogen (^15N) medium → all DNA became “heavy.”
• Then transferred to light nitrogen (^14N) medium and allowed to replicate.
• DNA extracted after each generation → spun in centrifuge to separate by density.

Observations:

Conclusion:

The results supported the semi-conservative model: Each new DNA molecule contained one old (^15N) and one new (^14N) strand. Conservative and dispersive models were disproved.

🧠 Quick Recap 

🧩 Memory Tip:

➡ “H–P–L” → Helicase (unzips), Polymerase (builds), Ligase (joins)
➡ “15 → 14 = Hybrid → Light” → key idea from Meselson & Stahl.