DNA Replication 🧬

Think of the double-helix as a clever zipper. When it “unzips,” each old strand guides the building of a brand-new partner strand. After the job is done, every daughter DNA holds one parental strand + one fresh strand. Scientists call this neat trick semiconservative replication.:contentReference[oaicite:0]{index=0}

1. Watson & Crick’s Big Hint 💡

• The perfect base pairing they discovered instantly suggested a copy mechanism: separate the strands, match up complements, and voilà — two DNAs!:contentReference[oaicite:1]{index=1}

2. Proof in the (💚+💙) Tube — Meselson & Stahl, 1958

1. Grew E. coli in heavy nitrogen medium (¹⁵NH₄Cl) so all DNA became “heavy.”
2. Shifted cells to normal nitrogen (¹⁴NH₄Cl) and collected DNA after each 20-min generation.
3. Spun DNA in CsCl density gradients. First generation showed a hybrid band (one heavy strand + one light). Second generation produced 50 % hybrid and 50 % light DNA. Perfect match for semiconservative replication!:contentReference[oaicite:2]{index=2}

Similar work with Vicia faba chromosomes confirmed the same pattern in plants.:contentReference[oaicite:3]{index=3}

3. The Cellular Toolkit ⚙️

• DNA-dependent DNA polymerase zips along the template and adds nucleotides only in the \(5′ \rightarrow 3’\) direction. It’s blazing fast — about 2 000 bp / s in E. coli! (Genome size ≈ \(4.6 \times 10^{6}\) bp).:contentReference[oaicite:4]{index=4}
• Energy comes from the high-energy bonds in dNTPs; when two phosphates pop off, the released energy drives chain growth.:contentReference[oaicite:5]{index=5}
• Replication fork = the tiny open stretch where new strands grow.
• Because polymerase needs \(5′ \rightarrow 3’\):
  • The leading strand (template \(3′ \rightarrow 5’\)) forms continuously.
  • The lagging strand (template \(5′ \rightarrow 3’\)) forms in pieces (Okazaki fragments).
  • DNA ligase stitches the fragments into one smooth strand.:contentReference[oaicite:6]{index=6}

4. Getting Started — Origins 🚀

• Replication kicks off at a specific origin of replication. Without an origin, DNA can’t copy itself — that’s why cloning vectors always carry one.:contentReference[oaicite:7]{index=7}

5. Replication in Bigger Cells 🧑‍🔬

• In eukaryotes, copying happens during the S-phase of the cell cycle. Timing must sync perfectly with mitosis; if division stalls but replication finishes, cells become polyploid (extra chromosome sets).:contentReference[oaicite:8]{index=8}

🚀 High-Yield NEET Nuggets

1. Semiconservative model — every new DNA keeps one parental strand.
2. Meselson-Stahl experiment — the gold standard evidence.
3. Directionality of polymerase (\(5′ \rightarrow 3’\)) & resulting leading vs. lagging strands.
4. Role of DNA ligase in sealing Okazaki fragments.
5. Origin of replication — essential start point (and why vectors need it).

Keep these gems in mind, and you’ll ace any DNA-replication question! 🎯✨