🧬 The Search for Genetic Material

1. A Quick Historical Snapshot 📜

Mendel had linked heredity to “factors,” and by the early 1900s scientists knew these factors lived on chromosomes. But which molecule carried the instructions? This mystery drove the classic experiments below — each shining new light on life’s instruction manual. :contentReference[oaicite:0]{index=0}

2. Griffith’s “Transforming Principle” 🐭

  • Smooth (S) Streptococcus pneumoniae wore a sugary coat and killed mice; rough (R) strains lacked the coat and were harmless. :contentReference[oaicite:1]{index=1}
  • Heating destroyed S bacteria, and the heat-killed cells no longer harmed mice. :contentReference[oaicite:2]{index=2}
  • Mixing heat-killed S with live R shocked everyone — the mice died, and living S bacteria appeared! Something from the dead S cells transformed the R cells. :contentReference[oaicite:3]{index=3}
  • Conclusion: a “transforming principle” passes traits between cells. What was it? 🤔

3. Avery, MacLeod & McCarty — Pinning It on DNA 🔬

  • They purified proteins, RNA, and DNA from heat-killed S bacteria. Only the DNA fraction converted harmless R cells into deadly S cells. :contentReference[oaicite:4]{index=4}
  • Protease and RNase (protein- & RNA-destroying enzymes) had no effect. DNase (DNA-destroying) blocked transformation — smoking-gun evidence that DNA was the hereditary molecule. :contentReference[oaicite:5]{index=5}

4. Hershey–Chase Blender Experiment 🦠

  • Bacteriophages were grown with either 32P (DNA label) or 35S (protein label). :contentReference[oaicite:6]{index=6}
  • When labeled viruses infected E. coli and the mixture was “blender-whirled,” only 32P (DNA) entered the bacteria; 35S stayed outside. :contentReference[oaicite:7]{index=7}
  • Therefore, DNA —not protein — carries viral instructions into the cell. :contentReference[oaicite:8]{index=8}

5. What Makes a Good Genetic Material? 💡

Any hereditary molecule must:

  1. Self-copy (Replication) 🌀
  2. Be chemically & structurally stable 🛡️
  3. Permit slow, inheritable change (Mutation) 🔄
  4. Show its effect as visible traits (Expression) 🌱

DNA and RNA can both replicate by base-pairing, but DNA’s chemistry makes it far less reactive (no 2′-OH, plus thymine instead of uracil), giving it superior stability. :contentReference[oaicite:9]{index=9}

6. DNA vs RNA — Why DNA Took Over 👑

  • RNA is single-stranded, catalytic, and mutates quickly — great for short-term messages but not for long-term archives. :contentReference[oaicite:10]{index=10}
  • DNA’s double helix, complementary strands, and use of thymine make it sturdier and easier to repair. :contentReference[oaicite:11]{index=11}
  • Hence, organisms store genes in DNA, while RNA acts as messenger, adapter, and sometimes (in certain viruses) a temporary genome. :contentReference[oaicite:12]{index=12}

7. The RNA World Hypothesis 🌍

Early life probably relied on RNA for both information and catalysis. Over time, DNA evolved from RNA, adding durability, while proteins took over most catalytic jobs. :contentReference[oaicite:13]{index=13}

🔥 High-Yield NEET Nuggets

  1. Griffith transformation proves hereditary material can move between cells. :contentReference[oaicite:14]{index=14}
  2. Avery–MacLeod–McCarty identify DNA as the transforming substance. :contentReference[oaicite:15]{index=15}
  3. Hershey–Chase blender test unequivocally confirms DNA as genetic material. :contentReference[oaicite:16]{index=16}
  4. Key properties of genetic material: replication, stability, mutation, expression. :contentReference[oaicite:17]{index=17}
  5. DNA vs RNA — chemical differences explain DNA’s dominance, yet RNA’s role in early evolution (“RNA World”) remains pivotal. :contentReference[oaicite:18]{index=18}