5  Discovery of DNA as Genetic Material

5.1 The Great Mystery

5.1.1 What Scientists Wondered

In the early 1900s, scientists knew that children inherited traits from their parents (thanks to Mendel!). But they had a big question:

What exactly carries genetic information from parents to children?

It’s like knowing that letters get delivered to your house, but not knowing who delivers them!

Scientists knew cells had several important parts:

  • Proteins (which do most of the work)

  • DNA (which was known but seemed too simple)

  • RNA (also known but mysterious)

  • Carbohydrates (sugars) and lipids (fats)

Most scientists thought proteins must carry genetic information because they seemed complex enough. DNA seemed too simple with only 4 “letters”!

They were wrong—and it took clever experiments to prove it!

5.2 The First Clue: Griffith’s Experiment (1928)

5.2.1 The Mystery of the Killer Bacteria

A British scientist named Frederick Griffith was studying bacteria that caused pneumonia (a lung disease).

He discovered two types of these bacteria:

S-type bacteria (Smooth):

  • Had a smooth, slimy coating

  • Made mice sick and killed them

  • Looked smooth and shiny in a dish

R-type bacteria (Rough):

  • Had no coating

  • Did NOT make mice sick

  • Looked rough and bumpy in a dish

Think of the S-type as wearing a protective raincoat, while the R-type bacteria were naked and harmless!

5.2.2 The Surprising Discovery

Griffith did four experiments:

Experiment 1: Give live S-type bacteria to a mouse

  • Result: Mouse dies 😢

Experiment 2: Give live R-type bacteria to a mouse

  • Result: Mouse lives! 😊

Experiment 3: Heat-kill the S-type bacteria (they’re dead!), then give them to a mouse

  • Result: Mouse lives! 😊

  • Makes sense—dead bacteria can’t hurt you!

Experiment 4: Mix dead S-type bacteria with live R-type bacteria, give to mouse

  • Result: Mouse dies! 😱

Wait, what?! The dead bacteria somehow made the harmless bacteria deadly!

5.2.3 What Happened?

Griffith found that:

  • Something from the dead S-type bacteria “transformed” the living R-type bacteria

  • The R-type bacteria became S-type bacteria and could now kill mice

  • Even worse, the transformed bacteria stayed S-type forever and passed this on to their babies!

Griffith called this transformation—something from the dead bacteria changed the living bacteria’s genetic information permanently!

But what was that “something”? Griffith didn’t know. He guessed it was some kind of “transforming principle.”

5.3 The Second Clue: Avery, MacLeod, and McCarty (1944)

5.3.1 Detective Work!

Sixteen years later, three scientists—Oswald Avery, Colin MacLeod, and Maclyn McCarty—decided to solve Griffith’s mystery.

They wanted to find out: What exactly is the “transforming principle”?

5.3.2 The Experiment Plan

They were like detectives eliminating suspects one by one!

Here’s what they did:

  1. Took dead S-type bacteria

  2. Separated the bacteria into different parts (proteins, DNA, RNA, carbohydrates, lipids)

  3. Tested each part to see which one could transform R-type bacteria into S-type

5.3.3 The Process

Test 1: Remove all the proteins

  • Mixed the remaining stuff with R-type bacteria

  • Result: Transformation still happened!

  • Conclusion: It’s NOT proteins! (This was shocking—everyone thought it was proteins!)

Test 2: Remove all the RNA

  • Mixed the remaining stuff with R-type bacteria

  • Result: Transformation still happened!

  • Conclusion: It’s NOT RNA!

Test 3: Remove all the carbohydrates and lipids

  • Mixed the remaining stuff with R-type bacteria

  • Result: Transformation still happened!

  • Conclusion: It’s NOT carbohydrates or lipids!

Test 4: Destroy only the DNA (using an enzyme called DNase)

  • Mixed the remaining stuff with R-type bacteria

  • Result: NO transformation happened!

  • Conclusion: IT’S DNA!!!

5.3.4 The Big Discovery

Avery, MacLeod, and McCarty proved that:

  • DNA is the genetic material

  • DNA carries information from one generation to the next

  • Proteins are NOT the genetic material (surprise!)

This was HUGE news! But some scientists still didn’t believe it. They needed more proof.

5.4 The Final Proof: The Hershey-Chase Experiment (1952)

5.4.1 Enter the Bacteriophages!

Eight years later, two scientists—Alfred Hershey and Martha Chase—did another brilliant experiment.

They studied bacteriophages (we can call them “phages” for short). Phages are viruses that attack bacteria.

Think of phages like tiny spaceships that:

  • Land on bacteria

  • Inject something inside

  • Use the bacteria to make more phages

  • Eventually burst the bacteria and release new phages

The Question: What do phages inject into bacteria—DNA or protein?

5.4.2 The Clever Trick: Radioactive Labels

Hershey and Chase used a brilliant trick—they “tagged” molecules with radioactive labels (like putting glow-in-the-dark stickers on things so you can track them!).

Key Facts About Phages:

  • Phages have a protein coat on the outside

  • Phages have DNA on the inside

  • Proteins contain an element called sulfur

  • DNA contains an element called phosphorus

5.4.3 The Experiment

They did two experiments:

Experiment 1: Label the Protein

  1. Grew phages with radioactive sulfur (this labels the protein)

  2. Let the phages infect bacteria

  3. Used a blender to separate the phage coats from the bacteria (yes, really—a blender!)

  4. Checked where the radioactive label was

Result: The radioactive sulfur stayed OUTSIDE the bacteria (in the phage coat) Conclusion: Protein doesn’t go into the bacteria

Experiment 2: Label the DNA

  1. Grew phages with radioactive phosphorus (this labels the DNA)

  2. Let the phages infect bacteria

  3. Used a blender to separate the phage coats from the bacteria

  4. Checked where the radioactive label was

Result: The radioactive phosphorus was found INSIDE the bacteria! Conclusion: DNA goes into the bacteria

5.4.4 The Final Verdict

After the bacteria were infected:

  • New phages were created

  • These new phages had radioactive phosphorus (DNA), NOT radioactive sulfur (protein)

This proved: DNA is the genetic material that gets passed from parent to offspring!

Think of it like this:

  • The phage is a syringe

  • The protein coat is the syringe body (stays outside)

  • The DNA is the medicine (gets injected inside)

  • The DNA contains the instructions to make new phages!

5.5 Why These Experiments Were Brilliant

5.5.1 They Used the Scientific Method

All three experiments followed great scientific thinking:

  1. Observation: Something interesting happens

  2. Question: Why does it happen?

  3. Hypothesis: Maybe it’s because of X

  4. Experiment: Test if X is true

  5. Conclusion: Based on results, accept or reject the hypothesis

5.5.2 They Were Creative

  • Griffith: Used living and dead bacteria in different combinations

  • Avery, MacLeod, McCarty: Systematically removed different molecules

  • Hershey and Chase: Used radioactive labels to track molecules

5.5.3 They Built on Each Other’s Work

Science is like building a tower—each scientist adds new blocks on top of previous discoveries:

  • Griffith found transformation (but didn’t know what caused it)

  • Avery, MacLeod, McCarty identified DNA as the transforming principle

  • Hershey and Chase confirmed DNA is the genetic material

5.6 The Impact

5.6.1 Changing Biology Forever

These discoveries:

  • Proved DNA carries genetic information

  • Shifted the focus of biology to DNA research

  • Led to the discovery of DNA’s structure (the double helix) in 1953

  • Started the age of molecular biology

  • Eventually led to the Human Genome Project

  • Made modern genetic engineering possible

5.6.2 Real-World Applications Today

Understanding that DNA is genetic material allows us to:

  • Identify criminals through DNA fingerprinting

  • Test for genetic diseases

  • Create GMO crops that resist pests

  • Develop personalized medicine

  • Study evolution and how species are related

  • Bring back extinct species (like trying to revive mammoths!)

5.7 Fun Facts! 🎉

  • Griffith never knew his work would lead to discovering DNA as genetic material—he died in 1941, before Avery’s team published their results!

  • Avery was 67 years old when he made his discovery—proving you’re never too old to do great science!

  • Martha Chase left science to raise a family but her contribution to the Hershey-Chase experiment was crucial

  • Alfred Hershey won the Nobel Prize in 1969 (for different work on phages)

  • The “blender” they used was actually called a Waring blender—the same kind used to make smoothies!

  • Before these experiments, most scientists bet on proteins being genetic material because proteins seemed more complex

5.8 Key Takeaways

  • Early Mystery: Scientists knew traits were inherited but didn’t know what molecule carried genetic information

  • Griffith (1928): Discovered “transformation”—something from dead bacteria could change living bacteria

  • Avery, MacLeod, McCarty (1944): Identified DNA as the “transforming principle”

  • Hershey-Chase (1952): Confirmed DNA (not protein) is injected into bacteria and carries genetic information

  • Radioactive labeling: Clever technique to track molecules

  • These discoveries proved DNA is the genetic material

  • This knowledge revolutionized biology and medicine

  • Science builds on previous discoveries—each experiment answered new questions

Sources: Information adapted from Nature Scitable, Khan Academy Biology, university molecular biology courses, and historical accounts of these famous experiments.