GEN 1.1 Race to Discover DNA- Pre AP Biology Study Notes - New Syllabus.

Race to Discover DNA

🌿 Introduction

DNA structure was not discovered suddenly.
It was a scientific race. Different scientists collected different pieces of evidence.
When all pieces were combined, the final model of DNA was formed.

📌 Important idea: Scientific models change when new evidence is discovered.

This topic is about how the DNA model improved over time.

🧠 Step 1 – What Is a Scientific Model?

Before jumping into the scientists, understand this clearly:

A scientific model:

• Explains a structure or process
• Is based on evidence
• Can change when new data appears
• Must match experimental results

DNA structure is a perfect example of model improvement.

🧪 Step 2 – Chargaff’s Observations (Chemical Evidence)

Scientist: Erwin Chargaff

He studied the chemical composition of DNA from different organisms.

What He Found

• Amount of Adenine (A) = Amount of Thymine (T)
• Amount of Guanine (G) = Amount of Cytosine (C)

This is called:

Chargaff’s Rule

A = T
G = C

Why This Was Important

• DNA bases follow a pattern
• Bases are paired in some specific way
• DNA is not randomly arranged

But at this stage:

• Shape of DNA was unknown
• 3D structure was not discovered

So only chemical ratios were known, not structure.

📸 Step 3 – Franklin’s Structural Evidence

Scientist: Rosalind Franklin

She used X-ray diffraction to study DNA.

This technique helps scientists understand molecular shape.

Her famous image was called Photo 51.

What Her Work Showed

• DNA has a helical shape
• It has a uniform width
• Nitrogenous bases are located near the center
• Sugar-phosphate backbone is on the outside

Why This Was a Big Step

• First strong evidence that DNA is a helix
• Confirmed DNA has repeating structure
• Gave clues about internal arrangement

But:

• Exact base pairing pattern was still not fully explained

Now scientists knew the shape, but not the complete model.

🧠 Step 4 – Watson and Crick’s Final Model

Scientists: James Watson and Francis Crick

They combined:

• Chargaff’s base ratio data
• Franklin’s X-ray diffraction evidence
• Model building approach

They physically built a 3D model to see if all evidence fits together.

Final Consensus Model (Double Helix)

Their model explained everything clearly.

Main Features:

1. DNA is a double helix
Two strands twisted around each other

2. Strands are antiparallel
They run in opposite directions

3. Sugar-phosphate backbone is on the outside

4. Nitrogen bases are inside

5. Base pairing follows Chargaff’s rule
A pairs with T
G pairs with C

This pairing is called complementary base pairing.

🔄 How the Model Changed Over Time

Each step solved a different puzzle piece.

Consensus means:
The scientific community accepted it because it explained all evidence.

🧠 Simple Memory Trick

Think in order:

C → F → W

Chargaff → Franklin → Watson & Crick

First numbers
Then shape
Then final structure

📦 Quick Recap 
DNA model improved step by step.
Chargaff discovered base ratios A=T and G=C.
Franklin proved DNA is a helix with bases inside.
Watson and Crick built the double helix model.
Scientific models change when new evidence emerges.