Biology · Heredity · Block 1
DNA
Structure & Replication
How a twisted ladder holds the instructions for life — and how it copies itself before a cell divides.
Two meters of DNA, folded into a space too small to see.
If you remember one thing from this block
A pairs with T.
C pairs with G.
Always.
This rule is the foundation of base pairing, replication, transcription, translation, mutations, and genetics.
What DNA is made of
From one nucleotide to the whole helix
Each nucleotide is phosphate + sugar + one base. The bases pair across the strands — A with T, C with G — and the whole thing twists into a double helix.
Why these pairs?
The shapes fit. And the bonds hold.
A and T are held together by 2 hydrogen bonds.
C and G are held together by 3 hydrogen bonds.
The information is in the order of the bases, not in the sugar or the phosphate. The sugar-phosphate backbone holds the strand together — it does not carry the message.
A twisted ladder. Two strands wound around each other. Same shape in every living thing on Earth.
She fired X-rays at crystallized DNA and photographed the shadow. The pattern showed a helix — the proof we needed.
How we figured out the structure
Four scientists, twelve years.
1940s — Erwin Chargaff measured the bases in DNA from many species. Same pattern every time: %A = %T and %C = %G. He didn't know why.
1952 — Rosalind Franklin took Photo 51, an X-ray diffraction image of crystallized DNA. The pattern was the fingerprint of a helix.
1953 — James Watson & Francis Crick saw Franklin's photo and the Chargaff numbers and built the model: two strands, antiparallel, A with T, C with G, twisted.
The structure explained the rules. Once you saw the shape, you understood why the pairing has to be A–T and C–G — and you could guess, immediately, how DNA copies itself.
Chargaff's rule, as a chain
%A = %T and %C = %G. All four percentages add to 100. Always.
Worked example
A DNA sample is 30% adenine. What are the other three?
Step 1. A pairs with T, so T = 30%.
Step 2. A + T = 30 + 30 = 60%. That leaves 40% for C and G together.
Step 3. C pairs with G, so they split that 40% evenly: C = 20%, G = 20%.
Check. 30 + 30 + 20 + 20 = 100. ✓
Part 2 of 2
But how does a cell copy it?
Before a cell divides, it has to make a complete second copy of all its DNA. Three steps. One famous experiment.
DNA replication, in three steps
1
Unzip
The two strands of the double helix come apart in the middle. The hydrogen bonds between the bases break. Each strand is now exposed.
2
Match
Free nucleotides floating in the cell pair with the exposed bases — A with T, C with G — and link end-to-end into a new strand alongside each old one.
3
Two helixes
When the matching is done, the cell has two complete double helixes where it had one before. Each new helix is half old, half new.
The replication fork
One double helix becomes two
Each daughter helix has one old strand (solid) and one new strand (dashed). The original is split between two copies.
The name for this
Semiconservative replication.
Semi means half. Half of each new DNA molecule is saved (conserved) from the original. The other half is brand new.
Two daughter helixes. Each is 50% old + 50% new. Neither is fully old, neither is fully new.
If a test question says "each new DNA molecule has one old strand and one new strand" — that's semiconservative.
How we proved it · 1958
Meselson and Stahl: the prettiest experiment in biology.
They grew bacteria for many generations in food made with heavy nitrogen (¹⁵N), so the DNA was heavy. Then they switched the bacteria to normal nitrogen (¹⁴N) and let them divide.
After one round of copying: every DNA molecule had medium weight — half heavy, half light. After two rounds: half medium-weight, half all-light.
That pattern only works if each new DNA molecule keeps one old strand and gets one new strand. The experiment ruled out every other way DNA could have copied itself. Semiconservative wins.
Why this is the foundation
Every time a cell divides
Mitosis & growth
Skin healing, bone growing, hair growing — every new cell gets a full DNA copy first. Replication is the step before mitosis.
Parents → children
Heredity
The reason you have your parents' traits: their DNA was copied (with rare mistakes) into the egg and sperm that made you. The pairing rule preserves the information.
From bacteria to whales
One universal code
The same four bases. The same pairing rule. The same replication machinery — give or take. All life on Earth runs on this chemistry.
The whole lesson, in two sentences
Structure
Two strands. Twisted into a ladder. Bases pair across the rungs: A with T, C with G. The order of the bases is the message.
Replication
The strands unzip. Each one is a template. New bases pair to the old. Two helixes where there was one — each half old, half new.
The chemistry is the copy mechanism. The shape tells you how it copies. That's why the structure was the breakthrough.
One more time
A pairs with T.
C pairs with G.
Always.
Everything in this block — and the next three blocks — follows from this one rule.
Write it down — all four
What you walk out of class knowing.
In your notebook, answer:
- What is the building block of DNA, and what three parts does it have?
- The base-pairing rule: which base pairs with which?
- What are the three steps of DNA replication?
- What does "semiconservative" mean — in your own words?