Semi-conservative replication & the Meselson-Stahl experiment
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Question
What is DNA replication?
Answer
The process of copying a DNA molecule to make **two identical molecules**, done before a cell divides.
Question
What does 'semi-conservative' replication mean?
Answer
Each new DNA molecule is made of **one original (parental) strand and one new strand**.
Question
Why does 'semi' (half) appear in the name?
Answer
Because **half** of each new molecule — one whole strand — is **conserved** from the original.
Question
What is a template strand?
Answer
An **old strand** used as a pattern to build a new complementary strand.
Question
How are new strands built against each template?
Answer
By **complementary base pairing** — A pairs with T, and C pairs with G.
Question
What does each daughter molecule contain after replication?
Answer
**One old (parental) strand and one new strand** — never two old or two new together.
Question
What were the three possible models of replication?
Answer
**Conservative**, **semi-conservative** and **dispersive**.
Question
How did Meselson and Stahl label the DNA?
Answer
They grew bacteria in **heavy ¹⁵N** (so all DNA was heavy), then switched them to **light ¹⁴N**.
Question
What did the single intermediate band in generation 1 show?
Answer
Every molecule was half-heavy and half-light (one old + one new strand) — this **ruled out the conservative model**.
Question
What did generation 2 (intermediate + light bands) show?
Answer
Some molecules were now fully light — this **ruled out the dispersive model**, leaving only semi-conservative.
Question
What conclusion did Meselson and Stahl reach?
Answer
That DNA replication is **semi-conservative**.
Question
On a Paper 1A diagram, how do you spot semi-conservative replication?
Answer
Each daughter molecule shows **one old (template) strand paired with one new strand**.
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Topic 4.1 hub
DNA replication
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