Lac operon worksheet - Name: The Lac Operon Page 1 of 3 The Lac ... - Free Printable

Let’s go step by step through each part of the worksheet. We’ll check what happens in each scenario based on how the lac operon works.

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1a. WT – Lactose is absent

In wild-type (normal) bacteria, when lactose is NOT present:

- The regulator gene `lacI` makes a repressor protein (shown as 🟠).
- This repressor binds to the operator region (`lacO`) and blocks RNA polymerase from transcribing the structural genes (`lacZ`, `lacY`, `lacA`).
- So, no β-galactosidase (□), permease (△), or transacetylase (◇) are made.
- Transcription arrow should be drawn only for `lacI` → because it’s always “on” (constitutive).
- No transcription arrow over `lacP-lacZ-lacY-lacA` — blocked by repressor.
- Repressor is active and bound to DNA at `lacO`.

✔ What’s already drawn:
- Arrow under `lacI` → correct.
- Repressor symbol below → correct.
- Repressor bound at `lacO` → correct.
- NO arrows over structural genes → correct.
- NO product symbols on right → correct.

✔️ This part is correctly filled out.

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1b. WT – Lactose is present

When lactose IS present:

- Lactose acts as an inducer — it binds to the repressor protein.
- This changes the shape of the repressor so it can’t bind to the operator anymore.
- Now RNA polymerase can move past the promoter and transcribe `lacZ`, `lacY`, `lacA`.
- So we get: β-galactosidase (□), permease (△), transacetylase (◇).
- Also, `lacI` still makes repressor — but now it’s inactive (crossed out).
- Transcription arrow goes over structural genes.
- Repressor is crossed out (inactivated by lactose).

✔ What’s already drawn:
- Arrow under `lacI` → correct.
- Crossed-out repressor → correct (shows it’s inactivated).
- Long arrow over structural genes → correct.
- Product symbols on right: □, △, ◇ → all correct.

✔️ This part is also correctly filled out.

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2. LacOᶜ mutant – Lactose is absent

This is a mutation in the operator region — called “operator constitutive” (`LacOᶜ`). That means the operator sequence is changed so the repressor CANNOT bind to it — even if lactose is absent.

So:

- `lacI` still makes repressor (🟠) — that’s fine.
- BUT the repressor cannot bind to the mutated operator (shown with red circle and slash — good!).
- Therefore, RNA polymerase can always transcribe the structural genes — even without lactose!
- So you WILL get β-galactosidase (□), permease (△), transacetylase (◇) — even though lactose is absent.
- Transcription arrow must go over structural genes.
- Repressor is made but doesn’t bind — so don’t cross it out (it’s not inactivated by lactose; it just can’t bind due to mutation).

✔ What’s already drawn:
- Arrow under `lacI` → correct.
- Repressor symbol below → correct (not crossed out — good!).
- Red circle/slash over `lacO` → shows mutation preventing binding → correct.
- Long arrow over structural genes → correct.
- Product symbols on right: □, △, ◇ → correct.

✔️ This part is ALSO correctly filled out!

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Wait — let me double-check everything one more time before finalizing.

In 1a: No lactose → repressor ON → blocks transcription → no products → ✔️
🔹 In 1b: Lactose present → repressor OFF (crossed out) → transcription ON → products made → ✔️
🔹 In 2: Operator mutant → repressor CAN’T bind → transcription ALWAYS ON → products made even without lactose → ✔️

All diagrams match expected biology.

There’s nothing missing or wrong here.

But wait — looking again at section 2: The instruction says “indicate which proteins produced bind to DNA and where”.

In section 2, the repressor is made (symbol shown), but does it bind to DNA? NO — because operator is mutated. So we should NOT show it bound anywhere. And indeed, there’s no repressor drawn bound to DNA — only floating below. Good.

Also, do any of the products (□, △, ◇) bind to DNA? No — they’re enzymes involved in metabolism, not DNA-binding proteins. So no need to show them binding.

Everything checks out.

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Final Answer:
The worksheet is already completely and correctly filled out for all three scenarios (1a, 1b, and 2). No corrections or additions are needed.