Justify experimental controls
Explain why a transformation experiment needs a positive control that should grow and a negative control that should not.
- Control vs. experimental group: You must know a control is a comparison group before you can tell a positive from a negative control.
- Selective media lets only some cells grow: Knowing that selective media kills cells without the resistance gene explains what each control should show.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
In a transformation experiment, a positive control is expected to grow on selective media (proving the steps work), and a negative control with no plasmid is expected to NOT grow (proving the media is really selecting).
Cells with NO plasmid are spread on a plate containing antibiotic, as the negative control. The diagram shows two plates. What should this negative-control plate show, and why?
Reviewed| Plate | What was added |
|---|---|
| Positive control | cells + plasmid with resistance gene |
| Negative control | cells, no plasmid |
- A.Colonies should grow, because every cell survives antibiotic
- B.No colonies should grow, because the cells lack the resistance gene the antibiotic selects for
- C.Colonies should grow, because the antibiotic feeds the cells
- D.No colonies should grow, because plasmids are poisonous to cells
Show the worked solution ▾
Answer: B. No colonies should grow, because the cells lack the resistance gene the antibiotic selects for
- Step 1: Check what the cells have: The negative-control cells got no plasmid, so they have no antibiotic-resistance gene.
- Step 2: Apply the selection: The antibiotic in the plate kills cells that lack the resistance gene, so these cells should die and leave no colonies.
Why it's right: With no plasmid the cells have no resistance gene, so the antibiotic kills them and the negative-control plate should stay clear.
- A: Cells without the resistance gene do not survive the antibiotic.
- C: Antibiotic kills unprotected cells; it does not feed them.
- D: The plasmid is not poisonous; the cells simply never received one.
Aligned to BI 6.1: negative control on selective media · reading level ~grade 9
- A transformation lab plate map labels one plate 'expected growth' and one 'expected no growth': those are the positive and negative controls.
Fill these in as you work through the lesson.
- Positive control (the should-work check):
- Negative control (the should-not-work check):
- Transformation (a cell taking up a plasmid):
- Selective media (a plate that only lets resistant cells grow):
On selective media, the positive control is expected to and the negative control is expected to , so each one checks a different part of the setup.
- What is a positive control supposed to show you?
- What is a negative control supposed to rule out?
- On a plate with antibiotic, why should cells given no plasmid stay empty?
A lab tries to transform bacteria with a plasmid carrying an antibiotic-resistance gene, then plates them on antibiotic media. Describe what a good positive control and a good negative control would be, and what each plate should look like.