In AP Biology, an auxotroph is an organism (usually a bacterium) that carries a mutation disabling its ability to synthesize a specific compound, such as an amino acid, so it can only grow when that compound is supplied in its medium.
An auxotroph is an organism that can't make some molecule it needs to live, like a particular amino acid or vitamin. The original ("wild-type") strain could build that molecule from scratch. A mutation broke the gene for one of the enzymes in that pathway, and now the organism has to get the finished product handed to it in the growth medium.
This ties straight into Topic 6.7 (Mutations). A change in the DNA sequence alters the protein produced, which changes the phenotype (EK 6.7.A.1). For an auxotroph, the broken protein is an enzyme. A point mutation, frameshift, or nonsense mutation can knock out that enzyme so it no longer works, and the organism loses a metabolic ability it used to have. The flip side, a prototroph, makes everything it needs on a minimal medium with no extras.
Auxotrophs live in Unit 6: Gene Expression and Regulation, specifically Topic 6.7. They're a clean, concrete example of the chain the CED wants you to trace: a DNA mutation changes a protein, and the changed protein changes the phenotype (EK 6.7.A.1, learning objective AP Bio 6.7.A). You can also link them to AP Bio 6.7.B, since whether losing that pathway hurts the organism depends entirely on the environment. On a plate full of the missing amino acid, an auxotroph is perfectly fine. On a minimal plate, it can't grow. That environment-dependent cost is exactly the idea behind "beneficial, detrimental, or neutral depends on the environmental context" (EK 6.7.B.1).
Keep studying AP® Biology Unit 6
Auxotrophic Mutant (Unit 6)
These are basically the same idea named two ways. The mutation is the cause; the auxotroph is the organism that results. If a strain has an auxotrophic mutation in its tryptophan pathway, that strain IS a tryptophan auxotroph.
Nonsense and Frameshift Mutations (Unit 6)
Auxotrophy is what those mutation types look like in real life. A nonsense or frameshift mutation in an enzyme gene can shut down that enzyme entirely, which is exactly how a working biosynthesis pathway breaks and turns a prototroph into an auxotroph.
Natural Selection and Environmental Context (Unit 7)
Losing a pathway isn't automatically bad. If the missing nutrient is always available, the auxotroph saves energy not building it and may even outcompete the wild type. The same mutation that's harmful in one environment can be favored in another (EK 6.7.B.1).
Auxotroph is a vocabulary anchor for cause-and-effect reasoning about mutations. Expect multiple-choice stems describing a bacterium that grows on "complete" medium but not "minimal" medium, then asking you to identify that the strain has a mutation in a biosynthesis gene. The skill is connecting genotype to phenotype: a broken gene means a broken enzyme means an organism that can't make a needed compound. No released FRQ uses the word "auxotroph" verbatim, but the underlying logic, that a DNA change alters a protein and therefore the phenotype, is exactly what Topic 6.7 free-response prompts reward. If asked, be ready to say whether the mutation is harmful, neutral, or beneficial and explain that it depends on the environment.
A prototroph is the wild-type that can synthesize everything it needs and grows on bare minimal medium. An auxotroph lost one of those abilities to a mutation and needs the missing compound supplied. Quick test: minimal medium only feeds prototrophs; auxotrophs need the extra ingredient.
An auxotroph is an organism that can't synthesize a compound it needs and must get it from the growth medium.
Auxotrophy is caused by a mutation that breaks an enzyme in a biosynthesis pathway, a direct example of how genotype changes phenotype (EK 6.7.A.1).
A prototroph makes everything it needs on minimal medium; an auxotroph requires the missing nutrient to be added.
Whether losing the pathway is harmful or neutral depends on the environment, which ties into EK 6.7.B.1.
On the test, a strain that grows on complete medium but not minimal medium is the classic signal of an auxotroph.
It's an organism, usually a bacterium, that carries a mutation disabling its ability to make a needed molecule like an amino acid, so it can only grow when that molecule is provided in its medium. It's a Topic 6.7 example of a mutation changing phenotype.
No. If the missing nutrient is always available in the environment, the auxotroph is fine and may even save energy by not building it. The cost only shows up when that nutrient is gone, which is the environment-dependent idea in EK 6.7.B.1.
A prototroph is the wild-type that can synthesize everything it needs and grows on plain minimal medium. An auxotroph lost one of those abilities to a mutation and needs the missing compound supplied to grow.
Any mutation that disables an enzyme in a biosynthesis pathway can do it, including point, nonsense, or frameshift mutations. The broken enzyme means the pathway stops, so the organism can no longer make its product.
Look for a strain that grows on complete (or supplemented) medium but fails to grow on minimal medium. That gap means it can't make something on its own, which signals an auxotroph.
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