Cystic Fibrosis

In AP Bio, cystic fibrosis is a genetic disorder caused by mutations in the CFTR gene, which encodes a chloride channel. The most common mutation, ΔF508, deletes three nucleotides and removes one amino acid, so the protein misfolds and chloride transport fails, leading to thick mucus.

What is Cystic Fibrosis?

Cystic fibrosis (CF) is a genetic disorder caused by mutations in the CFTR gene. CFTR codes for a gated ion channel that sits in the cell membrane and lets chloride ions (Cl⁻) move across an epithelium. When the channel doesn't work, salt and water balance gets thrown off, and the body produces thick, sticky mucus that clogs the lungs, pancreas, and other organs.

The most famous mutation is ΔF508, a deletion of three nucleotides in the CFTR gene. That deletion removes a single amino acid, a phenylalanine at position 508. The protein still gets made, but it folds wrong, so the cell flags it as defective and destroys it before it ever reaches the membrane. No channel at the surface means no chloride transport. This is the textbook AP example of how a small change in DNA sequence (genotype) cascades into a major change in phenotype (EK 6.7.A.1, EK 6.7.B.1).

Why Cystic Fibrosis matters in AP Biology

CF lives in Unit 6: Gene Expression and Regulation, specifically Topic 6.7 Mutations. It's the case study College Board reaches for when testing learning objective AP Bio 6.7.A (describe types of mutation) and AP Bio 6.7.B (explain how changes in genotype produce changes in phenotype). The big idea is the chain: mutation → altered protein → altered phenotype. CF shows that you can't just memorize "mutations are bad." You have to trace exactly how a specific DNA change breaks a specific protein and produces a specific symptom.

How Cystic Fibrosis connects across the course

Frameshift and Point Mutations (Unit 6)

CF is a menu of mutation types in one gene. The classic ΔF508 deletes three nucleotides, so the reading frame stays intact but one amino acid is lost. A single-nucleotide change in CFTR can instead create a premature stop codon (a nonsense mutation), and other insertions or deletions can cause frameshifts. Same gene, different mutation, different damage.

Chloride Channels and Membrane Transport (Units 2 and 6)

CFTR is a chloride channel, so CF ties the molecular genetics of Unit 6 back to the membrane transport ideas of Unit 2. The 2018 FRQ even noted the channel needs ATP binding to open, linking gene expression to active transport and cell function.

Genetic Variation and Natural Selection (Units 6 and 7)

CFTR mutations are detrimental in most environments, but carriers (one copy) may have had a survival edge against certain diseases. That's the core idea of AP Bio 6.7.C: whether a mutation is harmful, neutral, or beneficial depends on the environmental context, which is what natural selection acts on.

Is Cystic Fibrosis on the AP Biology exam?

Expect CF on multiple-choice as a mechanism question, not a memorization one. A stem will hand you a specific CFTR mutation and ask you to predict the cellular phenotype. For ΔF508, the answer is that the protein misfolds and gets degraded, so no functional channel reaches the membrane and chloride transport fails. If the stem describes a single nucleotide change that creates a premature stop codon, you classify it as a nonsense mutation and explain the truncated, nonfunctional protein. The 2018 Short FRQ Q6 framed CFTR as a gated ion channel that requires ATP binding to move Cl⁻, so be ready to connect the gene to the protein's actual job. The skill being tested is tracing genotype to phenotype, so always show the chain: DNA change → protein change → loss of function → symptom.

Cystic Fibrosis vs Sickle Cell Disease

Both are classic AP genetic-disorder examples, but the mutation types differ. Sickle cell comes from a point mutation (one nucleotide swap that changes one amino acid in hemoglobin), while the famous CF mutation, ΔF508, is a three-nucleotide deletion that removes an entire amino acid and causes the CFTR protein to misfold and get destroyed.

Key things to remember about Cystic Fibrosis

• Cystic fibrosis is caused by mutations in the CFTR gene, which codes for a chloride channel in the cell membrane.

• The most common mutation, ΔF508, deletes three nucleotides and removes a phenylalanine at position 508, so the protein misfolds and the cell degrades it before it reaches the membrane.

• Because no working channel reaches the surface, chloride transport fails and thick mucus builds up in the lungs and other organs.

• Different CFTR mutations cause CF different ways, including nonsense mutations that create a premature stop codon and produce a truncated, nonfunctional protein.

• CF is the AP Bio template for tracing genotype to phenotype: a DNA change alters a protein, and the broken protein produces the disease symptoms.

Frequently asked questions about Cystic Fibrosis

What is cystic fibrosis in AP Biology?

It's a genetic disorder caused by mutations in the CFTR gene, which codes for a chloride channel. When the channel doesn't work, chloride and water balance fails and thick mucus clogs the lungs and other organs. AP Bio uses it as the standard example of how a mutation changes a protein and therefore the phenotype (Topic 6.7).

Is the ΔF508 mutation a frameshift mutation?

No. ΔF508 deletes three nucleotides, which is a multiple of three, so the reading frame stays intact and only one amino acid (phenylalanine 508) is lost. A frameshift happens when the number of nucleotides inserted or deleted is not a multiple of three, which shifts every codon after it.

How is cystic fibrosis different from sickle cell disease?

Sickle cell comes from a point mutation, a single nucleotide substitution that changes one amino acid in hemoglobin. The common CF mutation, ΔF508, is a three-nucleotide deletion that removes a whole amino acid and makes the CFTR protein misfold so the cell destroys it.

Why does the ΔF508 mutation stop chloride transport?

Losing phenylalanine 508 makes the CFTR protein fold incorrectly. The cell recognizes the misfolded protein and degrades it, so the channel never reaches the membrane. With no functional channel at the cell surface, chloride ions can't cross the epithelium.

What kind of cystic fibrosis question shows up on the AP Bio exam?

You'll usually get a specific CFTR mutation and have to predict the protein and cellular outcome, like explaining why ΔF508 leads to a degraded protein and lost chloride transport. The 2018 Short FRQ described CFTR as an ATP-gated chloride channel, so connect the gene to the protein's actual function and trace the genotype-to-phenotype chain.