In AP Biology, a deletion (del) is a mutation in which one or more nucleotides are removed from a DNA sequence. If the number deleted isn't a multiple of three, it causes a frameshift that scrambles every codon downstream, often producing no functional protein.
A deletion is exactly what it sounds like: a chunk of DNA gets cut out. That chunk can be a single nucleotide, a few of them, or a whole gene. The CED groups deletions under the types of mutation in EK 6.7.A.1, alongside point mutations and insertions.
Why deletions are a big deal comes down to the reading frame. Your ribosome reads mRNA in groups of three nucleotides (codons), starting from a fixed point. Delete a number of nucleotides that isn't a multiple of three and you shove every codon after the cut out of position. That's a frameshift mutation, and it usually garbles the rest of the protein, often creating an early stop codon. The result is frequently no functional gene product, which is why a del allele can produce zero working protein (like a del allele making no ALD protein in a classic experiment setup).
Deletion lives in Unit 6: Gene Expression and Regulation, specifically Topic 6.7 Mutations. It directly supports AP Bio 6.7.A (describe the various types of mutation) and AP Bio 6.7.B (explain how changes in genotype may result in changes in phenotype). The throughline is genotype to phenotype: a deletion changes the DNA sequence, which can change the type or amount of protein, which can change the trait. Because deletions are a source of genetic variation, they also feed AP Bio 6.7.C, the link between DNA changes and natural selection. Whether a deletion is beneficial, detrimental, or neutral depends on context, so don't assume every mutation is bad.
Keep studying AP® Biology Unit 6
Frameshift mutations (Unit 6)
A deletion that removes a number of nucleotides not divisible by three IS a frameshift. Think of it like deleting a letter mid-sentence and re-spacing every word after it into nonsense. Insertions cause the same scramble in the opposite direction.
Cystic Fibrosis (Unit 6)
The most common cystic fibrosis allele is a three-nucleotide deletion in the CFTR gene. It removes one amino acid without shifting the frame, showing that not every deletion is a frameshift and that even small losses can break a protein's function.
Aneuploidy and chromosome number changes (Unit 6)
Deletion works at the nucleotide scale, but errors in meiosis can delete or duplicate entire chromosomes (aneuploidy, EK 6.7.B.2). Both are losses of genetic material, just at wildly different scales, and both can produce new phenotypes.
DNA repair mechanisms (Unit 6)
Deletions often arise when DNA repair fails after damage like double-strand breaks. The cell tries to stitch the break back together and sometimes loses bases in the process, which is how errors in repair (EK 6.7.B.1) create permanent mutations.
You'll most often meet deletions in multiple-choice questions that give you a DNA or mRNA sequence and ask you to predict the effect of removing nucleotides. The move tested is recognizing whether the deletion shifts the reading frame: count the bases removed and check if it's a multiple of three. No released FRQ uses the word "deletion" verbatim, but it supports the genotype-to-phenotype reasoning that free-response questions reward, like explaining why a mutation produces no functional protein or how it could affect an organism's fitness. Be ready to classify a mutation (deletion vs. substitution vs. insertion) and justify the phenotypic consequence.
A point mutation swaps one nucleotide for another and keeps the sequence the same length, so the reading frame stays intact and only one codon usually changes. A deletion removes nucleotides, which can shorten the protein or, if not a multiple of three, shift the frame and scramble everything downstream.
A deletion is a mutation that removes one or more nucleotides from a DNA sequence.
If the number of nucleotides deleted is not a multiple of three, it causes a frameshift that changes every codon after the cut.
Frameshift deletions often introduce an early stop codon and produce no functional protein, which is why a del allele can make zero working product.
A deletion that removes a multiple of three nucleotides shifts no frame but still removes amino acids, as in the common cystic fibrosis allele.
Deletions are a source of genetic variation and can be beneficial, detrimental, or neutral depending on the environment (EK 6.7.B.1).
On the exam, count the deleted bases and check divisibility by three to predict whether the reading frame shifts.
It's a mutation where one or more nucleotides are removed from a DNA sequence. Under CED EK 6.7.A.1, deletions can change the type or amount of protein produced, and if the number removed isn't a multiple of three, they cause a frameshift.
No. A deletion only causes a frameshift if the number of nucleotides removed is not a multiple of three. Deleting three (or six, or nine) nucleotides keeps the reading frame intact and just removes whole amino acids, like the common cystic fibrosis CFTR allele.
A point mutation substitutes one nucleotide for another and keeps the DNA the same length, usually changing only one codon. A deletion removes nucleotides, which shortens the sequence and can shift the entire reading frame downstream.
A frameshift deletion changes every codon after the deletion point, often creating an early stop codon. The ribosome then makes a short, garbled, nonfunctional protein, which is why a del allele can produce zero working product.
No. Whether a mutation is beneficial, detrimental, or neutral depends on the environmental context (EK 6.7.B.1). Deletions are a source of genetic variation that natural selection can act on, so some are harmful, some are neutral, and a few can even be advantageous.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.