---
title: "Deletion (del) — AP Biology Definition & Exam Guide"
description: "A deletion mutation removes one or more nucleotides from DNA, often shifting the reading frame and killing protein function. See how it shows up on the AP Bio exam."
canonical: "https://fiveable.me/ap-bio/key-terms/deletion-del"
type: "key-term"
subject: "AP Biology"
unit: "Unit 6"
---

# Deletion (del) — AP Biology Definition & Exam Guide

## Definition

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.

## What It Is

A **deletion** is exactly what it sounds like: a chunk of DNA gets cut out. That chunk can be a single [nucleotide](/ap-bio/key-terms/nucleotide "fv-autolink"), 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](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt "fv-autolink") 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](/ap-bio/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG "fv-autolink"), 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).

## Why It Matters

Deletion lives in **[Unit 6](/ap-bio/unit-6 "fv-autolink"): Gene Expression and Regulation**, specifically Topic **6.7 Mutations**. It directly supports **[AP Bio](/ap-bio "fv-autolink") 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.

## Connections

### 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)](/ap-bio/key-terms/cystic-fibrosis)

The most common [cystic fibrosis](/ap-bio/key-terms/cystic-fibrosis "fv-autolink") 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](/ap-bio/key-terms/aneuploidy "fv-autolink"), 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)](/ap-bio/key-terms/dna-repair-mechanisms)

Deletions often arise when DNA repair fails after damage like [double-strand breaks](/ap-bio/key-terms/double-strand-breaks "fv-autolink"). 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.

## On the AP Exam

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.

## Deletion (del) vs Point mutation (substitution)

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.

## Key Takeaways

- 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.

## FAQs

### What is a deletion mutation in AP Biology?

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.

### Does a deletion always 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.

### How is a deletion different from a point mutation?

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.

### Why does a deletion sometimes produce no functional protein?

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.

### Is a deletion mutation always harmful?

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.

## Related Study Guides

- [6.7 Mutations](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt)

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