DNA Mutations

Explore how permanent changes in the DNA sequence drive evolution, cause disease, and shape the diversity of life on Earth.

AP Biology

What Is a Mutation?

A mutation is any permanent change in the nucleotide sequence of DNA. Mutations can occur in a single base pair or involve large segments of a chromosome. They are the ultimate source of all new genetic variation in every population.

Why Mutations Matter

★ Evolution

Mutations introduce new alleles that natural selection can act upon, driving adaptation over generations.

★ Disease

Harmful mutations can cause genetic disorders like sickle cell anemia, cystic fibrosis, and cancer.

★ Diversity

Mutations generate the raw genetic variation that makes each organism unique within a population.

Two Major Categories

Point Mutations: Substitutions

Three Outcomes of Substitution

Silent

New codon codes for the same amino acid due to redundancy in the genetic code. No effect on protein.

Missense

New codon codes for a different amino acid. May alter protein folding and function (e.g., sickle cell).

Nonsense

New codon becomes a stop codon (UAA, UAG, UGA). Produces a truncated, usually nonfunctional protein.

Frameshift Mutations

Chromosomal Mutations

Somatic vs. Germline Mutations

	Somatic	Germline
Cells affected	Body (non-reproductive) cells	Eggs or sperm
Inherited?	No: dies with organism	Yes: passed to offspring
Evolutionary impact	None: not in gene pool	Drives evolution
Example	Skin cancer from UV damage	Inherited sickle cell allele

Original mutated and unmutated control lineage

A contextual tissue image accompanies an authored lineage from M to M1 and M2 and from M1 to M11 and M12, all carrying the mutation marker. A separate companion shows an unmutated control lineage. — A somatic mutation can be inherited by descendant cells within a tissue without entering the next organismal generation. Illustration is schematic where scale or molecular structure is simplified.

Causes: Mutagens and Errors

1

Spontaneous errors: DNA polymerase misincorporates a base during replication (roughly 1 error per \(10^9\) bases after proofreading).

2

Chemical mutagens: Substances like benzene, nitrous acid, or aflatoxin chemically modify bases, causing mispairing.

3

Radiation: UV light creates thymine dimers; X-rays break the DNA backbone, causing deletions and translocations.

4

Transposons: "Jumping genes" insert themselves into new locations, disrupting gene function or regulation.

Explain why DNA damage need not become a mutation.

A UV-induced lesion can be excised; an intact complementary strand guides gap filling, and ligase seals the remaining nick. DNA damage and a fixed sequence mutation are distinct events. The helix is illustrative and does not depict a measured structure or specific lesion.

An illustrative DNA helix sits alongside four independently authored stages: UV lesion, excision, template-guided filling and sealing of a nick. The complementary strand remains intact. — A UV-induced lesion can be excised; an intact complementary strand guides gap filling, and ligase seals the remaining nick. DNA damage and a fixed sequence mutation are distinct events. The helix is illustrative and does not depict a measured structure or specific lesion.

Effects: Beneficial, Neutral, Harmful

Beneficial

Increases fitness. Example: mutations conferring antibiotic resistance in bacteria or lactose tolerance in humans.

Neutral

No effect on fitness. Most mutations fall here: silent substitutions or changes in noncoding DNA.

Harmful

Decreases fitness. Example: BRCA1 mutations dramatically increase breast cancer risk.

How to Identify Mutation Type

1

Compare sequences: Align the original and mutated DNA (or mRNA) side by side.

2

Count the bases: Same number = substitution. More = insertion. Fewer = deletion.

3

Check the reading frame: If the indel is not a multiple of 3, it is a frameshift. If it is a multiple of 3, only one amino acid is added/removed.

4

Translate both codons: Use the codon table to determine if the amino acid changes (missense), stays the same (silent), or becomes a stop (nonsense).

5

Predict the effect: Consider protein structure. Does the change affect the active site? Folding? Regulation?

Example 1: Silent Mutation

Example 2: Nonsense Mutation

Example 3: Frameshift Deletion

Consider an mRNA sequence: AUG-GCA-UUC-GAA, coding for Met-Ala-Phe-Glu.

If the C in GCA is deleted, the sequence becomes: AUG-GAU-UCG-AA...

Before Deletion

AUG | GCA | UUC | GAA → Met-Ala-Phe-Glu

After Deletion

AUG | GAU | UCG | AA... → Met-Asp-Ser-???

Common Mistakes

✕ All mutations are harmful

Most mutations are neutral (in noncoding DNA or silent). Many beneficial mutations drive adaptation and evolution.

✕ Substitution = frameshift

Substitutions replace one base with another, keeping the reading frame intact. Only insertions/deletions (not multiples of 3) cause frameshifts.

✕ Somatic mutations are inherited

Only germline mutations pass to offspring. Somatic mutations affect only the individual and are not transmitted to the next generation.

Mutation Summary Table

Type	Mechanism	Effect on Protein	Severity
Silent	Base substitution	No change	None
Missense	Base substitution	Different amino acid	Variable
Nonsense	Base substitution	Premature stop	Usually severe
Frameshift	Insertion / deletion	All downstream changed	Usually severe
Chromosomal	Large rearrangement	Multiple genes affected	Often severe

Question 1 of 5

A mutation changes the mRNA codon GAA to GAG. Both codons code for glutamic acid. What type of mutation is this?

Since both GAA and GAG code for the same amino acid (glutamic acid), the protein is unchanged. This is a silent (synonymous) mutation, made possible by the degeneracy of the genetic code.

Question 2 of 5

An insertion of two nucleotides into a coding region would most likely result in:

Two nucleotides is not a multiple of 3, so the reading frame shifts. Every codon downstream of the insertion is altered, scrambling the entire amino acid sequence. This is a frameshift mutation.

Question 3 of 5

A skin cell acquires a UV-induced mutation that causes uncontrolled division. This mutation is somatic. Which statement is true?

Somatic mutations occur in body (non-reproductive) cells. They are not passed to offspring and do not enter the gene pool. However, they can cause the affected cells to proliferate abnormally, leading to cancer.

Question 4 of 5

The sickle cell allele is caused by a single base substitution in the beta-globin gene. In regions where malaria is endemic, heterozygous carriers have a survival advantage. This is an example of:

The sickle cell allele is harmful in homozygotes (causes sickle cell disease) but beneficial in heterozygotes (confers malaria resistance). This trade-off maintains the allele in the population through heterozygote advantage, a form of balancing selection.

Question 5 of 5

A mutation changes a codon from UGG (tryptophan) to UGA (stop). What type of mutation is this, and what is the most likely consequence?

Changing UGG (the only codon for tryptophan) to UGA (a stop codon) is a nonsense mutation. Translation terminates prematurely, producing a shortened protein that is almost certainly nonfunctional. No reading frame is shifted because it is a substitution, not an insertion or deletion.