6.7 Mutations
- Syllabus
- 2025
- Topic
- 6.7
- Level
- —
Learning objectives
6.7A—Describe the various types of mutationDescribe the various types of mutation.• Alterations in a DNA sequence are mutations that can cause changes in the type or amount of the protein produced and the consequent phenotype. DNA mutations can be beneficial, detrimental, or neutral based on the effect or the lack of effect they have on the resulting nucleic acid or protein and the phenotypes that are conferred by the protein.- i. Point mutations occur when one nucleotide has been substituted for a diff erent nucleotide.- ii. Frameshift mutations occur when one or more nucleotides are ins erted or deleted, causing the reading frame to be shifted.- iii. Nonsense mutations occur when there is a point mutation that causes a pr emature stop.- iv. Silent mutations occur when the change in the nucleotide sequence has no eff ect on the amino acid sequence.- Exclusion: Knowledge of specific mutations and their effects is beyond the scope of the AP Exam.6.7B—Explain how changes in genotype may result in changes in phenotypeExplain how changes in genotype may result in changes in phenotype.• Errors in DNA replication or DNA repair mechanisms as well as external factors, including radiation and reactive chemicals, can cause random mutations in the DNA.- i. Whether a mutation is beneficial, detrimental, or neutral depends on the environmental context.- ii. Mutations are a source of genetic variation.• Errors in mitosis or meiosis can result in changes in phenotype.- i. Changes in chromosome number resulting from nondisjunction often result in new phenotypes caused by triploidy (aneuploidy).- ii. Changes in chromosome number often result in disorders with developmental limitations.- iii. Alterations in chromosome structure lead to genetic disorders.- Exclusion: Knowledge of specific disorders related to changes in chromosome number is beyond the scope of the AP Exam.6.7C—Explain how alterations in DNA sequences contribute to variation that can be subject to natural selectionExplain how alterations in DNA sequences contribute to variation that can be subject to natural selection.• Changes in genotype may affect phenotypes that are subject to natural selection. Genetic changes that enhance survival and reproduction can be selected for by environmental conditions.- i. The horizontal acquisitions of genetic information in prokaryotes via transformation (uptake of DNA), transduction (viral transmission of genetic information), conjugation (cell-to-cell transfer of DNA), and transposition (movement of DNA segments within and between DNA molecules) increase genetic variation.- ii. Related viruses can recombine genetic information if they infect the same host cell.- iii. Reproductive processes that increase genetic variation are evolutionarily conserved and are shared by various organisms.
How Mutation Types Change Genetic Information
A mutation is an alteration in a DNA sequence. Its effect depends on how the altered nucleotide sequence changes the resulting nucleic acid or protein, so a mutation may be beneficial, detrimental, or neutral.
| Mutation type | DNA change | Immediate consequence |
|---|---|---|
| Point substitution | One nucleotide is replaced by another | One codon may change |
| Frameshift | One or more nucleotides are inserted or deleted | The reading frame shifts, changing downstream codons |
| Nonsense | A point mutation creates a premature stop codon | Translation ends early |
| Silent | The nucleotide sequence changes but the encoded amino acid does not | Amino-acid sequence is unchanged |
A changed codon can alter the type or amount of protein produced, which may alter phenotype. The size of the DNA change alone does not determine severity: a single substitution can create a premature stop, while another substitution can be silent.
A point mutation is defined by nucleotide substitution, whereas a frameshift is defined by a shifted reading frame after insertion or deletion. The AP Exam does not require knowledge of specific named mutations and their effects.
From Genotype Change to Phenotype Change
A genotype change can affect phenotype when it changes a gene product, its amount, chromosome content, or chromosome structure. The phenotypic outcome depends on both the biological consequence of the change and the environmental context.
| Source of change | Genetic consequence | Possible route to phenotype |
|---|---|---|
| DNA replication or repair error | Random DNA mutation | Altered protein type, amount, or function |
| Radiation or reactive chemical | Random DNA mutation | Altered gene product and cell function |
| Nondisjunction in mitosis or meiosis | Altered chromosome number | Altered gene dosage and development |
| Altered chromosome structure | Changed arrangement or amount of genetic information | Disrupted gene function or regulation |
Sequence pathway: DNA change → altered RNA or protein → altered cell function → possible phenotype change. Chromosome pathway: segregation or structural error → changed chromosome content → changed gene dosage or function → possible phenotype change.
Mutations supply genetic variation. The same genotype change can be beneficial, detrimental, or neutral depending on the environment because environmental conditions determine whether the resulting phenotype affects performance, survival, or reproduction.
A mutation does not guarantee an observable phenotype, and an external mutagen does not direct a useful change. Specific disorders caused by chromosome-number changes are outside the required AP scope.
How Genetic Variation Becomes Material for Selection
Altered DNA sequences create genetic variation. When a genetic difference changes a heritable phenotype, environmental conditions can favor variants that improve survival and reproduction.
| Source of variation | What occurs |
|---|---|
| Transformation | A prokaryote takes up DNA |
| Transduction | A virus transfers genetic information |
| Conjugation | DNA moves by cell-to-cell transfer |
| Transposition | A DNA segment moves within or between DNA molecules |
| Viral recombination | Related viruses exchange genetic information in the same host cell |
| Reproductive processes | Conserved processes generate new genetic combinations |
These processes increase genetic variation; they do not guarantee an advantage. If a resulting phenotype increases survival or reproductive success in a particular environment, individuals carrying that genetic change can contribute more descendants, so the variant may become more common.
DNA alteration or genetic-information exchange → heritable variation → possible phenotypic difference → differential survival and reproduction in an environment → change in variant frequency.
Natural selection does not create mutations because organisms need them. Variation arises first; selection then acts on phenotypic consequences in a particular environmental context.