7.8 Mutations
4 min read•august 22, 2024
Mutations are changes in DNA that can be inherited. They're the building blocks of genetic diversity and evolution. From tiny point mutations to big chromosomal changes, these genetic tweaks can have huge impacts on organisms.
Mutations can happen spontaneously or be caused by environmental factors. Some are harmless, while others can lead to diseases or give organisms new advantages. Understanding mutations is key to grasping how genes work and evolve over time.
Mutations and Their Types
Definition and Types of Mutations
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- Mutations are changes in the nucleotide sequence of DNA that can be inherited by future generations if they occur in germline cells
- Point mutations involve the substitution, insertion, or deletion of a single nucleotide within a gene sequence
- Substitution mutations can be either transitions (purine to purine or pyrimidine to pyrimidine) or transversions (purine to pyrimidine or vice versa)
- Insertion mutations occur when one or more nucleotides are added to the DNA sequence
- Deletion mutations involve the removal of one or more nucleotides
- Frameshift mutations result from insertions or deletions that alter the reading frame, leading to a completely different amino acid sequence downstream of the mutation
Large-Scale Mutations
- Mutations can also involve larger segments of DNA
- Duplications copy entire genes or chromosomal regions
- Inversions flip the orientation of a DNA segment
- Translocations move a segment of DNA to a different location in the genome
Causes of Mutations
Spontaneous Mutations
- Mutations can occur spontaneously due to errors during DNA replication
- Base mispairing (adenine pairing with cytosine instead of thymine)
- Slippage of the DNA polymerase enzyme, leading to insertions or deletions
- Defects in DNA repair mechanisms can increase the frequency of mutations by allowing DNA damage to persist and be passed on to daughter cells
Environmental Factors and Mutagens
- Environmental factors can cause DNA damage that leads to mutations if not properly repaired
- Ultraviolet radiation from the sun can cause pyrimidine dimers
- Ionizing radiation (X-rays, gamma rays) can break DNA strands
- Chemical mutagens can induce mutations by altering the structure of DNA bases or disrupting the DNA double helix
- Alkylating agents (ethyl methanesulfonate) add alkyl groups to bases
- Intercalating agents (ethidium bromide) insert between base pairs
- Viruses and transposable elements can introduce mutations by inserting their genetic material into the host genome
Impact of Mutations on Proteins
Silent and Missense Mutations
- Silent mutations do not change the amino acid sequence of the protein and generally have no effect on protein function
- Missense mutations result in the substitution of one amino acid for another
- Can alter the structure, stability, or function of the protein depending on the location and nature of the substitution
- Sickle cell anemia is caused by a missense mutation in the hemoglobin gene
Nonsense and Frameshift Mutations
- Nonsense mutations introduce a premature stop codon, leading to the production of a truncated and often non-functional protein
- Frameshift mutations can drastically alter the amino acid sequence downstream of the mutation, often resulting in a completely non-functional protein
- Duchenne muscular dystrophy is caused by frameshift mutations in the dystrophin gene
Regulatory Mutations
- Mutations in regulatory regions of genes can affect the expression level of the protein
- Altered cellular processes and potentially disease states
- Mutations in the promoter region of the BRCA1 gene can increase the risk of breast cancer
Mutations in Evolution and Disease
Mutations as a Source of Genetic Variation
- Mutations are the ultimate source of genetic variation within populations, providing the raw material for natural selection to act upon
- Beneficial mutations that increase an organism's fitness can become more prevalent in a population over time, contributing to adaptive evolution
- Mutations in the CCR5 gene provide resistance to HIV infection
- Neutral mutations do not significantly affect an organism's fitness and can accumulate in populations through genetic drift
- Deleterious mutations that decrease an organism's fitness are generally selected against but can persist in populations at low frequencies
Mutations in Genetic Disorders and Disease
- Germline mutations in genes associated with essential cellular processes or development can lead to genetic disorders that are passed on to offspring
- Cystic fibrosis is caused by mutations in the CFTR gene
- Huntington's disease is caused by a trinucleotide repeat expansion in the huntingtin gene
- Somatic mutations acquired during an individual's lifetime can contribute to the development of cancers and other age-related diseases
- Mutations in the p53 tumor suppressor gene are found in many types of cancer
- The accumulation of mutations in populations over long periods of time can lead to the formation of new species through the process of speciation