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Types of Mutations and DNA Repair Mechanisms
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Mutations are permanent changes in the DNA sequence, and they can range from harmless variation to major causes of disease. In medical science, understanding mutation types helps explain inherited disorders, cancer development, drug resistance, and differences in patient response to treatment. DNA is constantly exposed to damage from normal metabolism, radiation, chemicals, and replication errors, so cells need reliable repair systems to preserve genetic information. When repair fails or is overwhelmed, mutations can accumulate and alter cell function.
Different mutations affect DNA and proteins in different ways, including single base substitutions, insertions, deletions, and larger chromosomal changes. Cells respond with specialized repair pathways such as mismatch repair, base excision repair, nucleotide excision repair, homologous recombination, and nonhomologous end joining. Each pathway recognizes a specific kind of lesion and uses enzymes to remove damage and restore the correct sequence as accurately as possible. These mechanisms are clinically important because defects in repair genes are linked to disorders such as xeroderma pigmentosum, Lynch syndrome, and hereditary breast and ovarian cancer.
Key Facts
- Point mutation: one nucleotide is changed, and it may be silent, missense, or nonsense.
- Frameshift mutation: insertion or deletion of nucleotides not in multiples of 3 shifts the reading frame and often changes every codon downstream.
- Missense mutation changes one amino acid to another, while nonsense mutation changes a codon into a stop codon.
- DNA replication follows complementary base pairing: A pairs with T, and G pairs with C.
- Mutation rate can be expressed as mutations per base pair per generation, and repair systems greatly reduce the final error rate after replication.
- Double strand break repair uses homologous recombination for high fidelity repair and nonhomologous end joining for faster but more error prone repair.
Vocabulary
- Mutation
- A mutation is a permanent change in the nucleotide sequence of DNA.
- Mismatch repair
- Mismatch repair is the pathway that corrects base pairing errors left behind after DNA replication.
- Base excision repair
- Base excision repair removes small damaged bases such as those altered by oxidation, deamination, or alkylation.
- Nucleotide excision repair
- Nucleotide excision repair removes bulky DNA lesions such as thymine dimers caused by ultraviolet light.
- Homologous recombination
- Homologous recombination is a high accuracy repair process that fixes double strand breaks using a matching DNA template.
Common Mistakes to Avoid
- Confusing mutation with DNA damage, because DNA damage is the chemical lesion itself while a mutation is the permanent sequence change that remains after faulty repair or replication.
- Assuming all mutations change the protein sequence, because silent mutations can leave the amino acid sequence unchanged due to redundancy in the genetic code.
- Forgetting that frameshifts depend on the number of bases added or removed, because insertions or deletions in multiples of 3 do not shift the reading frame.
- Thinking every repair pathway works on every lesion, because each system is specialized and using the wrong pathway can lead to failed repair or genomic instability.
Practice Questions
- 1 A coding DNA sequence begins as ATG-GAA-TTC-CCA. If the second codon changes from GAA to TAA, what type of mutation occurred and what is the likely effect on the protein?
- 2 A DNA strand contains 12 bases in a coding region. If 1 base is deleted near the start of the sequence, how does this affect the reading frame compared with deletion of 3 bases?
- 3 Why are defects in homologous recombination genes such as BRCA1 or BRCA2 strongly associated with cancer risk, even if the original mutation does not immediately kill the cell?