Gene Mutation, DNA Repair, & Homologous Recombination – Chapter 11 Study Notes

Gene Mutation, DNA Repair, & Homologous Recombination

Germ Line and Somatic Mutations

Mutations are changes in the DNA sequence that can occur in different cell types, with distinct consequences for inheritance and organismal health.

• Germ-line mutations: Occur in reproductive cells (sperm and egg) and can be passed to offspring, affecting future generations.

• Somatic mutations: Occur in non-reproductive cells; these mutations cannot be inherited but may affect the individual if they occur in critical genes.

• Somatic cells divide by mitosis, so only the direct descendants of the mutated cell will carry the mutation.

• Germ-line mutations can result in half of the gametes carrying the mutation, potentially affecting all cells of the offspring.

Example: A mutation in a germ cell may lead to a genetic disorder in the next generation, while a somatic mutation may cause cancer in the individual.

Point Mutations

Point mutations involve changes to a single nucleotide or a small number of nucleotides within a gene. These can have varied effects depending on their type and location.

• Substitution: Replacement of one nucleotide with another.

• Addition (Insertion): Addition of one or more nucleotides.

• Deletion: Removal of one or more nucleotides.

• Frameshift mutation: Insertions or deletions that alter the reading frame of the gene, often resulting in a nonfunctional protein.

Table: Types and Consequences of Point Mutations

Type	Consequence
Synonymous (Silent)	No amino acid change
Missense	Change in amino acid
Nonsense	Creates stop codon, truncates protein
Frameshift	Wrong sequence after mutation
Promoter (Regulatory)	Changes timing/level of transcription
Polyadenylation	Improper mRNA processing
Splice site	Improperly removes/excludes exons
DNA replication mutation	Changes number of short repeats

Additional info: Frameshift mutations often result in premature stop codons and nonfunctional proteins.

Mutation Hotspots

Some regions of the genome are more prone to mutations, known as mutation hotspots.

• CpG dinucleotides: Regions where cytosine is followed by guanine; these are often methylated and prone to mutation.

• Trinucleotide repeats: Repeated sequences of three nucleotides can expand and cause disorders (e.g., Huntington's disease).

• Large genes, such as those involved in muscular dystrophy and neurofibromatosis, have higher mutation rates.

Example: Dystrophin gene mutations are associated with Duchenne muscular dystrophy.

Base-Pair Substitution Mutations

Base-pair substitutions involve replacing one nucleotide with another, which can be classified as transitions or transversions.

• Transition: Purine to purine (A ↔ G) or pyrimidine to pyrimidine (C ↔ T).

• Transversion: Purine to pyrimidine or vice versa (A/G ↔ C/T).

Equation: $\text{Transition:} \quad A \leftrightarrow G, \quad C \leftrightarrow T$ $\text{Transversion:} \quad A \leftrightarrow C, \quad A \leftrightarrow T, \quad G \leftrightarrow C, \quad G \leftrightarrow T$

Types of Coding-Sequence Substitution Mutations

Substitution mutations in coding regions can have different effects on protein structure and function.

• Silent (synonymous) mutation: No change in amino acid sequence.

• Missense mutation: Changes one amino acid to another, potentially altering protein function.

• Nonsense mutation: Converts a codon to a stop codon, resulting in truncated protein.

Example: Sickle cell anemia is caused by a missense mutation in the β-globin gene.

Frameshift Mutations

Frameshift mutations result from insertions or deletions that are not multiples of three nucleotides, altering the reading frame.

• Can produce entirely different amino acid sequences downstream of the mutation.

• Often result in premature stop codons and nonfunctional proteins.

Example: Tay-Sachs disease is caused by a frameshift mutation in the HEXA gene.

Regulatory Mutations

Mutations in regulatory regions can affect gene expression without altering the protein sequence.

• Promoter mutations: Change the amount of protein produced by affecting transcription initiation.

• Splicing mutations: Affect the removal of introns, potentially leading to abnormal mRNA and protein products.

• Polyadenylation mutations: Disrupt proper mRNA processing and stability.

Example: β-thalassemia can result from splicing mutations in the β-globin gene.

Mutation Consequences: Synonymous Mutations

Even mutations that do not change the protein sequence (synonymous) can have biological effects.

• May affect mRNA stability, splicing, or translation efficiency.

• Can influence disease risk or phenotype.

Additional info: Recent research shows that synonymous mutations can contribute to disease by affecting gene regulation.

Summary Table: Types of Mutations and Their Effects

Mutation Type	Effect
Germ-line	Heritable, affects offspring
Somatic	Non-heritable, affects individual
Point (Substitution)	May be silent, missense, or nonsense
Frameshift	Alters reading frame, often severe
Regulatory	Changes gene expression

Key Terms

• Mutation: Any change in the DNA sequence.

• Germ-line mutation: Mutation in reproductive cells, heritable.

• Somatic mutation: Mutation in body cells, not heritable.

• Point mutation: Change affecting a single nucleotide.

• Frameshift mutation: Insertion/deletion altering the reading frame.

• Transition: Purine to purine or pyrimidine to pyrimidine substitution.

• Transversion: Purine to pyrimidine or vice versa substitution.

• Synonymous mutation: Mutation that does not change the amino acid.

• Missense mutation: Mutation that changes one amino acid.

• Nonsense mutation: Mutation that creates a stop codon.

Additional info: The notes continue with DNA repair mechanisms and homologous recombination, which are essential for maintaining genome stability and preventing disease. These topics include direct repair, base excision repair, nucleotide excision repair, mismatch repair, and recombination pathways such as homologous recombination and non-homologous end joining.