Mutations and DNA Damage Repair

Mutations and DNA Damage

Consequences of Mutations

  • Severe Impact: A single nucleotide change can have significant implications for a cell or an entire organism.

  • Sickle Cell Anemia: This condition is a classic example, resulting from an A to T nucleotide change. This specific alteration replaces glutamic acid with valine at a critical position in the protein.

  • Sickle Cell Trait vs. Disease: Both conditions are worsened under low oxygen. The distinction lies in the genetic inheritance (heterozygous for trait, homozygous for disease), implying differences in the severity and manifestation of symptoms.

Causes and Types of Nucleotide Changes in DNA

  • Spontaneous Chemical Changes: Occur due to thermal collisions between DNA and other molecules within the cell.

    • Depurination: Common spontaneous event where guanine (G) and adenine (A) bases are lost from the DNA backbone. If unrepaired, this can lead to deletions or substitutions during replication, as the DNA polymerase might insert an incorrect base or delete a base entirely opposite the depurinated site.

    • Deamination: Can affect cytosine (C), converting it to uracil (U). Unrepaired deaminated C (now U) will pair with adenine (A) during replication, effectively changing a G-C pair to an A-T pair in subsequent DNA generations.

  • Replication Errors: Mistakes made by DNA polymerase during DNA synthesis.

  • Malfunctioning DNA Repair Enzymes: Inability of the cell's repair mechanisms to correct existing damage.

  • Types of Mutations:

    • Deletions: Removal of one or more nucleotides.

    • Additions (Insertions): Addition of one or more nucleotides.

    • Substitutions: Replacement of one nucleotide with another.

  • Frameshift Mutations: Both deletions and additions typically lead to frameshift mutations, altering the reading frame of the genetic code and subsequently changing the entire downstream amino acid sequence encoded by the gene.

  • UV Radiation-Induced Damage: Ultraviolet (UV) rays from sunlight cause DNA damage by creating thymine dimers, where two adjacent thymine bases on the same DNA strand become covalently bonded. Unless repaired, these dimers can lead to errors (deletions or substitutions) during DNA replication.

    • Xeroderma Pigmentosum: Humans lacking the enzymes necessary to repair thymine dimers suffer from this disease, which is characterized by an extreme sensitivity to sunlight and a high incidence of skin cancer.

Basic Mechanism of DNA Repair

Most types of DNA damage create structural anomalies not found in undamaged DNA, which serve as signals for repair enzymes.
The repair process generally involves three fundamental steps:

  1. Excision: Damaged nucleotides are removed from the DNA strand by specific enzymes. Different enzymes are specialized for different types of damage.

  2. Resynthesis: A DNA polymerase synthesizes a new, correct DNA segment to fill the gap created by excision. This step utilizes the undamaged complementary strand as a template. The same DNA polymerase often handles resynthesis for various types of damage.

  3. Ligation: DNA ligase seals the nicks in the sugar-phosphate backbone, covalently joining the newly synthesized segment to the original DNA strand. The same ligase typically performs this function for various repairs.

DNA Replication Error Correction: Mismatch Repair

  • Negligible Error Rates: DNA replication is highly accurate, but errors can still occur.

  • Correction Necessity: Errors made during DNA replication must be promptly corrected to prevent them from becoming permanent mutations.

  • Mismatch Repair System: This specialized system identifies and eliminates replication errors, ensuring the restoration of the original DNA sequence. It selectively repairs the newly synthesized strand, which contains the error, using the parent strand as a template.

Repair of Double-Stranded DNA Breaks (DSBs)

Causes: Double-stranded breaks can result from various factors, including ionizing radiation, oxidizing agents, and mishaps during DNA replication (e.g., at replication forks).

Homologous Recombination (Homologous End Joining)

  • Flawless Repair: This mechanism provides a highly accurate repair pathway for double-stranded DNA breaks.

  • Conditions for Repair: It requires the presence of an identical DNA molecule (sister chromatid) derived from replication, typically occurring during the S phase of interphase when DNA synthesis takes place.

  • Mechanism: The damaged DNA strand invades the intact identical DNA molecule, using it as a template to resynthesize the missing or damaged segments, ensuring an accurate repair without loss of genetic information.

Dual Role of Homologous Recombination

While homologous recombination is crucial for DNA repair, an analogous mechanism also plays a vital role in generating genetic diversity:

  1. Crossing Over in Meiosis: Takes place between similar (but not identical) DNA sequences in homologous chromosomes during prophase I of meiosis.

  2. Genetic Diversity: Unlike repair, where identical DNA is used, crossing over involves paternal and maternal chromosomes which are similar but not identical. This exchange generates novel combinations of DNA sequences, contributing to genetic variation.

Genetic Rearrangements: Transposons and Viruses

Insertion of foreign DNA sequences into chromosomes can cause genetic rearrangements, leading to mutations or the generation of novel proteins.

Transposons (Jumping Genes / Mobile Genetic Elements)

  • Definition: DNA segments that can move from one position to another within a cell's genome, a process known as transposition.

  • Impact: Transposons can alter gene order on chromosomes, add new sequences, cause mutations, or lead to the generation of novel proteins.

  • Intracellular Movement: Transposons cannot leave the cell; they move with the cell as it divides.

  • Components for Movement: They encode the necessary machinery for their transposition.

  • Transposase: Often contain a gene for the transposase enzyme, which catalyzes the movement (transposition) of the transposon.

  • Additional Features: Frequently carry antibiotic resistance genes and possess characteristic inverted repeats at their ends, which are recognized by the transposase.

  • Direct Repeats (Footprints): After a transposon