L11

DNA Rearrangement by Genetic Recombination

Genetic Recombination

  • Definition: Exchange of genetic material between two DNA molecules; results in DNA rearrangement.

  • Roles/Uses:

    • Biological: Repair mechanisms, natural genetic diversity.

    • Experimental: Gene mapping and creating transgenic organisms.

Biological Roles of Recombination

  • DNA Repair: Corrects errors and damages in DNA.

  • Creation of New Gene/Allele Combinations: Achieved through crossing over during meiosis.

  • Formation of New Genes: Example: Immuno-globulin rearrangement.

  • Integration of Specific DNA Elements: Involves inserting new genetic material into existing DNA.

Experimental Uses of Recombination

  • Gene Mapping:

    • Distance between genes estimated via recombination frequency.

  • Transgenic Cells and Organisms: Creation of organisms with modified genes.

Types of Genetic Recombination

  • General/Homologous Recombination:

    • Exchange occurs between homologous DNA sequences.

  • Site-Specific Recombination:

    • Involves specific limited stretches of DNA similarity.

  • Transposition:

    • Movement of mobile DNA elements between donor and target sites with minimal sequence similarity.

General or Homologous Recombination

  • Mechanism:

    • Occurs between two homologous DNA sequences.

    • Can be between sister chromatids or homologous chromosomes (during meiosis).

Model for Homologous Recombination: Single-Strand Break (SSB) Model

  • Key Steps:

    1. Alignment of homologous DNA molecules.

    2. Break Introduction in one strand of each molecule.

    3. Strand Invasion: Single-stranded region pairs with a complementary strand from the homologous DNA.

    4. Result: Formation of a Holliday Junction and possible branch migration.

    5. Cleavage: Holliday junction is resolved into different products: splice (crossover) or patch (non-crossover).

Figure Representation of Holliday Junction

  • Electron micrographs and three-dimensional representations depict the structure and dynamics of the Holliday junction.

Model for Homologous Recombination: Double-Strand Break (DSB)

  • Steps:

    1. Double Strand Break: Initiation of recombination.

    2. Exonucleases create 3' overhangs.

    3. Base Pairing of exposed 3' end with the intact homolog.

    4. Formation of two Holliday junctions requires resolution.

Bacterial Genetics and Recombination

  • E. coli:

    • Only has one haploid chromosome.

    • Major role of homologous recombination is DNA repair.

    • Mechanisms include transformation, transduction, and conjugation.

Chi (c) Sequences in E. coli

  • Function: Crossover hotspots, enhances recombination frequency.

  • Consensus Sequence: 5' - GCTGGTGG - 3'.

  • Presence of multiple chi sequences (>1000) in E. coli genome enhances recombination.

Homologous Recombination in Eukaryotes

  • Mechanisms:

    • Involves centromere homologs, crossing-over during meiosis (chiasma formation).

Gene Conversion Consequence of DSBs

  • Mismatch Repair System: Recognizes and repairs mis-paired bases, can lead to nonreciprocal transfer of genetic information termed gene conversion, impacting Mendelian inheritance patterns.