Recombination

Introduction to DNA Recombination

  • Definition and Significance

    • DNA recombination is a crucial mechanism for DNA repair, particularly for addressing double strand breaks (DSBs).

    • DSBs happen when both phosphodiester bonds are severed in a double helix.

  • Key Learning Outcomes

    • Explain the role of DNA recombination as a mechanism for DNA repair.

    • Explain the mechanism of DNA recombination and its role in meiosis.

    • Explain the role of independent assortment in creating genetic variation.

DNA Recombination in DNA Repair

  • Mechanism of Fixing Double Strand Breaks

    • When DSBs occur after DNA replication but before mitosis/meiosis, replicated chromosomes can align, forming a template for repair.

    • Steps in the Repair Mechanism:

    • Begin with the double strand break.

    • Trim the ends of the DNA strands to create segments capable of complementary base pairing.

    • Strand Invasion: A DNA polymerase binds to the invading strand (example: orange strand in the presentation) and begins to extend it using the template strand.

    • This process creates a new DNA segment that is identical to the original.

    • Finally, a ligation event occurs, producing two identical DNA helices.

    • Note: The repaired strands are identical due to both being derived from the same chromosome before the break.

DNA Recombination During Meiosis

  • Comparison to Mitosis

    • In mitosis, each replicated chromosome aligns individually on the metaphase plate.

    • During meiosis, homologous chromosomes align together, creating opportunities for genetic diversity.

  • Mechanism of Recombination in Meiosis

    • Chromosomal Alignment: Maternal (M) and paternal (P) homologous chromosomes are aligned; differences in alleles may exist.

    • Example: one chromosome may carry a dominant allele (A) while the corresponding homologous chromosome may carry a recessive allele (a).

    • Crossing Over: Occurs during metaphase I.

    • An intentional double strand break is created by the cell, and then a nuclease digests the 5' ends of the strands leading to strand exchange.

    • Template usage: During this repair, the genes/alleles are not identical, allowing for variation (e.g., dominant B used to repair recessive b).

    • Formation of Holiday Junctions: Named after the scientist who discovered them, these junctions can slide along the chromosome until resolved.

    • Resolution leads to altered chromosomes, with varying allele combinations from the original parental contributions.

    • Outcome of Crossing Over: New allele combinations arise (e.g., recessive only a and dominant A and B only). This creates genetic variations that contribute to offspring diversity.

Role of Independent Assortment in Genetic Variation

  • Principle Established by Gregor Mendel

    • While Mendel focused on phenotypes in pea plants, he highlighted critical genetic principles:

    • The principle of segregation for alleles of a single gene.

    • The principle of independent assortment for different genes.

  • Independent Assortment Explained

    • Chromosome arrangements during meiosis dictate assortments.

    • Example parental genotypes:

    • Parent one: Dominant for both Y (yellow) and R (round) alleles.

    • Parent two: Recessive for both y and r alleles.

    • There are varied outcomes based on chromosome alignments in meiosis:

    • All dominant or all recessive alleles may align or can assort independently to create all combinations.

    • Implication of Recombination and Independent Assortment:

    • The genetic variability produced through these mechanisms leads to diverse offspring.

    • In species with multiple chromosomes, like Homo sapiens with 22 pairs, the number of possible genetic combinations increases dramatically.

  • Conclusion on Diversity

    • Combining recombination during meiosis with independent assortment enhances the genetic diversity within populations, ensuring that offspring are genetically distinct from their parents.