homologous recombination

Examples of homologous recombination in the following topics:

• Genetic Linkage and Distances

  • As chromosomes condensed and paired with their homologs, they appeared to interact at distinct points.
  • It is now known that the pairing and interaction between homologous chromosomes, known as synapsis, does more than simply organize the homologs for migration to separate daughter cells.
  • When synapsed, homologous chromosomes undergo reciprocal physical exchanges of DNA at their arms in a process called homologous recombination, or more simply, "crossing over."
  • He also assumed that the incidence of recombination between two homologous chromosomes could occur with equal likelihood anywhere along the length of the chromosome.
  • A recombination frequency of 0.5 indicates that 50 percent of offspring are recombinants and the other 50 percent are parental types .

• Gene rearrangement within genomes

  • During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes.
  • Crossing-over (homologous recombination) is one such mechanism by which DNA variations can occur, and genes can be rearranged.
  • In meiosis and mitosis, recombination occurs between similar molecules (homologs) of DNA.
  • In both meiotic and mitotic cells, recombination between homologous chromosomes is a common mechanism used in DNA repair.
  • Recombination can occur between DNA sequences that contain no sequence homology.

• Genetic Linkage and Violation of the Law of Independent Assortment

  • Genes that are located on separate non-homologous chromosomes will always sort independently .
  • Homologous chromosomes possess the same genes in the same linear order.
  • Like genes on the homologs align with each other.
  • The process of crossover, or recombination, occurs when two homologous chromosomes align during meiosis and exchange a segment of genetic material.
  • The result is two recombinant and two non-recombinant chromosomes.

• Genetic Maps

  • The exchange of DNA between homologous pairs of chromosomes is called genetic recombination, which occurs by the crossing over of DNA between homologous strands of DNA, such as nonsister chromatids.
  • Linkage analysis involves studying the recombination frequency between any two genes.
  • The greater the distance between two genes, the higher the chance that a recombination event will occur between them, and the higher the recombination frequency between them .
  • Because genetic maps rely completely on the natural process of recombination, mapping is affected by natural increases or decreases in the level of recombination in any given area of the genome.
  • Some parts of the genome are recombination hotspots, whereas others do not show a propensity for recombination.

• Meiosis II

  • The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.
  • The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover .
  • In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I.
  • In anaphase I, the homologous chromosomes are separated.

• Genetic Variation

  • crossing over (or recombination) between chromatids of homologous chromosomes during meiosis
  • However, existing genes can be arranged in new ways from chromosomal crossing over and recombination in sexual reproduction.

• Chromosomal Structural Rearrangements

  • Both are identified during meiosis by the adaptive pairing of rearranged chromosomes with their former homologs to maintain appropriate gene alignment.
  • If the genes carried on two homologs are not oriented correctly, a recombination event could result in the loss of genes from one chromosome and the gain of genes on the other.
  • When one homologous chromosome undergoes an inversion, but the other does not, the individual is described as an inversion heterozygote .
  • To maintain point-for-point synapsis during meiosis, one homolog must form a loop, and the other homolog must mold around it.
  • This inversion pairing is essential to maintaining gene alignment during meiosis and to allow for recombination.

• Gene Duplications and Divergence

  • Gene duplication can occur as the result of an error in recombination or through a retrotransposition event.
  • Duplications can arise from unequal crossing-over that occurs during meiosis between misaligned homologous chromosomes.
  • The product of this recombination is a duplication at the site of the exchange and a reciprocal deletion.
  • Ectopic recombination is typically mediated by sequence similarity at the duplicate breakpoints, which form direct repeats.
  • This can apply to genes and proteins, such as nucleotide sequences or protein sequences that are derived from two or more homologous genes.

• Homologous Structures

  • Homologous traits of organisms are therefore explained by descent from a common ancestor.
  • If we go all the way back to the beginning of life, all structures are homologous!
  • In genetics, homology is measured by comparing protein or DNA sequences.
  • As a result, hybrid or mosaic structures can evolve that exhibit partial homologies.
  • Analogy is different than homology.

• Meiosis I

  • The tight pairing of the homologous chromosomes is called synapsis.
  • The homologous pairs orient themselves randomly at the equator.
  • Crossover occurs between non-sister chromatids of homologous chromosomes.
  • The result is an exchange of genetic material between homologous chromosomes.
  • Early in prophase I, homologous chromosomes come together to form a synapse.