homology

Examples of homology in the following topics:

• 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.

• Distinguishing between Similar Traits

  • For example, the bones in the wings of bats and birds have homologous structures .
  • Similar traits can be either homologous or analogous.
  • Homologous structures share a similar embryonic origin; analogous organs have a similar function.
  • The wings of a butterfly and the wings of a bird are analogous, but not homologous.
  • Some structures are both analogous and homologous: the wings of a bird and the wings of a bat are both homologous and analogous.

• Use of Whole-Genome Sequences of Model Organisms

  • Sequencing genomes of model organisms allows scientists to study homologous proteins in more complex eukaryotes, such as humans.
  • Sequencing genomes allows scientists to identify homologous proteins and establish evolutionary relationships.
  • Furthermore, if a newly-discovered protein is homologous to a known protein, through homology, scientists can make an educated guess as to how the new protein functions.
  • Research on many proteins that are important to humans is done by examining their homologs in yeasts.
  • Saccharomyces cerevisiae, a yeast, is used as a model organism for studying signaling proteins and protein-processing enzymes which have homologs in humans.

• Gene rearrangement within genomes

  • During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes.
  • It is known that this 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 at their arms .
  • In meiosis and mitosis, recombination occurs between similar molecules (homologs) of DNA.
  • Recombination can occur between DNA sequences that contain no sequence homology.

• Genomic DNA and Chromosomes

  • Matched pairs of chromosomes in a diploid organism are called homologous ("same knowledge") chromosomes.
  • Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus.
  • Each copy of a homologous pair of chromosomes originates from a different parent; therefore, the genes themselves are not identical.
  • The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity.
  • There are 23 pairs of homologous chromosomes in a female human somatic cell.

• 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.
  • In preparation for the first division of meiosis, homologous chromosomes replicate and synapse.
  • Like genes on the homologs align with each other.
  • At this stage, segments of homologous chromosomes exchange linear segments of genetic material .

• Comparing Meiosis and Mitosis

  • In meiosis I, the homologous chromosome pairs become associated with each other and are bound together with the synaptonemal complex.
  • Chiasmata develop and crossover occurs between homologous chromosomes, which then line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad.
  • When the tetrad is broken up and the homologous chromosomes move to opposite poles, the ploidy level is reduced from two to one.
  • In this case, the duplicated chromosomes (only one set, as the homologous pairs have now been separated into two different cells) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles.

• 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 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.