homologous structure

Examples of homologous structure in the following topics:

• Homologous Structures

  • Homology is the relationship between structures or DNA derived from the most recent common ancestor.
  • If we go all the way back to the beginning of life, all structures are homologous!
  • As a result, hybrid or mosaic structures can evolve that exhibit partial homologies.
  • The opposite of homologous structures are analogous structures, which are physically similar structures between two taxa that evolved separately (rather than being present in the last common ancestor).
  • Describe the connection between evolution and the appearance of homologous structures

• Distinguishing between Similar Traits

  • Similar traits can be either homologous structures that share an embryonic origin or analogous structures that share a function.
  • For example, the bones in the wings of bats and birds have homologous structures .
  • Homologous structures share a similar embryonic origin; analogous organs have a similar function.
  • Some structures are both analogous and homologous: the wings of a bird and the wings of a bat are both homologous and analogous.
  • Bat and bird wings are homologous structures, indicating that bats and birds share a common evolutionary past.

• Vestigial Structures

  • These unused structures without function are called vestigial structures.
  • Vestigial structures are often homologous to structures that function normally in other species.
  • In some cases the structure becomes detrimental to the organism.
  • The vestigial versions of a structure can be compared to the original version of the structure in other species in order to determine the homology of the structure.
  • Homologous structures indicate common ancestry with those organisms that have a functional version of the structure.

• Convergent Evolution

  • However, the wings of bats and insects have evolved from very different original structures.
  • Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye.
  • Traits arising through convergent evolution are analogous structures, in contrast to homologous structures, which have a common origin, but not necessarily similar function.
  • The British anatomist Richard Owen was the first scientist to recognize the fundamental difference between analogies and homologies.
  • Bat and pterosaur wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions.

• Chromosomal Theory of Inheritance

  • During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
  • The sorting of chromosomes from each homologous pair into pre-gametes appears to be random.

• Gene rearrangement within genomes

  • Most recombination is naturally occurring and the rearrangement of DNA can occur by several different mechanisms and result in structural variation.
  • 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.

• Chromosomal Structural Rearrangements

  • Structural rearrangements of chromosomes include both inversions and translocations, which may have detrimental effects on an organism.
  • Cytologists have characterized numerous structural rearrangements in chromosomes, but chromosome inversions and translocations are the most common.
  • To maintain point-for-point synapsis during meiosis, one homolog must form a loop, and the other homolog must mold around it.
  • Not all structural rearrangements of chromosomes produce nonviable, impaired, or infertile individuals.
  • Describe the various types of structural rearrangements of chromosomes and how they can impact an organism

• Meiosis I

  • The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate.
  • 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.
  • Early in prophase I, homologous chromosomes come together to form a synapse.

• Genomic DNA and Chromosomes

  • Before discussing the steps a cell must undertake to replicate, a deeper understanding of the structure and function of a cell's genetic information is necessary.
  • Matched pairs of chromosomes in a diploid organism are called homologous ("same knowledge") chromosomes.
  • 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.

• Constructing an Animal Phylogenetic Tree

  • Phylogenetic trees are constructed according to the evolutionary relationships that exist between organisms based on homologous traits.
  • In particular, they clarify whether certain traits are homologous (found in the common ancestor as a result of divergent evolution) or homoplasy (sometimes referred to as analogous: a character that is not found in a common ancestor, but whose function developed independently in two or more organisms through convergent evolution).
  • Lophotrochozoa is named for two structural features, each common to certain phyla within the clade.
  • Some lophotrochozoan phyla are characterized by a larval stage called trochophore larvae, and other phyla are characterized by the presence of a feeding structure called a lophophore.