If Two Genes Are Homologous What Do You Know About Them

7.13C: Homologs, Orthologs, and Paralogs

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9312

Learning Objectives

• Distinguish homologs, orthologs and paralogs

Homology forms the basis of organization for comparative biology. A homologous trait is oft called a homolog (also spelled homologue). In genetics, the term "homolog" is used both to refer to a homologous poly peptide and to the factor ( DNA sequence) encoding it. Equally with anatomical structures, homology between poly peptide or Dna sequences is defined in terms of shared ancestry. Two segments of DNA can accept shared ancestry because of either a speciation issue (orthologs) or a duplication consequence (paralogs). Homology amongst proteins or DNA is oftentimes incorrectly concluded on the basis of sequence similarity. The terms "pct homology" and "sequence similarity" are often used interchangeably. As with anatomical structures, high sequence similarity might occur because of convergent evolution, or, equally with shorter sequences, because of chance. Such sequences are like, only not homologous. Sequence regions that are homologous are as well chosen conserved. This is not to be dislocated with conservation in amino acid sequences in which the amino acid at a specific position has been substituted with a different one with functionally equivalent physicochemical backdrop. I tin can, nonetheless, refer to partial homology where a fraction of the sequences compared (are presumed to) share descent, while the residual does not. For example, partial homology may result from a gene fusion event.

Figure: Example of Homologous DNA: This is the sequence alignment of a homologous protein from two unlike species The "*" represent a conserved amino acid in the ii proteins.

Homologous sequences are orthologous if they were separated by a speciation event: when a species diverges into two separate species, the copies of a single factor in the 2 resulting species are said to be orthologous. Orthologs, or orthologous genes, are genes in different species that originated by vertical descent from a unmarried cistron of the concluding common antecedent. For case, the plant Flu regulatory protein is nowadays both in Arabidopsis (multicellular higher plant) and Chlamydomonas (single cell green algae). The Chlamydomonas version is more circuitous: information technology crosses the membrane twice rather than one time, contains additional domains, and undergoes alternative splicing. Withal, it can fully substitute the much simpler Arabidopsis protein, if transferred from algae to plant genome by means of gene engineering science. Significant sequence similarity and shared functional domains indicate that these 2 genes are orthologous genes, inherited from the shared ancestor. Orthologous sequences provide useful information in taxonomic nomenclature and phylogenetic studies of organisms. The pattern of genetic divergence can be used to trace the relatedness of organisms. Two organisms that are very closely related are likely to display very similar Deoxyribonucleic acid sequences between two orthologs. Conversely, an organism that is further removed evolutionarily from another organism is likely to brandish a greater divergence in the sequence of the orthologs being studied.

Homologous sequences are paralogous if they were separated by a gene duplication event: if a gene in an organism is duplicated to occupy two different positions in the aforementioned genome, then the two copies are paralogous. Paralogous genes often belong to the same species, but this is not necessary. For example, the hemoglobin gene of humans and the myoglobin gene of chimpanzees are paralogs. Paralogs can be divide into in-paralogs (paralogous pairs that arose afterwards a speciation event) and out-paralogs (paralogous pairs that arose before a speciation event). Betwixt species out-paralogs are pairs of paralogs that exist between 2 organisms due to duplication before speciation. Inside species out-paralogs are pairs of paralogs that exist in the same organism, but whose duplication event happened subsequently speciation. Paralogs typically accept the same or like role, but sometimes do not. Due to lack of the original selective pressure level upon ane re-create of the duplicated factor, this re-create is gratis to mutate and acquire new functions. Paralogous sequences provide useful insight into the mode genomes evolve. The genes encoding myoglobin and hemoglobin are considered to be ancient paralogs. Similarly, the four known classes of hemoglobins (hemoglobin A, hemoglobin A2, hemoglobin B, and hemoglobin F) are paralogs of each other. While each of these proteins serves the same basic office of oxygen transport, they have already diverged slightly in function: fetal hemoglobin (hemoglobin F) has a college affinity for oxygen than developed hemoglobin. Nonetheless, function is not always conserved. Human angiogenin diverged from ribonuclease, for instance, and while the 2 paralogs remain like in tertiary structure, their functions inside the prison cell are now quite different.

Central Points

• A homologous gene (or homolog) is a gene inherited in two species by a common antecedent. While homologous genes can be similar in sequence, similar sequences are not necessarily homologous.
• Orthologous are homologous genes where a factor diverges afterwards a speciation event, but the gene and its main part are conserved.
• If a factor is duplicated in a species, the resulting duplicated genes are paralogs of each other, even though over time they might become different in sequence limerick and function.

Key Terms

• conserved: In biology, conserved sequences are similar or identical sequences that occur within nucleic acid sequences (such as RNA and Dna sequences), protein sequences, protein structures.
• selective force per unit area: Whatsoever cause that reduces reproductive success in a proportion of a population, potentially exerts evolutionary pressure or pick pressure level.

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Source: https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/7%3A_Microbial_Genetics/7.13%3A_Bioinformatics/7.13C%3A_Homologs%2C_Orthologs%2C_and_Paralogs

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