This web page was produced as an assignment for Genetics 564, an undergraduate capstone course at UW-Madison.
What is Homology? [1,2]
Homology between two sequences or structures is considered when there is enough similarities that it can not be due to mere chance that the two characteristics arose by themselves, rather the abundance of similarities are result of sharing a common ancestor. This is the easiest explanation for how two traits or sequences evolved to be so alike. Homology can be utilized when observing structures such as the arm of a a human in comparison the fin of a whale, or homology can be looked at in terms of DNA sequences and protein amino acids.
Gene homology [2] arises when there is a gene duplication at some point in the evolutionary tree, and either the duplicated gene is altered or conserved in the genome. These duplicated genes are known as homologs and this broad term encompasses two different varieties: orthologs and paralogs. Orthologs occur as a result of a speciation event and will retain their function, while paralogs are due to gene duplication events and typically have slightly different functions. Gene homology can be studied through programs such as BLAST and FASTA. Both FASTA and BLAST are used to analyze similarities between sequences, drawing conclusions on significance of relatedness. These programs that perform similarity searches are effective because if there is a significance in similarities between two sequences, homology can be inferred indicating that they share a common ancestor.
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Structural homology [1] is concerned with the structure of two different species that may have resulted from a common ancestor. An example of this can be seen when observing the bone structure of the arm of human and the fin of a whale. The similar structure shows that the two species might share a common ancestor, although at some point during evolutionary history they diverged resulting in the differences in functionality. This concept helps scientists observe the relatedness of species without looking at its genome. Structural homology is not always the most effective system in predicting relatedness between species but can be complimented in some cases with gene homology. Conversely to homologous structures is analogous structures, which states that a structure that shares similar function with another species arose independently.
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KIF59 Homologs in Common Organisms
Homo sapiens (Humans)Protein: KIF5A (isoform 1)
Accession Number: NP_004975.2 Length: 1032 aa |
Pan Troglodytes (Chimpanzee)Protein: KIF5A
Accession Number: XP_509167.3 Length: 1032 aa |
Canis Lupus (Grey Wolf) |
Bos Taurus (Cow) |
Mus Musculus (Mouse) |
Rattus Norvegicus (Brown Rat) |
Xenopus Tropicalis (Western Clawed Frog) |
Danio Rerio (Zebrafish) |
Drosophila Melanogaster ( Fruit Fly) |
Caenorhabditis Elegans (Nematod Worm)
Conclusion
Homology is the assumption that certain sequences or characteristics are not due to chance but rather the result of having an evolutionary common ancestor. Both structure and gene homology play a role in observing the relatedness between species and coming to this conclusion about a species on the evolutionary tree. We explored this concept in detail by using FASTA, one of the gene homology programs readily available, to investigate the relatedness of KIF5A amongst several different species. It can be noted that out of the wide range of species discovered, there are several model organisms that have been identified where the KIF5A gene has been conserved. This provides options for researching the gene of interest in the context of SPG10.
References:
[1] The Editors of Encyclopedia Britanica. (September 8, 2019). Homology. Retrieved from https://www.britannica.com/science/homology-evolution
[2] Weber, C. (April 27, 2015) Homology and Genes. Retrived from https://www.cell.com/current-biology/pdf/S0960-9822(04)00287-8.pdf
[1] The Editors of Encyclopedia Britanica. (September 8, 2019). Homology. Retrieved from https://www.britannica.com/science/homology-evolution
[2] Weber, C. (April 27, 2015) Homology and Genes. Retrived from https://www.cell.com/current-biology/pdf/S0960-9822(04)00287-8.pdf