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homologous and heterologous

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  1. 1. Heterologs & Homologs Course: B.Sc Biochemistry Subject: Basic of Bioinformatics Unit: III
  2. 2. Heterologs & Homologs Heterologs :- Heterologs differ in both origin and activity. • Genes that are "unique" in activity and sequence are said to be heterologous. Note that genes initially defined as heterologous by syntax (letter matching) may actually be homologous by activity. Homologs :- Homologs have common origins but may or may not have common activity. • Genes that share an arbitrary threshold level of similarity determined by alignment of matching bases are termed homologous. Homology is a qualitative term that describes a relationship between genes and is based upon the quantitative similarity.
  3. 3. Sequence Similarity Similarity is a quantitative term that defines the degree of sequence match between two compared sequences. • For example, two aligned genes or segments of sequence that are homologous may have varying degrees of similarity based upon identical base matches in the alignment. In the first sequence alignment in the following figure, the sequences are obviously identical and therefore exhibit 39 matches out of 39 positions aligned, or 100% similarity. In the second alignment the aligned sequences contain 28 matches out of 39 possible. The quantitative match or degree of similarity is then 28/39 or 72%. In both cases the sequences are homologous.
  4. 4. A • atgcctgaaggcctattgtttcccagtcgattggctgct... ||||||||||||||||||||||||||||||||||||||| 39 of 39 matches atgcctgaaggcctattgtttcccagtcgattggctgcg... B • atgcctgaaggcctattgtttcccagtcgattggctgct... |||||| |||||| |||||||| |||||| || 28 of 39 matches atgccteggcttatattgtatcccagtccattggcagcg...
  5. 5. Analogs & Orthologs Analogs: - Analogs have common activity but not common origin. • Genes or proteins that display the same activity but lack sufficient similarity to imply common origin are said to have analogous activity. The implication is that analogous proteins followed evolutionary pathways from different origins to converge upon the same activity. Thus, analogous genes or proteins are considered a product of convergent evolution. Analogs have homologous activity but heterologous origins. • Example :- Brain & Computer Orthologs :- Orthologs are homologs produced by speciation. • Orthologs are genes in different species that evolved from a common ancestral gene by speciation. Normally, orthologs retain the same function in the course of evolution. OR • any gene pairwise relation where the ancestor node is a speciation event. Often have similar function
  6. 6. Paralogs Paralogs: - Paralogous are homologous produced by gene duplication. • Paralogs are genes related by duplication within a genome. Orthologs retain the same function in the course of evolution, whereas paralogs evolve new functions, even if these are related to the original one. OR • any gene pairwise relation where the ancestor node is a duplication event. Paralogs tend to have different functions
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  9. 9. Sequence Alignment – PHYLOGENY 3.
  10. 10. Books and Web References • Books Name : 1. Introduction To Bioinformatics by T. K. Attwood 2. BioInformatics by Sangita 3. Basic Bioinformatics by S.Ignacimuthu, s.j. • • • 10
  11. 11. Image References • 1. • 2. mple1.png • 3. s/nature12027-f1.2.jpg