Orthologs,Paralogs & Xenologs

Molecular Evolution Topic (Homology Sequencing Types)

1. Orthologs, Paralogs, & Xenologs Muhammad Osama Zafar BS-BioTech (4 th – Semester) Gulab Devi Educational Complex (GDEC), Lahore Course Title : Molecular Evolution

2. Sequence Homology Sequence homology is the biological homology between DNA, RNA or Protein Sequences Defined in terms of shared ancestry in the evolutionary history of life Homlogous genes( Homologues ) are derived from a common ancestors Homology among DNA, RNA, or proteins is typically inferred from their  nucleotide  or  amino acid  sequence similarity.

3. Types of Sequence Homology Significant similarity is strong evidence that two sequences are related by evolutionary changes from a common ancestral sequence.   Alignments  of multiple sequences are used to indicate which regions of each sequence are homologous. Two segments of DNA can have shared ancestry because of three phenomena: a  Speciation  event ( ORTHOLOGS ) a  Duplication event ( PARALOGS ) a  horizontal(lateral) gene transfer event ( XENOLOGS )

4. Orthologs Homologous sequences are orthologous: if they are inferred to be descended from the same ancestral sequence separated by a  speciation  event: when a species diverges into two separate species The copies of a single gene in the two resulting species are said to be orthologous. Orthologs, or orthologous genes, are genes in different species that originated by vertical descent from a single gene of the last common ancestor. The term "ortholog" was coined in 1970 by the Molecular Evolutionist  Walter Fitch .

5. Example (ORTHOLOGS) EyeLess Gene : Controls the Eye development & formation

6. Paralogs Paralogous genes are genes that are related via duplication events in the last common ancestor (LCA) of the species being compared. They result from the mutation of duplicated genes during separate speciation events. When descendants from the LCA share mutated homologs of the original duplicated genes then those genes are considered paralogs These are the homologous genes in one species derived from the gene duplication

7. Example (PARALOGS) Drospophilla

8. Xenologs Homologs resulting from  horizontal gene transfer  between two organisms. Xeno is a Greek word that means “Foreigner ”. Can have different functions if the new environment is vastly different for the horizontally moving gene. In general, though, xenologs typically have similar function in both organisms. The term was coined by Walter Fitch Results from a Horizontal Gene Transfer(HGT) between two organisms. A direct transfer between two species.

9. Example (XENOLOGS)

10. Summary