📘 Notes on Sequence and Structural Homology (With Examples)
1. Sequence Homology
- Definition:
Sequence homology means similarity in DNA, RNA, or protein sequences because of a common ancestor.
(Roman Urdu: Jab DNA, RNA ya protein ke sequences ek asal gene se aayen aur miltay julday hon → ise sequence homology kehtay hain.)
Types of Sequence Homology
- Orthologs
- Sequences in different species, evolved from a common ancestral gene during speciation.
- Usually retain the same function across species.
- Examples:
- Hemoglobin gene in Humans and Mice → both transport oxygen.
- Cytochrome c in Humans and Yeast → both used in electron transport chain.
- Actin gene in Fruit fly and Humans → both involved in cell structure.
- (Roman Urdu: Mukhtalif species mein jo gene ek asal se aaye aur same kaam karte rahein → Orthologs.)
- Paralogs
- Sequences within the same species, produced by gene duplication.
- They may evolve new or specialized functions.
- Examples:
- Hemoglobin and Myoglobin in Humans → both bind oxygen but with different roles (hemoglobin → blood transport, myoglobin → muscle storage).
- Alpha- and Beta-globin genes in Humans → arose by duplication, both part of hemoglobin.
- HOX genes in Vertebrates → duplicated many times to control body plan.
- (Roman Urdu: Aik hi species mein gene duplicate ho kar naye roles hasil kar lein → Paralogs.)
Importance of Sequence Homology
- Evolutionary history → shows divergence of species.
- Gene function prediction → if an unknown gene is homologous to a known one, its function can be predicted.
- Disease studies → homologous disease genes in animals help study human genetic disorders.
- Example: BRCA1 gene (breast cancer susceptibility) studied in mice, yeast, and humans.
- (Roman Urdu: Agar insaan ka gene kisi aur jaanwar mein bhi mil jaye to usay study karke diseases ka ilaj dhoondha jata hai.)
2. Structural Homology
- Definition:
Structural homology = similarity in 3D structure of proteins or macromolecules due to common ancestry.
(Roman Urdu: Proteins ki 3D shape agar milti ho aur asal ek hi ho to ise structural homology kehtay hain.)
Examples of Structural Homology
- Serine Proteases: Trypsin, Chymotrypsin, and Elastase → different substrates, but same 3D catalytic structure.
- Hemoglobin (humans) and Leghemoglobin (plants) → both bind oxygen, similar structure despite different organisms.
- Green Fluorescent Protein (GFP in jellyfish) and GFP-like proteins in corals → same fold and fluorescence.
- Lysozyme in Humans and Hen Egg White Lysozyme → similar fold and antimicrobial activity.
(Roman Urdu: Agar sequence thoda alag bhi ho lekin 3D shape same ho to function bhi milta julta hota hai.)
Applications of Structural Homology
- Protein function prediction → unknown protein’s structure compared with known proteins.
- Drug design → drugs made to fit specific protein structures (e.g., HIV protease inhibitors).
- Evolutionary biology → shows how proteins evolved new functions but kept structural cores.
✅ Summary
- Sequence Homology → similarity in DNA/RNA/protein sequences.
- Orthologs = across species (same function).
- Paralogs = within same species (may evolve new functions).
- Structural Homology → similarity in 3D protein structures (helps in predicting function & drug design).
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