12.7 Sequence Homology
Sequence homology is a fundamental concept in bioinformatics and evolutionary biology that examines the similarities between DNA, RNA, and protein sequences to infer common ancestry and functional relationships.
Sequence Homology
Definition: Sequence homology is the presence of shared sequences or structural features between two or more biological molecules (DNA, RNA, or protein) that indicates a common ancestral origin. It is a conclusion drawn from sequence similarity; high sequence similarity suggests homology.
Basis of Homology: The underlying principle is that two sequences are homologous if they have evolved from a common ancestral sequence. Over time, sequences in different species diverge due to mutations, but they may retain significant similarities if they are related.
Significance and Applications: Studying sequence homology is crucial for:
- Evolutionary Relationships: Determining how closely related different species are (phylogenetics). The more similar the sequences, the more recent their common ancestor.
- Functional Conservation: Predicting the function of an unknown gene or protein. If a new sequence is homologous to a sequence with a known function, it is likely to have a similar function.
- Genetic Diversity: Understanding variations within and between populations.
- Identifying Important Regions: Highly conserved sequences (regions with little change over evolutionary time) often indicate functionally or structurally important parts of a molecule, such as an enzyme's active site.
Example of Conserved Sequence
The diagram illustrates sequence homology for the BauA protein, a protective antigen in mice. The high degree of conservation (similarity) across the 25 samples suggests that this protein has a critical function that has been maintained through evolution.
Possible Questions and Answers
Q: What is sequence homology? A: It is the inference of a shared evolutionary origin between two biological sequences (DNA, RNA, or protein) based on their similarity.
Q: Why is studying sequence homology important? A: It helps scientists understand evolutionary relationships, predict the function of newly discovered genes or proteins, and identify functionally critical regions within a sequence.
Q: What does a high degree of sequence conservation imply? A: It implies that the sequence has an important biological function that has been preserved by natural selection over evolutionary time.