Annotation, orthologues and comparative genomics
Genome annotation locates features and proposes functions using sequence evidence and other data. Orthologues diverge through speciation; paralogues diverge through duplication. Conservation can support a functional hypothesis, but similarity and ancestry do not establish an exact function.
What a learner can do afterwards
- Distinguishes genes related by speciation from genes related by duplication, and says why the difference matters for function.
- Uses conservation across distant species as evidence of function, and states the limits of that argument.
- Explains why an annotation transferred by similarity is a hypothesis rather than a result.
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An assembly is useful before every gene is annotated. Annotation identifies features and attaches biological information. A match to a studied gene can suggest a function. Similarity alone does not confirm that function. Record transferred functions as predictions and test the proposed activity in its biological context.
Orthologues are homologous genes whose ancestral copies separated through speciation. Paralogues separated through gene duplication. Paralogues can occur in different species when duplication preceded speciation. The ancestral event defines the relationship, not present location. Comparing gene and species trees helps infer these events; uncertain trees can leave the relationship unresolved.
Orthology can guide function prediction without guaranteeing the same function. Duplicated copies may retain, divide or change ancestral activities. Orthologues can also differ in activity, expression or regulation. Use ancestry together with experimental evidence and biological context before transferring detailed claims.
Less sequence change than a suitable neutral model predicts can suggest purifying selection: harmful changes tend to be removed. This supports functional constraint without identifying an exact function. A low mutation rate can also preserve sequence, so unchanged DNA alone is not proof.
Conserved non-coding DNA may regulate expression or have another function. Conservation alone identifies neither a regulatory role nor its target gene. Test a regulatory proposal by changing the region and measuring expression against comparison samples. A structural role is also a function.
Speciation defines orthology; duplication defines paralogy. Conservation and similarity support hypotheses, not automatic functional conclusions.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. "Understanding and building phylogenetic trees" by Khan Academy. "Discovering the tree of life" by California Academy of Sciences.
More to read
Location and activity answer different questions
A genome map records locations. An assembly provides reconstructed DNA sequence. Annotation describes features and attaches biological information to them. A gene's position does not establish its activity. In a species comparison, researchers identify corresponding regions before interpreting their similarities and differences. Functional annotations can help that interpretation, but an assembly remains useful when many genes still have unknown functions.
Check the sequence before interpreting absence
An apparent difference between species may reflect incomplete data. An unresolved repeat can leave gene-copy number uncertain. A missing assembly region can hide a gene that is present in the organism. Check coverage of the relevant regions and inspect the sequence comparison before calling a gain or loss. Missing observation and biological absence are different explanations that need different evidence.
Document a function prediction
When similarity suggests a function, record the studied gene that supplied the evidence. State which activity is predicted and which details have been measured in the new organism. Expression patterns and biological conditions may differ even when sequences resemble one another. A test can support part of a prediction without supporting every detail. Clear evidence records help later users distinguish observations from transferred claims.
Follow the ancestral event
Classify a gene relationship by the event separating its ancestral copies. Speciation defines orthology; duplication defines paralogy. Present-day species names do not replace this history. A duplication can occur before species separate, leaving paralogues in different species. Comparing gene and species trees helps infer the events. Gene loss and uncertain trees can complicate that reconstruction, so unresolved relationships should remain marked as uncertain.
Compare DNA with biological output
Genomics examines genome sequences. Proteomics examines proteins in biological samples. Tissues sharing a genome can produce different protein profiles because expression, RNA processing and protein modification vary. Sequence conservation therefore does not guarantee identical output in every condition. Choose measurements that test the proposed role. A non-coding region may need an expression experiment rather than a test of a protein product.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.