The big idea: Some sequences in genes stay the same, or nearly the same, across species and over long periods of evolution. Two hypotheses explain why.
The points to remember
- A conserved sequence is identical or similar across a species or a group of species.
- A highly conserved sequence stays identical or similar over long periods of evolution.
- Hypothesis 1, function: the gene's product only works with this sequence, so mutations are selected against.
- Hypothesis 2, mutation rate: this region mutates more slowly than others.
Remember it as: Same in many species: needed for the job, or slow to change.
Real example: histone H4. Histones are the proteins DNA wraps around. The histone H4 of a pea plant and of a cow differ at only 2 of their 102 amino acids, even though plants and animals separated over a billion years ago.
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Two hypotheses: Both hypotheses predict conserved sequences; they differ in why.
Functional requirement
- The product (protein or RNA) needs an exact shape to work
- Most mutations in the sequence are harmful
- Natural selection removes them
- So the sequence stays the same
Slower mutation
- Some regions of DNA mutate less often
- Fewer changes arise in the first place
- So the sequence changes slowly
- Even without selection against changes
Conserved, not unchanging: Conserved sequences can still change a little. 'Highly conserved' means very few changes over a very long time.
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How it is asked: Defining conserved sequences, or outlining the two hypotheses for them.
IB-style questionOutline[2 marks]
Outline two hypotheses that could account for highly conserved sequences in genes.
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