Conserved and highly conserved sequencesSpec D1.3.10
In short
Conserved sequences are base sequences that are identical or similar across a species or a group of species; highly conserved sequences remain identical or similar over long periods of evolution. One hypothesis is that the gene products have strict functional requirements, so natural selection removes almost any change. Another hypothesis is that these sequences have slower rates of mutation.
- Conserved sequence
- A base sequence (or amino acid sequence) that is identical or similar across a species or a group of species.
- Highly conserved sequence
- A sequence that has remained identical or similar over long periods of evolution, so it is shared by distantly related groups.
Examples of highly conserved genes include those for histone proteins, which package DNA, and for ribosomal RNA, which are similar in organisms as different as yeast and humans.
Hypotheses to account for conservation
| Hypothesis | Explanation |
|---|---|
| Functional requirements for the gene products | The protein or RNA must have a very precise structure to work, for example because it binds many other molecules. Almost any mutation makes it less functional, so individuals carrying it are selected against and the mutation is removed from the population. The sequence stays the same because changes are eliminated, not because they do not happen. |
| Slower rates of mutation | Some sequences may mutate less often than others, for example because of their position in the genome or more effective repair, so fewer changes arise in the first place. |
For protein-coding genes the two hypotheses make different predictions. If functional requirements are the cause, base substitutions that do not change the amino acid (possible because the code is degenerate) should still build up at a normal rate, while substitutions that change the amino acid are rare. If the mutation rate is slower, both kinds of change should be rare. In mammals, histone genes show almost no amino acid changes but a normal rate of the silent changes, which supports the functional-requirements hypothesis for these genes.
Name both hypotheses when asked to account for conserved sequences: functional requirements of the gene product (with selection removing changes) and a slower rate of mutation.
Quick check
What is a single-nucleotide polymorphism (SNP)?
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A position in the genome where a single base differs between individuals, resulting from a base substitution mutation.
Why does deleting one base usually have a larger effect than substituting one base?
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It shifts the reading frame, changing every codon after the mutation.
Give one chemical mutagen and one form of mutagenic radiation.
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Benzo[a]pyrene (in tobacco smoke); ultraviolet light (or X-rays, gamma rays).
Why are somatic mutations not inherited?
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They are not in germ cells, so they are not passed into gametes.
HL only What does the guide RNA do in CRISPR–Cas9 editing?
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It binds by complementary base pairing to the target DNA sequence and brings Cas9 to it.
Written and checked against the IB Biology HL specification · Updated October 2026