4.3.5Additional lessons
Conserved Sequences — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 40 of 101
Part 1 of 6What Conservation Means
If a sequence is shared by distant species, it has probably been important for survival all that time.
Conserved sequence
- A stretch of DNA or protein that is almost identical in many species, even those that diverged hundreds of millions of years ago.
Why it persists
- Changes in these regions usually damage an essential function, so organisms carrying them die or fail to reproduce.
Purifying selection
- Natural selection removes harmful variants, keeping the sequence unchanged generation after generation.
Neutral regions
- Sequences with little function can accumulate mutations freely, so they differ much more between species.
Part 2 of 6Examples of Conserved Genes
Histone H4
- This protein packages DNA, and the pea and cow versions differ at only two of about 102 amino acids.
Ribosomal RNA genes
- Every cell needs ribosomes for translation, so rRNA genes are conserved in bacteria, archaea and eukaryotes.
Cytochrome c
- This electron carrier in respiration has a highly similar amino acid sequence across animals, plants and fungi.
Hox genes
- Homeobox sequences that help set the body plan along the head to tail axis are shared by insects and mammals.
| Gene | Role | Why it is conserved |
|---|---|---|
| Histone H4 | DNA packaging | Needed to form nucleosomes |
| rRNA | Ribosome structure | Needed for protein synthesis |
| Cytochrome c | Electron transport | Needed for ATP production |
Part 3 of 6Using Conserved Sequences
Evolutionary trees
- Fewer differences between two species' conserved genes suggest a more recent common ancestor, so sequences build phylogenies.
Molecular clock
- Differences accumulate at a roughly steady rate, so counting them estimates the time since two lineages split.
Finding function
- A newly sequenced human gene resembling a conserved mouse or yeast gene probably performs a similar role.
Primers and probes
- PCR primers are designed to match conserved regions so that one pair can amplify the gene from many species.
Part 4 of 6Comparing Two Sequences
A short procedure turns raw sequences into a measure of relatedness.
1
Align the same gene from the two species so that corresponding positions are matched.
2
Count the positions at which the bases or amino acids are identical.
3
Divide the identical positions by the total compared and multiply by 100 to get percentage identity.
4
Interpret the result: very high identity in a functional gene suggests strong selection and a close or essential relationship.
Part 5 of 6Worked Example
Question
- Two 20-amino-acid stretches of cytochrome c from species X and Y are aligned and differ at 2 positions.
- Calculate the percentage identity.
- Positions compared = 20.
- Positions that differ = 2, so identical positions = 20 - 2 = 18.
- Percentage identity = 18 / 20 x 100.
Answer 90% identity, which is high and consistent with a strongly conserved, essential protein.
Part 6 of 6Exam Checklist & Tips
4.3.5 – Key Points for the Exam
- Conserved sequence A base or amino acid sequence that remains largely unchanged between species over evolutionary time.
- Purifying selection Selection that removes harmful mutations and so preserves a sequence.
- Percentage identity The proportion of aligned positions that are the same in two sequences.
- Molecular clock Use of the rate of sequence change to estimate when species diverged.
Common Mistakes
- Saying conserved sequences never mutate; mutations occur but harmful ones are eliminated.
- Assuming all DNA is conserved, when non-functional regions diverge quickly.
- Stating that similar sequences prove the species look alike rather than share ancestry.
- Confusing the conserved sequence with the species' complete genome.
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