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4.3.5Additional lessons

Conserved Sequences — IB Biology HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 40 of 101

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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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
  • 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
  • Why it persists Changes in these regions usually damage an essential function, so organisms carrying them die or fail to reproduce.
Purifying selection
  • Purifying selection Natural selection removes harmful variants, keeping the sequence unchanged generation after generation.
Neutral regions
  • 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
  • Histone H4 This protein packages DNA, and the pea and cow versions differ at only two of about 102 amino acids.
Ribosomal RNA genes
  • Ribosomal RNA genes Every cell needs ribosomes for translation, so rRNA genes are conserved in bacteria, archaea and eukaryotes.
Cytochrome c
  • Cytochrome c This electron carrier in respiration has a highly similar amino acid sequence across animals, plants and fungi.
Hox genes
  • Hox genes Homeobox sequences that help set the body plan along the head to tail axis are shared by insects and mammals.
GeneRoleWhy it is conserved
Histone H4DNA packagingNeeded to form nucleosomes
rRNARibosome structureNeeded for protein synthesis
Cytochrome cElectron transportNeeded for ATP production

Part 3 of 6Using Conserved Sequences

Evolutionary trees
  • Evolutionary trees Fewer differences between two species' conserved genes suggest a more recent common ancestor, so sequences build phylogenies.
Molecular clock
  • Molecular clock Differences accumulate at a roughly steady rate, so counting them estimates the time since two lineages split.
Finding function
  • Finding function A newly sequenced human gene resembling a conserved mouse or yeast gene probably performs a similar role.
Primers and probes
  • 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.

  1. Positions compared = 20.
  2. Positions that differ = 2, so identical positions = 20 - 2 = 18.
  3. 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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