Mutations land anywhere, at random, over billions of years. So you would expect DNA sequences to drift apart until nothing matched. Yet some stretches are almost identical in you, in yeast and in bacteria. Something is holding them still — and biologists have two competing explanations for what.
📘 What you need to know
A conserved sequence is a section of DNA or RNA that shows minimal mutation over time, and is identical or very similar across a species or group of species.
Sequences with almost no change over long periods of evolution are highly conserved sequences.
Examples include the sequences for DNA helicases, tRNA and ribosomes, and for the respiratory proteins cytochrome c and ferredoxin.
Two hypotheses explain conservation: functional requirements and slower mutation rates.
Functional requirements: these genes code for proteins essential to survival, so a mutated version kills the cell and is never passed on. Natural selection maintains the sequence.
Slower mutation rates: DNA repair and proofreading are more active in coding regions, so errors are corrected before they show up.
Mutation rate is the number of changes to a DNA sequence over time — measured per gene per generation, or per genome per generation.
What “conserved” actually means
Line up the same gene from several species and compare the bases. In most regions you will find plenty of differences, accumulated over millions of years. But in some regions the sequence barely changes at all — sometimes not by a single base between organisms as different as a human and a yeast cell.
Those regions are conserved. Where the similarity holds over very long evolutionary periods, they are highly conserved.
These sequences are illustrative, not real data — but the pattern is exactly what sequence comparisons show.
Which sequences are conserved?
Look at the list and a theme jumps out: every one of them is part of a process that no cell can live without.
Sequences involved in DNA replication, transcription and translation — including those for DNA helicases, tRNA and ribosomes.
Sequences for proteins involved in cellular respiration — including cytochrome c and ferredoxin.
Notice the pattern. These are the oldest, most fundamental jobs a cell does. They were working before animals, plants and fungi separated, which is why the same sequences turn up in all of them.
Two hypotheses for why
Biologists have proposed two mechanisms. They are not mutually exclusive — both may contribute — but you should be able to state each clearly.
The two hypotheses predict the same data, which is why both are still discussed. Good science often looks like this.
Hypothesis 1: functional requirements
Conserved and highly conserved sequences sit inside genes coding for proteins that an organism cannot survive without — the machinery of transcription, translation and respiration.
Follow the logic through. If one of those genes mutates, the essential protein is faulty. The process it drives cannot happen, so the cell dies. A dead cell passes nothing to the next generation, so that mutation never appears in later organisms. Every generation, the same filter operates, and the sequence appears frozen.
Put another way: natural selection maintains the conserved sequence by necessity. The mutations do occur — they just never make it through.
Hypothesis 2: slower mutation rates
The second suggestion is that these regions genuinely mutate less. DNA repair and proofreading mechanisms are very active in coding regions and in genes with high functionality, so errors there are spotted and corrected far more often.
On this view, the mutations are not surviving because they were never allowed to become permanent in the first place — they are corrected before they can show up in sequenced DNA. Error correction is much less active in non-coding DNA, which is exactly where higher mutation rates are found.
How mutation rate is measured
base pair changes per gene per generation • or per genome per generation
Do not present these as right and wrong. IB wants you to recognise that two hypotheses can fit the same evidence, and to say what each one claims. If you can add that they might both contribute, better still.
Worked examples
WORKED EXAMPLE
Spotting the conserved gene
Gene P is 96% identical between humans and yeast. Gene Q is 41% identical between the same two species. Identify which is conserved and suggest what this implies about the protein it codes for.
Step 1: Compare the similarityGene P (96%) has barely changed since humans and yeast last shared an ancestor.
Step 2: Apply the definition
Minimal change over long evolutionary time = a conserved sequence.
Step 3: Suggest the implication
Its protein is likely to be essential for survival, so any mutation is lethal and never passed on.
Gene P is conserved; its protein is probably essential“Suggest” invites a reasoned inference, not a certainty — keep the word “likely” in.
WORKED EXAMPLE
Explaining conservation of cytochrome c
The sequence coding for cytochrome c is highly conserved across a very wide range of species. Explain this using the functional requirements hypothesis.
Step 1: State the protein’s role
Cytochrome c is involved in cellular respiration, which every one of these organisms depends on.
Step 2: Consider a mutation
A change to the sequence would change the amino acid sequence, the folding and the shape, so the protein would not work.
Step 3: Follow the consequence
Respiration would fail and the cell would die, so the mutation is not passed to future generations.
Step 4: State the result
Natural selection removes every variant, so the original sequence is maintained.
Mutations in it are lethal, so only the unchanged sequence is inheritedNotice this is the same sequence-shape-function chain you used for mutations.
WORKED EXAMPLE
Calculating a mutation rate
Comparing the same gene in two species reveals 6 base pair differences that have accumulated over 3 million years of separate evolution. Calculate the mutation rate in base pair changes per million years.
Step 1: Set up the calculation
Rate = number of changes ÷ time
Step 2: Substitute6 ÷ 3 = 22 base pair changes per million yearsGive the units. A bare number will not score on a “calculate the rate” question.
💡 Exam tip
Define conserved sequence properly: a section of DNA or RNA showing minimal mutation over time, similar or identical across species.
Have two examples ready from different processes — one from protein synthesis (tRNA, ribosomes, DNA helicase) and one from respiration (cytochrome c, ferredoxin).
Be able to state both hypotheses and label them: functional requirements, and slower mutation rates.
In the functional requirements answer, always include the step “so it is not passed on”. That is where the mark usually sits.
Link conservation back to common ancestry — shared sequences are evidence that species are related.
For mutation rate calculations, quote the units: per gene per generation, or per genome per generation.
⚠ Common mix-up
Saying conserved sequences never mutate. Under the first hypothesis they do — the mutations are simply lethal and never inherited.
Thinking conservation is a choice. Nothing is protecting these sequences on purpose; selection and repair produce the effect.
Confusing conserved with non-coding. Non-coding regions usually show higher mutation rates, because proofreading is less active there.
Only giving one hypothesis. The syllabus names two; a question asking for hypotheses expects both.
Mixing up conserved sequences and CpG hotspots. One mutates unusually little, the other unusually often.
Forgetting the units in a mutation rate answer.
That completes Mutations & Gene Editing. Test yourself by joining it to the previous chapter: a base changes → a codon changes → an amino acid changes → a protein changes → an organism changes → a population changes. Every page in these two chapters is one step of that sentence.
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