SCIENCE · INVESTIGATE AND EXPLAIN

Separate a sequence change from a trait claim

Compare synonymous substitutions, amino-acid substitutions and gained stop signals.

Learning outcome: Classify the modeled coding consequence without inferring an organism-level effect from a codon alone.

Starting recap

Several codons can specify the same amino acid. A stop codon is a termination signal, not an amino acid.

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Try

Changing the third base of TAC to A creates TAA, which corresponds to the stop codon UAA in the standard code.

Open the experiment

Predict

Do all single-base substitutions change an amino acid? Do all amino-acid substitutions produce a visible trait?

Your prediction and reason

Experiment

Compare GAA→GAG, GAA→GAC and TAC→TAA. Also choose the original third base to obtain no sequence change. Capture examples and classify the evidence.

Trace a coding-triplet substitution

Worked teaching model · synthetic values · One internal coding-strand DNA triplet is read 5′ to 3′ in a fixed reading frame using the standard genetic code. Only the third base can change. Initiation, RNA processing, expression level, protein folding and organism phenotype are not simulated.

Original coding triplet: GAA Third base after edit: G

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Trace the coding interpretation

Both DNA and RNA are shown 5′ to 3′ in the same fixed coding frame
Case	Coding DNA	mRNA	Standard-code meaning
Original	GAA	GAA	Glutamate
Edited	GAG	GAG	Glutamate

Synonymous substitution. DNA edit → RNA codon under the stated orientation → translation assignment. Protein activity and organism traits remain untested.

Calculated values on the same fixed scale
Row	Quantity	Value (indicator (0 no, 1 yes))
1	DNA sequence changed	1
2	Translation outcome changed	0
3	Stop signal gained	0

GAA → GAG: Synonymous substitution.

Coding DNA GAG corresponds to mRNA GAG in the stated orientation. Standard-code result: Glutamate. A coding change does not by itself establish a harmful, beneficial or visible trait.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Similar edits can have different coding outcomes

A substitution replaces one base with another without changing sequence length. GAA→GAG is synonymous in the standard code: both specify glutamate. GAA→GAC changes the assignment to aspartate. TAC→TAA changes tyrosine to a stop signal. A stop signal is not an amino acid; if encountered during translation in this frame, it ends the chain.

The NCBI genetic-code reference provides the assignments used here. The model does not include insertions or deletions, which can shift the downstream reading frame when their length is not a multiple of three.

Classify before interpreting

Choose TAC with third base C. The sequence is unchanged, so this is not a synonymous substitution; there was no substitution at all. Next choose T. TAT differs from TAC while both assign tyrosine, so this is synonymous. Then choose A and inspect the gained stop signal.

The classification is a statement about sequence and code. It is not a diagnosis of harm or benefit. A change near the end of a protein differs from an early stop; an amino-acid substitution in a critical binding site differs from one at a tolerant position. RNA processing and expression can also matter, including for some synonymous changes.

Plan evidence beyond the lookup

If a sequence change is proposed to explain altered enzyme activity, identify the relevant gene and transcript, verify the sequence, compare expression and protein abundance, and measure activity under matched conditions. A comparison with an otherwise similar reference helps, but a whole-organism trait can involve other genes and environment.

Use the captures to support only the modeled consequence. Write a separate hypothesis about possible function and an observation that could contradict it. Avoid labeling every substitution harmful or assuming that a familiar amino acid guarantees normal function.

Distinguish a mismatch from a completed sequence change

An incorrect new base can form a mismatch with the template. Proofreading or subsequent mismatch repair may correct it. If a mismatch persists through another copying round, a changed base pair can become established in a descendant DNA molecule. Repair mechanisms also respond to DNA damage; copying errors are not the only source of sequence changes. Background: OpenStax, DNA repair.

Try the replication and proofreading example. Its forced C opposite template A is a mismatch, not already a normally paired C–G substitution. The codon lookup on this page instead starts from specified DNA sequences and asks what their coding assignments would be.

Reason across two copying rounds: a DNA segment has an unrepaired A–C mismatch. Assume the strands separate and each is copied accurately, with no repair before copying. What base pair is produced from each template base?

Check the descendant pairs

Template A directs T, producing A–T. Template C directs G, producing C–G. One lineage can therefore retain the original pairing and the other a changed pairing. This is an idealized sequence argument, not a prediction of which cells survive or what trait they express. The codon context, gene function and other evidence are still needed to infer a consequence.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Give two reasons that the same type of coding change can have different effects in different genes or positions. Explain why synonymous does not guarantee no biological effect.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Connect a coding sequence to a translation outcome Next: Count chromosome sets separately from DNA copies

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