Compare original DNA fragments without inferring protein effects from incomplete context
Align the stated locus, locate a change and keep length separate from chromosome count.
Section B17.1 · Variation
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall genes, alleles, genotype and phenotype from the inheritance pathway. Use the stated organism, units and sample; the records in these lessons are invented.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Can two equal-length DNA fragments have different sequences?
What you will learn
- Identify altered bases in an aligned DNA record
- Separate sequence change from its unknown functional outcome
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Reference | AGCTTAGC |
| Same-length comparison | AGCTTAAC: position 7 G → A |
| Stated insertion | AGCTATAGC: nine bases |
| Missing context | Protein function, chromosome number and transmission |
Identify altered bases in an aligned DNA record
Comparing DNA sequences needs a stated locus and an alignment rule. The original reference AGCTTAGC and comparison AGCTTAAC each contain eight bases. Read positions from one at the left. The records differ at position seven, where G in the reference is A in the comparison. Equal sequence length does not mean equal sequence information, and one base difference does not imply one whole chromosome was added or removed.
The second illustrative record inserts A after the fourth base, changing AGCTTAGC to AGCTATAGC. This supplied alignment has nine bases after the insertion. The later reference bases shift one displayed position to the right, so blindly comparing every subsequent index can count multiple apparent mismatches for a single stated insertion event. The requirement is the general sequence-change definition; recognising these illustrative event types does not require a full molecular-effect catalogue.
Separate sequence change from its unknown functional outcome
Base addition, removal or replacement can illustrate a sequence change, but a fragment alone does not show a complete gene’s coding arrangement or the resulting amino-acid sequence. A change in fragment length is not necessarily a change in chromosome number. The original before/after examples are deliberately bounded and fictional, not laboratory observations from learners or diagnostic variants.
For any conclusion, name what the supplied evidence supports: sequence identity, changed positions under a defined alignment, or a stipulated event. Keep unknown effects unknown. Some genetic variants have no apparent effect on the measured feature; others may affect function. Do not infer harm, benefit, inheritance or a whole-population frequency from these fragment comparisons. Functional evidence and reproductive context would be needed for those different questions.
Sources for this lesson
Pause and explain
An explicitly inserted base shifts later positions. What should the comparison preserve?
Worked example
Compare AGCTTAGC with AGCTTAAC at the same stated locus.
Show the worked solution
- Align the two eight-base sequences and count positions from one.
- Only position seven differs, G → A; the fragment length stays eight.
Answer One supplied substitution differs at position 7, G → A. The effect on a protein or phenotype is not established.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare AGCTTAGC and AGCTTAAC and identify the changed position and base.
Give me a hint
Count aligned positions from the left, starting at one.
Compare my reasoning
- Positions one to six agree.
- Position seven changes G to A; position eight agrees.
Position 7 changes G → A; both fragments contain eight bases.
Look for these in your work
- Correct position and bases
- Length retained
A stipulated A insertion after base four changes AGCTTAGC to AGCTATAGC. Find the new length and the number of supplied insertion events.
Give me a hint
Follow the stated event rather than counting shifted indices as new mutations.
Compare my reasoning
- The reference has eight bases and one base is added.
- The stated record supplies one insertion event.
Nine bases after one supplied insertion event. Later index shifts do not establish multiple additional mutation events.
Look for these in your work
- New length nine
- One event and alignment reasoning
A report uses this short fragment change to assert the protein is harmful, a chromosome was lost and every offspring inherits it. Evaluate the assertions.
Give me a hint
The record supplies neither whole-gene function nor chromosome/reproductive context.
Compare my reasoning
- A fragment change does not establish a protein outcome or chromosome-number loss.
- Inheritance depends on reproductive lineage and transmission evidence.
All three assertions outrun the fragment record. Functional, chromosome-complement and reproductive evidence would be needed respectively.
Look for these in your work
- Three unsupported inferences identified
- Relevant evidence requirements
Common mistakes
- Calling equal length equal sequence
- Counting an inserted base as a new chromosome
- Predicting protein effects without gene context
What can you explain now?
An explicitly inserted base shifts later positions. What should the comparison preserve?
Compare with the explanation
The supplied event and alignment
A single insertion should not be mistaken for many independently stated events.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, identify altered bases in an aligned dna record and separate sequence change from its unknown functional outcome. Keep units, the stated sample and the limits of the evidence clear.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.