Lesson 2 of 9 / Mutation, cell division and cancer
Change a chromosome, not just one base
How do number and structure changes differ?
I can explain numerical and structural chromosomal aberrations, including trisomy 21.
I can explain gene mutations and numerical/structural chromosomal aberrations.Syllabus
Syllabus 8876, Core 2(d). Explain gene mutations and numerical/structural chromosomal aberrations.
About 6 min
Make a guess. You are not marked.
Trisomy 21 means three copies of what?
Show the answer
Chromosome 21 only.
It is aneuploidy: one extra copy of one chromosome, not of the whole set.
The key idea
Chromosomal aberrations alter chromosome number or large-scale structure. Aneuploidy changes the number of particular chromosomes; structural changes rearrange or change chromosome segments.
Explore the idea
Compare chromosome amount and order
Use this chromosome-segment order as the reference.
Explanation
A numerical aberration changes chromosome number. Aneuploidy means an abnormal number of particular chromosomes rather than a whole extra chromosome set. In the common trisomy-21 form of Down syndrome, cells carry three copies of chromosome 21 rather than the usual two.
Failure of chromosomes to separate correctly can produce gametes with an extra or missing chromosome. Fertilisation involving such a gamete can give an abnormal chromosome number in the resulting zygote. This differs from substituting a base within one gene.
Structural aberrations alter chromosome segments. Deletion removes a segment; duplication adds a repeat of a segment; inversion reverses a segment's orientation; translocation transfers a segment to another chromosomal location, commonly a non-homologous chromosome.
Consequences depend on which genes, dosage and regulatory relationships are altered. A balanced rearrangement can retain the total amount of DNA while still causing problems in some contexts. Down syndrome can also arise from extra chromosome-21 material in a translocation; do not claim every case has the same chromosome arrangement.
Step by step
- 1
Check number first
Count copies of the relevant chromosome, not the total number of chromatids.
- 2
Check segment order and amount
Loss, repetition, reversal and transfer identify distinct structural changes.
- 3
Connect to consequences
Altered gene dosage or disrupted sequences can change development or function.
Worked example
Read a segment model
A chromosome segment order changes from A-B-C-D-E to A-D-C-B-E. What structural aberration is shown?
One way to explain it
An inversion of the B-C-D segment. The segment remains present but its order is reversed.
Why this answer works
- Compare amount and order separately.
- Reversal without a lost or repeated segment supports inversion.
Watch out for this
A student says: "Trisomy 21 means that every chromosome occurs in three copies." What is wrong with this?
Show the answer
It refers to an extra copy of chromosome 21. Aneuploidy affects particular chromosomes, unlike an extra complete set.
Check your understanding
A-B-C-D becomes A-B-C-C-D. Which change is represented?
Show the answer
Duplication.
Segment C appears twice.
Quick recall
How do number and structure changes differ?