Structural chromosomal abnormalities Flashcards
What is translocation?
= exchange of two segments between non-homologous chromosomes
- Due to inappropriate non-homologous end joining
- Sticks a chunk of different chromosome onto the free-end of chromosome
There is no net gain or loss of genetic material
Involve any chromosome and any size fragment
Double strand breaks of DNA separating the chromosome will be fixed by non-homologous end joining.
What is the philadelphia chromosome?
What is the difference in the chromosomes in normal meiosis and in a result of translocation?
What is the result of unbalanced reciprocal translocation?
- Miscarriage
- Learning difficulties, physical disabilities
- Tend to be specific to each individual so risks and features vary
What is robertsonian translocation?
- Involves the acrocentric chromosome (little satellite arms that contain identical sets of genes)
- When you get a double strand break, the little satellite arms get chopped off both chromosome - and rather than being stuck back on, they’re lost and the 2 acrocentric chromosomes get stuck together
- = robertsonian chromosome
What robertsonian trnaslocation is most common?
- Robertsonian translocations 13;14 and 14;21 are common
- 21;21 translocation leads to 100% risk of down syndrome in foetus
So if 46 chromosomes present including Robertsonian, then must be unbalanced –> suffer clinical problems
P arms encode rRNA (multiple copies so not deleterious to lose some)
What is the difference between metacentric, submetacentric and acrocentric?
How is 2 bivalent chromosomes, quadrivalent chromosomes and trivalent chromosomes made?
What are the outcomes of translocations?
- Very difficult to predict
- Some unbalanced outcomes may lead to spontaneous abortion of conceptus so early that not seen as problem
- Or unbalanced outcomes leading to miscarriage
- May result in live-born baby with various problems
What are deletion events?
- Terminal deletion - loss of telomere chunk at end
- Interstitial deletion - loss of chunk of chromosome in the middle
- Inversion
- Duplication
- Ring chromosome
Deletions lead to a region on monosomy
What are microdeletions?
= no abnormality visible on metaphase spread
- Only a few genes may be lost or gained
- High resolution banding show microdeletions
When does deletion duplication events occur?
due to unequal crossing over (non-allelic homologues combination)
- Not aligned correctly
What type of samples can you check for chromosomal abnoramlities?
Pre natal
- Cell-free foetal DNA
- Chorionic villus sampling
- Amniocentesis
Postnatal
- Blood
- Saliva
How can chromosome staining be used to detect chromosomal abnormalities?
Most common = G-banding
- 2 sorts of chromatin stain differently:
- Euchromatin - GC-rich, loosely packed and genes active
- Heterochromatin - AT-rich, tightly packed, genes inactive
Can be seen under microscope
Takes a few days to get the cells divided at the metaphase stage
How can FISH be used to detect chromosomal abnormalities?
Fluorescent in situ hybridisation
Hybridisation = single stranded nucleic acid strand binds to a new single stranded nuclei acid strand - DNA binding to DNA or DNA binding to RNA
Requires cultured cells in metaphase spread
- Using fluorescent probe
- probe = ssDNA
- Denature probe and target DNA
- Mix probe and target DNA
- Probe binds to target
Not to be used to detect microdeletions
How can array CGH be used to detect chromosomal abnormalities?
- Array comparative genomic hydrazination
- For detection of sub-microscopic chromosomal abnormalities
- Uses extracted DNA
- Looks for microdeletions and microduplications
How can QF-PCR be used to detect chromosomal abnormalities?
Quantitative fluorescence polymerase chain reaction
To detect Trisomy’s 13, 18 and 21
Uses microsatellites (= repetitive sequence in non-genic regions varying between individuals -di, tri, tetra, penta, hexa) AKA short tandem repeats
- A healthy individual will be disomic for that chromosome - 2 peaks
- 2 peaks of different size of 3 peaks = trisomic
Method
- Isolate DNA from individual
- Design primers specific to flanking sequences
- PCR amplification (of DNA fragment of known sequence)
- Gel electrophoresis to genotype
- Homozygotes = single product of specific size
- Heterozygotes = two different sized products
How can you test for downs syndrome performing PCR and primers?
Perform PCR using primers for microsatellite known to be on chromosome 21 to test for downs
- Should be 2 copies of microsatellite (one from mother, one from father)
- If homozygous, there will be a single peak of high signal
- If heterozygous, there will be 2 peaks of similar lower signal
What is non-invasive pre-natal testing?
Cell free foetal DNA from maternal blood sample
Trisomy testing
