structural chromosomal abnormalities Flashcards
define translocation
exchange of segment between non-homologous chromosomes
occurs by inappropriate non homologous end joining
what is non homologous end joining
DNA repair mechanism
rejoins broken chromosomes
BUT
can go wrong and stick to incorrect chromosome = exchange of material
what can carriers of balanced chromosomes sometimes develop
philadelphia chromsomes
how are unbalanced individuals produced
tetravalent forms rather than bivalent.
can lead to miscarriage/learning difficulties/physical disabilites
specific to each individual therefore risks vary
describe and explain a robertsonian translocation
involves only acroscentric chromosomes long arms on both attach together = full long arms balanced carries has 45 chromosomes if 46 = unbalanced p arms encode rRNA
common robertsonian translocations = 13;14, 14;21 = common
21;21 = 100% Down syndrome risk
what are the outcomes of translocations
difficult to predict
approximate probabilities of producing possible gametes
some unbalanced outcomes may lead to spontaneous abortion of concepts = not seen as problem
unbalanced outcomes may lead to miscarriage
describe structural changes - deletions
terminal deletion = loss of telomeric chunk
interstitial deletion = loss of chromosomes in middle
- cause region of monosomy
- haploid insufficiency of some genes
- contiguous gene syndrome
- phenotype specific for size and place on deletion
describe micro deletion
only few genes lost or gained
velocardial facial syndrome/wolf-hisichron
most deletion/duplication occur due to unequal crossing over = exchange of genetic material between homologous chromosomes BUT haven’t aligned properly
what samples do we use to detect chromosomal abnormalities
pre natal = amniocentesis, chorionic villus sampling and cell-free fetal DNA
post natal = blood/saliva
chromosome staining to detect chromosomal abnormalities
use G banding
G = giasma
bands occur due to chromatin
euchromatin = GC rich, loosely packed, genes active
heterochromatin = AT rich, tightly packed, genes inactive
stain differently
can be seen under microscope
takes few days to do so. blood sample needed and to be cultured
looks for aneuploidies/translocations/large deletions
use of FISH for detecting chromosomal abnormalities
single stranded nucleic acid binds to new single stranded nucleic acid strand
- fluorescent probe
- denature probe and target DNA
- mix probe and target DNA
- probe binds to target
single stranded DNA = labelled with fluorescent molecule
20-100 bases in length
FISH takes several days
uses metaphase chromosomes
looks for aneuploidies, translocations and large deletions
use of array comparative genomic hybridisation
for detection of submicroscopic abnormalities
patient DNA labelled green
control DNA labelled red
determines how many copies particular genomic region patient has
uses fluorescent probe to differentiate between patient and control
uses extracted DNA
looks for micro deletions and microduplication
use of QF-PCR (quantitative fluorescence)
trisomies 13, 18, 21
use micro satellites = short repeated sequences, number of sequences vary
PCR:
- primers anneal at 50-65 degrees
- DNA polymerase extends strand from primer at 72 degrees
- each cycle = x2 results
QF-PCR
perform PCR using primers for micro satellites on chromosome 21
should be 2 copies of micro satellites
homozygous = single peak of high signal
heterozygous = 2 peaks of similar, lower signal
uses fluorescent probes for specific micro satellites on specific chromosomes
use extracted DNA
quick process - 48 hours and looks for aneuploidies
describe non-invasive pre natal testing and NGS
cell free fetal DNA maternal blood sample trisomy testing next generation sequencing high chance indicator for invasive tests
