cytogenetics Flashcards
what are cytogenetic techniques?
the process of staining and examining chromosomes under a light microscope
at what phase are chromosomes best examined?
metaphase
uses of cytogenetic techniques
prenatal screening
newborn screening for highly penetrant disorders
diagnostic and carrier testing for inherited disorders
what is karyotyping?
pairing and arranging chromosomes from largest to smallest
telocentric chromosomes
centromere is present at the end of the chromosomes
acrocentric chromosomes
centromere is almost terminal. one large and one small arm
sub-metacentric chromosomes
centromere is not at the middle position so the arms are unequal and L shaped
metacentric chromosomes
centromere is at the middle position, arms are equal and V shaped
advantages of chromosome cytogenetics
can detect : whole-chromosome aneuploidy large deletions or duplications balanced translocations or inversions triploidy
disadvantages of chromosome cytogenetics
not sufficient enough to detect and identify all chromosomal aberrations present at metaphase
unable to detect smaller chromosomal aberrations
needs experienced cytogeneticist
time consuming
what does FISH stand for
fluorescent in situ hybridisation
what is FISH
a method used for visualising specific locations on a chromosome
the aim is to detect chromosomal abnormalities
FISH procedure
fixation of the cell
design of probes - must be complementary to region of interest either ds RNA or ds DNA
DNase used to form nicks in the DNA
special nucleotides are labelled with fluorescent proteins and are used to seal the nicks
probe binds to chromosomal DNA sequence
results analysed on fluorescent microscope
if probe doesnt bind it suggests mutation
FISH uses
in karyotyping: detects chromosome copy number fluorescent tag can be designed to be complementary to a whole chromosome in disease: can detect deletion
explain chorionic villus sampling
obtain cells from placenta
chromosomes in placenta are the same as the fetal chromosomes
if a mosaic result is found, an amniocentesis is recommended
transabdominal approach in CVS
thin needle through the abdomen
transcervical approach in CVS
slim plastic catheter through cervix
when is an amniocentesis done
15-23 weeks of pregnancy
amniocentesis
obtain sample of amniotic fluid which contains cells from the fetal skin
insert needle through abdomen
can then create karyotype
and can also measure level of alpha-fetoprotein which is used to detect open neural tube defects
what is non-invasive prenatal testing
blood test at 10 weeks of pregnancy
blood contains cell free DNA and DNA from the placenta
screens for Down Syndrome, trisomy 18 and 13 and sex chromosome conditions
not a diagnostic test only tells the risk of the fetus developing the condition
What is used to arrest cells in metaphase?
Mitosis spindle inhibitor
Eg colchicine
Why is hypotonic saline added during metaphase cytogenic prep?
Makes the cell swell and causes the chromosomes to move apart
What is phytohemagglutinin and why is it added during metaphase cytogenic prep?
A mitogen
It induces the cells to divide
What are the steps in metaphase cytogenetic prep?
Add phytohemagglutinin and culture medium to sample
Culture at 37 degrees Celsius for 3 days
Add colchicine and hypotonic saline
Fix cells
Spread cells onto slide by dropping at a 45 degree angle
Digest with trypsin and stain with giemsa
What is aneuploidy?
When one or more individual chromosome are present as an extra copy or are missing q
What does trisomy mean?
3 copies of a particular chromosome
What is trisomy 21?
Down syndrome
Extra chromosome 21
What is monosomy?
When a chromosome is lacking from an otherwise diploid state
What is nondisjunction?
When paired chromosomes fail to separate during anaphase of meiosis 1 or sister chromatids fail to separate at either meiosis 2 or mitosis
Causes daughter cells to contain incorrect amount of chromosomes
When do most cases of nondisjunction arise?
During maternal meiosis
What is anaphase lag?
When a chromosome or chromatid is delayed in its movement during anaphase and fails to be incorporated into a daughter nucleus
