cytogenetics

Question 1

what are cytogenetic techniques?

Answer 1

the process of staining and examining chromosomes under a light microscope

Question 2

at what phase are chromosomes best examined?

Answer 2

metaphase

Question 3

uses of cytogenetic techniques

Answer 3

prenatal screening
newborn screening for highly penetrant disorders
diagnostic and carrier testing for inherited disorders

Question 4

what is karyotyping?

Answer 4

pairing and arranging chromosomes from largest to smallest

Question 5

telocentric chromosomes

Answer 5

centromere is present at the end of the chromosomes

Question 6

acrocentric chromosomes

Answer 6

centromere is almost terminal. one large and one small arm

Question 7

sub-metacentric chromosomes

Answer 7

centromere is not at the middle position so the arms are unequal and L shaped

Question 8

metacentric chromosomes

Answer 8

centromere is at the middle position, arms are equal and V shaped

Question 9

advantages of chromosome cytogenetics

Answer 9

can detect :
whole-chromosome aneuploidy 
large deletions or duplications 
balanced translocations or inversions
triploidy

Question 10

disadvantages of chromosome cytogenetics

Answer 10

not sufficient enough to detect and identify all chromosomal aberrations present at metaphase
unable to detect smaller chromosomal aberrations
needs experienced cytogeneticist
time consuming

Question 11

what does FISH stand for

Answer 11

fluorescent in situ hybridisation

Question 12

what is FISH

Answer 12

a method used for visualising specific locations on a chromosome
the aim is to detect chromosomal abnormalities

Question 13

FISH procedure

Answer 13

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

Question 14

FISH uses

Answer 14

in karyotyping:
detects chromosome copy number 
fluorescent tag can be designed to be complementary to a whole chromosome 
in disease:
can detect deletion

Question 15

explain chorionic villus sampling

Answer 15

obtain cells from placenta
chromosomes in placenta are the same as the fetal chromosomes
if a mosaic result is found, an amniocentesis is recommended

Question 16

transabdominal approach in CVS

Answer 16

thin needle through the abdomen

Question 17

transcervical approach in CVS

Answer 17

slim plastic catheter through cervix

Question 18

when is an amniocentesis done

Answer 18

15-23 weeks of pregnancy

Question 19

amniocentesis

Answer 19

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

Question 20

what is non-invasive prenatal testing

Answer 20

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

Question 21

What is used to arrest cells in metaphase?

Answer 21

Mitosis spindle inhibitor

Eg colchicine

Question 22

Why is hypotonic saline added during metaphase cytogenic prep?

Answer 22

Makes the cell swell and causes the chromosomes to move apart

Question 23

What is phytohemagglutinin and why is it added during metaphase cytogenic prep?

Answer 23

A mitogen

It induces the cells to divide

Question 24

What are the steps in metaphase cytogenetic prep?

Answer 24

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

Question 25

What is aneuploidy?

Answer 25

When one or more individual chromosome are present as an extra copy or are missing q

Question 26

What does trisomy mean?

Answer 26

3 copies of a particular chromosome

Question 27

What is trisomy 21?

Answer 27

Down syndrome

Extra chromosome 21

Question 28

What is monosomy?

Answer 28

When a chromosome is lacking from an otherwise diploid state

Question 29

What is nondisjunction?

Answer 29

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

Question 30

When do most cases of nondisjunction arise?

Answer 30

During maternal meiosis

Question 31

What is anaphase lag?

Answer 31

When a chromosome or chromatid is delayed in its movement during anaphase and fails to be incorporated into a daughter nucleus