Cytogenetic analysis

Question 1

what groups are metacentric chromosomes?

Answer 1

centromere in the middle of the chromosome, group H large metacentrics, group F small

Question 2

what are acrocentric chromosomes?

Answer 2

centromere at end of chromosome. group D and G

Question 3

what are submetacentric chromosomes?

Answer 3

centromere neither at end or middle

Question 4

in g-banding, where cell cycle are the cells arrested?

Answer 4

in metaphase of mitosis

Question 5

what does killing cells with fixative do?

Answer 5

blocks chromosome condensation and renders sample inert

Question 6

what bands does the trypsin digest create?

Answer 6

pale bands

Question 7

what do you stain it with after trypsin digest?

Answer 7

Leishmans dye

Question 8

what sequences are dark and light bands rich in?

Answer 8

dark= AT rich. light= GC rich

Question 9

what is open chromatin associated with?

Answer 9

darker bands

Question 10

why may the banding structure differ between 2 homologues?

Answer 10

due to stage of cell cycle that cell was in when fixative was added, tissue type

Question 11

what can make your chromosomes longer?

Answer 11

more mature cells

Question 12

what does a longer trypsin incubation cause?

Answer 12

a paler band as chromosome more collapsed so Leichmans cant get into collapsed chromosome

Question 13

what else can influence banding resolution?

Answer 13

slide aging, staining time and chromosome spread

Question 14

difference between direct and indirect FISH

Answer 14

indirect is when the flourscent dies are added after sample and probes hybridised. more sesitive but slower. direct is much quicker but less sensitive so longer probes required

Question 15

what is DAPI?

Answer 15

intercalating agent and used as counter stain

Question 16

how is FISH carried out?

Answer 16

heat metaphase spread sample to 75-78 degrees to make DNA ss but not destroy the chromatin structures. then wash to remove unbound probes

Question 17

what is chromosome enumeration dues for detecting?

Answer 17

common aneuploidies

Question 18

how is chromosome enumeration carried out?

Answer 18

probes made that are specific to certain alpha satellite sequences of common aneuplodies.

Question 19

what does a bright signal allow?

Answer 19

allowing rapid hybridisation times and less ambiguity

Question 20

what are microdeletion probes used for?

Answer 20

used to check an abnormality detected by G-banding eg Cri du chat. used for detection of small translocation, inversion, deletion, insertion, duplication in parent chromosome which causes unbalanced foetal karyotype

Question 21

what is whole chromosome painting used for?

Answer 21

if unsure of origin of fragment of DNA or if dont know where breakpoint is

Question 22

how does dTTP excess arrest cell cycle?

Answer 22

inhbits ribonuclease reductase enzyme at high concentration, reduces dCTP conc and becomes rate limiting-arrests DNA synthesis so cells remain in S phase

Question 23

how is the thymidine block released?

Answer 23

addition of dCTP to bypass the need for ribonucleotide reductase, or by washing and centrifuging and replacing media to dilute out the excess dTTP

Question 24

what happens once the cells are released from the block?

Answer 24

they proceed through mitosis in a synchronous manner

Question 25

what other chemical can be an S phase block?

Answer 25

FdU- blocks dTMP synthesis so dTTP conc becomes rate limiting

Question 26

what enzyme breaks down sister chromatid cohesion?

Answer 26

separase

Question 27

what does colcemid do to the alpha and beta hetrodimer tubules?

Answer 27

destabilises the polymerisation and stops the microtubules from forming

Question 28

what does colcemid block the cell cycle in?

Answer 28

metaphase since spindles not formed

Question 29

what is an accurate test?

Answer 29

the test detects the genetic condition you are looking for correctly

Question 30

what is a precise test?

Answer 30

the test produces the same result when repeated

Question 31

what is a specific test?

Answer 31

false positive rate is low- excludes normal patients

Question 32

what is a sensitive test?

Answer 32

false negative rate is low- correctly identifies all the people with the given disorder

Question 33

what do you need to see in 3 out of the 4 chromsomes to have a quality metaphase spread?

Answer 33

see the banding resolution of certain bands in both homologues

Question 34

what is QA3 used for?

Answer 34

oncology

Question 35

what isQA4 used for?

Answer 35

if a foetus is expected to have a large structural chromosome rearrangement or aneupoloidy

Question 36

what is QA5 used for?

Answer 36

a potential abnormality detected from the second pregnancy scan. looking to exclude aneuplodies and more subtle structural rearrangements

Question 37

what is QA6 used for?

Answer 37

looking for subtle structural arrangments and microdeletions when analysing parental blood samples

Question 38

what can chronic villus samples also contain?

Answer 38

maternal contamination