Chromosome Abnormalities

Question 1

In what phase of the cell cycle are chromosomes examined?

Answer 1

Metaphase

Question 2

What is Karyotyping?

Answer 2

This is systematic sorting of chromosomes.

Question 3

What is G banding staining?

Answer 3

This is where trypsin digests proteins and then a dye is added. This leads to dark AT rich bands and light GC bands.

Question 4

What are the advantages of cytogenetic analysis?

Answer 4

This leads to accurate diagnosis and prognosis. This is useful if a prenatal diagnosis is to be made because there can be plans and better clinical management.

Question 5

What is the most common way in which abnormalities are found prenatally?

Answer 5

Ultrasound scans. When abnormalities are found this is when more invasive tests are offered.

Question 6

What is Aneuploidy?

Answer 6

This is loss or gain of an entire chromosome. This is either a trisomy or a monosomy.

Question 7

State a common trisomy.

Answer 7

Down’s syndrome. This is trisomy 21.

Question 8

What is it called when an individual has an entire additional set of chromosomes?

Answer 8

Polyploidy.

Question 9

What is the cause of Aneuploidy?

Answer 9

Non dysjunction during one of the mitotic cell divisions.

Question 10

What is anaphase lag?

Answer 10

This is where chromosome loss occurs because of a defect with the spindle or chromosome attachment.

Question 11

What is X chromosome inactivation?

Answer 11

This is the name for the fact that in human cells only have one X chromosome active which ensures that all of the cells have the same X complement active.

Question 12

What is mosaicism?

Answer 12

This is where there are two different cell lines within one body due to mitotic nondysjunction.

Question 13

What is the name for the condition in which there are homologous chromosomes present from one parent?

Answer 13

Uni parent disomy (UPD)

Question 14

State the difference between Isodisomy and Heterodisomy

Answer 14

Isodisomy is where the two chromosomes are identical and Heterodisomy is where they are 2 homologous chromosomes from one parent.

Question 15

What is the most common cause of UPD?

Answer 15

Trisomy rescue is the most common cause but it must then be followed by mitotic error.

Question 16

What is a reciprocal translocation?

Answer 16

This is where there is movement from one chromosome to another, following two break arrangements. This can lead to either a balanced or unbalanced gamete. The region of monosomy/trisomy is what affects the phenotype.

Question 17

What is a robertsonian translocation?

Answer 17

This is where two apocentric chromosomes fuses loosing the p arms so that only the q arms are present.

Question 18

What risks do robertsonian translocations lead to?

Answer 18

If der 21,21 then can only have a child with downs. If others then this causes a risk of Aneuploidy at meiosis.

Question 19

When can deletions occur in chromosomes?

Answer 19

This can occur during pairing and recombination during meiosis. Micro deletions can only be seen with FISH.

Question 20

What is necessary in order for us to carry out FISH?

Answer 20

We need to know what we are looking for so that we can use an appropriate probe.

Question 21

What can microarray methodology (aCGH) be used to identify?

Answer 21

This is a process in which the whole genome is examined using DNA. This is not useful for balanced translocations but can identify loss or gain of material.

Question 22

What is a problem with aCGH?

Answer 22

It can lead to uncertain results and this causes dilemma. It also does not identify balanced rearrangements and can miss mosaicism.

Question 23

What is the future for DNA testing of foetuses?

Answer 23

The future is non invasive procedures that use fetal DNA in maternal blood from 9 weeks gestation. This is challenging as this DNA deteriorates rapidly.

Question 24

What are the problems with NGS? (Next generation sequencing)

Answer 24

This gives the full DNA sequence, however it produces a lot of data. Rate of analysis is the current bottleneck. It gives a high diagnostic yield and it reduces the cost of testing individual genes.

Question 25

What is an advantage of whole exome sequencing?

Answer 25

This is just targeted to exomes and gives a reduced amount of data compared to whole genome which is advantageous for the analysis stage.

Question 26

What epidemic molecule leads to activation of genes (so they are expressed)?

Answer 26

Acetyl. Methyl leads to inactivation.