Chromosome Abnormalities Flashcards
In what phase of the cell cycle are chromosomes examined?
Metaphase
What is Karyotyping?
This is systematic sorting of chromosomes.
What is G banding staining?
This is where trypsin digests proteins and then a dye is added. This leads to dark AT rich bands and light GC bands.
What are the advantages of cytogenetic analysis?
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.
What is the most common way in which abnormalities are found prenatally?
Ultrasound scans. When abnormalities are found this is when more invasive tests are offered.
What is Aneuploidy?
This is loss or gain of an entire chromosome. This is either a trisomy or a monosomy.
State a common trisomy.
Down’s syndrome. This is trisomy 21.
What is it called when an individual has an entire additional set of chromosomes?
Polyploidy.
What is the cause of Aneuploidy?
Non dysjunction during one of the mitotic cell divisions.
What is anaphase lag?
This is where chromosome loss occurs because of a defect with the spindle or chromosome attachment.
What is X chromosome inactivation?
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.
What is mosaicism?
This is where there are two different cell lines within one body due to mitotic nondysjunction.
What is the name for the condition in which there are homologous chromosomes present from one parent?
Uni parent disomy (UPD)
State the difference between Isodisomy and Heterodisomy
Isodisomy is where the two chromosomes are identical and Heterodisomy is where they are 2 homologous chromosomes from one parent.
What is the most common cause of UPD?
Trisomy rescue is the most common cause but it must then be followed by mitotic error.
What is a reciprocal translocation?
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.
What is a robertsonian translocation?
This is where two apocentric chromosomes fuses loosing the p arms so that only the q arms are present.
What risks do robertsonian translocations lead to?
If der 21,21 then can only have a child with downs. If others then this causes a risk of Aneuploidy at meiosis.
When can deletions occur in chromosomes?
This can occur during pairing and recombination during meiosis. Micro deletions can only be seen with FISH.
What is necessary in order for us to carry out FISH?
We need to know what we are looking for so that we can use an appropriate probe.
What can microarray methodology (aCGH) be used to identify?
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.
What is a problem with aCGH?
It can lead to uncertain results and this causes dilemma. It also does not identify balanced rearrangements and can miss mosaicism.
What is the future for DNA testing of foetuses?
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.
What are the problems with NGS? (Next generation sequencing)
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.
What is an advantage of whole exome sequencing?
This is just targeted to exomes and gives a reduced amount of data compared to whole genome which is advantageous for the analysis stage.
What epidemic molecule leads to activation of genes (so they are expressed)?
Acetyl. Methyl leads to inactivation.
