Chromosomal Abnormalities Flashcards
Which is the stabilising histone?
H1
What is the term for chromatin switching back and forth between active and inactive?
Epigenetic modification
Which 2 processes help with chromatin activation?
DNA demethylation and histone acetylation
Which 2 processes help with chromatin deactivation?
DNA methylation and histone deacetylation
What does acrocentric mean?
The p arm is small in comparison to the q arm.
As the number of the chromosome increases, the size of the chromosome…
Except for…
decreases, except chromosome 22 is bigger than chromosome 21
What groups can chromosomes be split into and which are acrocentric?
groups A-G, groups D and G are acrocentric
What does a dark band on a chromosome mean?
AT rich, gene poor
What does a light band on a chromosome mean?
GC rich, gene rich
What are the different staining methods for chromosomes?
solid staining, G banding, C banding (centromeres) and Q banding (fluorescent)
What are constitutional abnormalities?
abnormalities that have been present since birth
Define aneuploidy
The loss or gain of whole chromosomes.
Define trisomies
Having 3 copies of a particular chromosome.
Which 3 trisomies are the only ones compatible with life and which syndromes do they relate to?
21: Down syndrome
13: Patau syndrome
18: Edwards syndrome
Define monosomies
Having only 1 copy of a particular chromosome.
What is the only monosomy compatible with life?
monosomy X Turner syndrome (45,X) - only affects females
Define polyploidy
Sets of chromosomes greater than the normal diploid number.
Gain of a whole haploid set(s) of chromosomes
Write the karyotype for triploidy
69,XXX
Define mosaicism
Cells within the same person have a different genetic makeup.
What is anaphase lag?
Chromosomes get left behind at cell division due to defects in the spindle function or attachment.
What is hypotonia and what is it a sign of?
floppy baby: low muscle tone
Down syndrome
What do patients with Down Syndrome have an increased prevalence of?
leukaemia and early Alzheimers
What features define Edwards syndrome?
small lower jaw, low-set ears, rocker-bottom feet and overlapping fingers
Where does with error usually occur that causes Edwards syndrome and what is the average lifespan?
maternal meiosis II error
lifespan: 5-15 days
What are the features of Patau syndrome?
polydactyly, failure of forebrain to develop, facial defects and the majority die as neonates
What are the features of Turner syndrome?
puffy feet, short stature, heart defects, infertility, redundant skin on back of neck and absent paternal X chromosome in most cases
How do X chromosomes vary from autosomes?
only one X chromosomes is ever active in a human cell
What are PAR1 and PAR2?
Short, pseudo-autosomal regions that X and Y chromosomes have in common.
What are PAR1 and PAR2 needed for?
Pairing in meiosis
What is Uniparental Disomy (UPD)?
presence of homologous chromosomes from only one parent in a cell
Define isodisomy
2 identical chromosomes from one parent present
Define heterodisomy
2 homologous chromosomes from one parent present
What is it called when only part of the chromosome is involved in UPD?
segmental UPD
How do you get acquired UPD?
when mitosis goes wrong in yourself (not the parents)
How many histones make up the unit that DNA wraps around?
8 (octamer)
What is genomic imprinting?
Epigenetic phenomenon by which certain genes are expressed dependent upon their parent of origin.
If an allele is imprinted then it is silenced.
What are the 4 ways in which UPD can arise?
trisomy rescue, monosomy rescue, gamete complementation and mitotic error
What are reciprocal translocations?
The exchange of genomic material between 2 non-homologous chromosomes.
What are the features of reciprocal translocation?
usually unique to a family, produce balanced and unbalanced gametes and if formed from meiotic disjunctions then quadrivalents form before segregation
What are Robertsonian translocations?
Translocation caused by breaks at or near the centromeres of two acrocentric chromosomes. The reciprocal exchange of parts gives rise to one large chromosomes and one extremely small chromosome that may be lost. Leaves only 45 chromosomes since 2 have fused together.
Do Robertsonian translocations affect the phenotype?
Not directly because the only genes lost are on the short arm which are common to many chromosomes and therefore will still be present within the cell.
How can chromosome deletions occur?
From uneven pairing and recombination in meiosis.
Can chromosome deletions be balanced?
No
They are always unbalanced because genetic material is lost.
What tests would we do for prenatal aneuploidy screening?
FISH probes for common aneuploidies (quick) and full karyotype (slow)
When and how can non-invasive prenatal testing occur?
using foetal DNA in maternal blood at 9 weeks gestation
