Chromosomal Abnormalities Flashcards
What is meant by the karyotype?
It is the chromosome set of an individual species in terms of number and structure of chromosomes in the cell nuclei
How do you prepare a karyotype?
- Collect roughly heparinized venous blood (can use amniotic cells, CVS)
- isolate white cells
- Culture in presence of phytohaemagglutinin (stimulates T-lymphocyte growth/ differentiation)
- After 48 hours add colchicine (causes mitotic arrest- metaphase)
- Place in hypotonic saline
- Place on slide
- Fix and stain
What are ideograms?
Graphical representations of chromosomes
Chromosomes have some common structural features
How many chromosomes do humans have?
22 pairs of normal chromosomes, 1 pair of sex chromosomes
46, XX
How are bands indentified?
Bands caused by differently stained
Bands originally identified with low level of resolution (only a few bands visible per chromosome e.g., 1,2,3)
Improved tech more bands visible
Named as sub-bands e.g., 11,12,21,22,23 etc
Further improvements leads to sub-sub-bands e.g., 11.1, 11.2 etc
Improved resolution helps identify smaller aberration
What does bphs stand for?
bands per haploid set (bands DO NOT represent genes or families of genes)
What stain do you use for karyotypes?
giemsa
Right now, for karyotypes what stage of mitosis is preferred?
prophase (over the usual metaphase)
What is giesma staining the basis of?
basis of nomenclature
What is another method of staining karyotypes?
Karyotype fluorescent stain
What is a nucleosome?
When the DNA is wrapped around the histone proteins
When nucleosomes condense, what is formed?
A chromatin fibre
What is formed when chromatin condenses?
Chromosome
How do very long DNA molecules fit into the cell?
The DNA is wrapped around histones, forming nucleosomes which are then coiled into the chromatin fibre which is then further condensed to fit into the chromosome
Which cells do not contain chromatin?
Red blood cells
During what phase of mitosis are chromsomes usually karyotypes and why?
During metaphase, as the chromosomes are more condensed
Which cells are commonly chosen to be karyotyped and why?
White blood cells as they easily enter into mitosis
What is the short arm of a chromosome called?
The p arm
What is the long arm of the chromosome called?
The q arm
What connects the p arm with the q arm?
A centromere
What is added when karotyping to prevent mitosis from progressing past the metaphase?
Colchicine
How are the chromosomes released from the cell?
During karyotyping, a hypotonic solution is added, which causes the cell to swell and burst, releasing all the chromosomes
What is used during karyotyping to fix the chromosomes in place on the glass slide?
Carnoys fluid
what is carnoys fluid?
A combination of acetic acid and ethanol used to fix the chromosomes in place
What are the tips of the chromosome called?
Telomere
What does giemsa staining produce?
Recognizable banding patterns on the chromosomes
How are the bands on chromosomes numbered?
The bands closest to the centromere is 1, and they increase in number the further you get from the centromere
What do dark bands on the chromosomes represent (G-dark)?
Heterochromatin - this is where the DNA is wrapped more tightly around the histone proteins and is more condensed
What do light bands on chromosomes represent (G-light)?
Euchromatin - where the DNA is not packed as tightly around the chromatin so it is more open
What is meant by metacentric chromosomes?
Where the p arm and the q arm are the same length
What are submetacentric chromosomes?
Where the p arm is shorter than the q arm
What is meant by acrocentric chromosomes?
Where the short p arms have been reduced to a stump and replaced by non-coding satellites
What type of translocation can acrocentric chromosomes undergo?
Robertsonian translocations
What is a robertsonian translocation?
Where there is translocation between two acrocentric chromosomes which results in the formation of one chromosome with normal lengthed arms, and one where the two satellite sections have combined = loss of chromosome
Which chromosomes do Robertsonian translocations usually occur in?
13, 14, 15, 21 and 22
What is meant by a de novo abnormality?
A chromosomal abnormality which is not inherited from the parents
During what stage does cross-over occur?
During prophase of meiosis 1
What is the purpose of meiosis?
To reduce the diploid number to haploid so that diploid zygotes can be formed
To ensure there is genetic variation amongst off spring
What is the purpose of mitosis?
For growth, repair and replace exhausted daughter cells - helps to maintain diploid number
Describe how crossing over occurs?
The pairs of homologous chromosomes align, chiasma form and sections of DNA are exchanged
What is a recombinant chromatid?
A chromatid in which sections of DNA have been swapped during cross over
What is monosomy?
A loss of a chromosome - so now the chromosome only consists of one chromatid
What is trisomy?
Where there is a gain of a chromosome, 3 copies of chromosomes
What is the purpose of meiosis?
To achieve reduction from diploid (46->23)
To ensure genetic variation in the gametes
Enables random assortment of homologues and recombination
What is the most common cause of numerical chromosomal abnormalities?
Non-dysjunction
What is non-dysjunction?
The failure of homologous chromosomes to separate properly during cell division
When does non-dysjunction occur?
Meiosis I= all daughter cells affected
Meiosis II= half affected
Always occurs in either +1 or -1 chromosome
What does aneuploidy mean?
abnormal number of chromosomes
What are the three stages in which chromosomal abnormalities can occur?
Mitosis, meiosis 1 or meiosis 2
What is Trisomy 21?
Down syndrome
What causes trisomy 21?
failure of the 21st chromosome to separate during egg or sperm development
(maternal non-disjunction)
What can be used to rapidly detect pre-natal Trisomy?
Quantitative Flourescence PCR
What is the cause of 85-90% of trisomy 21?
Maternal dysjunction
What increases the risk of maternal dysjunction?
Increasing maternal age
Why does maternal non-dysjunction increase with age?
Vulnerability of oogenesis
Paused in utero in prophase I until puberty
Secondary oocyte arrests in metaphase II
Only competes if fertilized
One primary oocyte yields only one ovum
Finite number of primary oocytes
When does most aneuploidy caused by non-disjunction arise?
Oogenesis
Likely due to degradation of factors which hold homologous chromatids together
What is the paternal age effect?
Vulnerability of male meiosis
No equivalent to oocyte mitotic arrest
Primary spermatocytes undergo roughly 23 mitotic divisions per year and potentially accumulate defects
Paternal age is not a risk factor for increased aneuploidy
What is a risk factor for aneuploidy in males?
Smoking
What is the karyotype of someone with trisomy 21?
47 + 21, or 47, XX+21
What causes turners syndrome?
X monosomy
What causes Kleinfelder’s disease?
XXY - trisomy where you have an extra X chromsome
What is the most common form of aneuploidy?
Sex chromosome imbalance
How does the body tolerate an imbalance in sex chromsomes?
(X-inactivation) The excess X chromosomes get inactivated
Low gene content of Y chromosomes
How are excess X chromosomes silenced?
It is packed into a transcriptionally inactive heterochromatin
What is a Barr body?
An inactive X chromosome
If excess X chromosomes are silenced, why can some XXY and XXX have effects?
Some genes may be located in the PAR regions - genes in this region behave like autosomal genes and therefore are not inactivated like the rest of the X chromosome
Are trisomies compatible with life?
Most trisomies are not compatible with life
What is mosaicism?
Presence of 2 or more populations of cells with different genotypes
X-inactivation results in mosaic expression
Results in generally milder phenotype
Is everyone thought to be mosaic?
yes
Is there a way that aneuploidy is more survivable?
Yes, if they are mosaic
What are PAR regions?
Pseudoautosomal regions - contain pseudoautosomal genes that behave like autosomal genes and therefore are not silenced when excess X or Y chromosomes are
What causes single chromosome abnormalities?
Can be the result of unequal crossover
Breaks in chromosome
Can occur at ends of chromosomes (or around centromere)
What do single chromosome abnormalities cause?
Can cause reproductive problems
Carriers are often unaffected
What is it which allows crossing over between X and Y chromosomes?
Psuedoautosomal regions
What translocation results in the formation of the Philadelphia gene?
t(9;22)(q34;q11)
What condition can the fusion of the ABL and BCR gene lead to?
Chronic Myeloid Leukemia
What is haploinsufficiency?
A dominant phenotype in diploid organisms that are heterozygous for a loss of function allele
aka an allele is inactivated so the gene overall does not function
What is genomic imprinting?
Changes to the DNA or chromatin that are inherited but do not affect the DNA sequence
the process by which only one copy of a gene in an individual (either from their mother or their father) is expressed, while the other copy is suppressed.
How is genomic imprinting achieved?
Through DNA methylation
Carriers of balanced translocations have increased risk of what?
Creating gametes with unbalanced chromosome translocations, leading to infertility, miscarriages or children with abnormalities
What is the severity of the phenotype dependant on?
The gene content of the affected segment
Is trisomy or monosomy usually more tolerated?
Trisomy
What are some signs and symptoms of patients with Williams syndrome?
Long philtrum - space between nose and lip
Short, upturned nose
Arched eyebrows
Funny, cocktail party personalities
Why is Williams syndrome detected using targeted FISH (fluorescent in situ hybridisation)?
The deletion is too small to be detected using karyotyping
What are the three types of structural chromosomal abnormalities that can occur?
Deletion, duplication and inversion
What can be seen when higher levels of karotyping resolution is done?
Sub-bands and sub-sub bands
Most aneuploidy caused by non-disjunction arises in what process?
Oogenesis
Is paternal age a risk factor for aneuploidy?
No
What three conditions are affected by the paternal age effect?
Apert syndrome
Crouzon syndrome
Pfeiffer syndrome
How many times per chromosome per cycle of meiosis does cross over occur?
1-3 times
When does crossing over occur and what does it do?
Prophase I
Increases genetic diversity
Pairs of chromosomes align
Chiasma form and crossover occurs
What does unequal crossing over cause?
Duplication or deletion
What syndrome is 7q11.23 deletion?
Williams syndrome
What are some signs and symptoms of 7q11.23 duplication syndrome?
Delayed speech development Autistic behaviours that affect social interaction and communication Dilatation of the aorta Flat eyebrows Broad nose and short philtrum
what are the two causes of mosaicism?
Mitotic disjunction during embryonic development
Loss of extra chromosome in early development
What is paracentric inversion?
Inversion not around the centromere
What is pericentric inversion?
Inversion of a section of the chromosome, around the centromere
What is an example of a disease of chromosomal deletions?
Cri-du-chat syndrome (46,XY,del(5p))
and
Velocardiofacial/ DiGeorge syndrome (22q11.2 del)
They are microscopic, easily detectable in microscope
How do you see micro-deletions?
seen in very high resolution banding; molecular genetics
What are the classes of chromosome?
Metacentric= short arm p, long arm q (same length)
Submetacentric= short arm is shorter
Acrocentric= stalk and satellite instead of short arm
What is Robertsonian translocation?
Occurs between acrocentric chromosomes
Can be homologous or non homologous
Most common 13 and 14; 14 and 15; 14 and 21
Most people show no effects
Can cause problems in offspring
