Cytogenetics Flashcards
(36 cards)
An exact multiple of the haploid chromosome number (n) is termed [], and any other chromosome number is termed [].
An exact multiple of the haploid chromosome number (n) is termed euploid, and any other chromosome number is termed .
The most common mechanism by which aneuploidy occurs is [].
The most common mechanism by which aneuploidy occurs is meiotic nondisjunction.
Often, chromosome abnormalities are the result of structural rearrangements rather than differences in chromosome number. Such rearrangements are defined as[] if the resultant chromosome has the normal complement of genetic material or [], if there is additional or missing genetic information.
Often, chromosome abnormalities are the result of structural rearrangements rather than differences in chromosome number. Such rearrangements are defined as balanced if the resultant chromosome has the normal complement of genetic material or unbalanced, if there is additional or missing genetic information.
Unbalanced rearrangements cause [] and [] involving part or all of a chromosome. Balanced rearrangements include[] (either [] or []), and [].
Unbalanced rearrangements cause deletions and duplications involving part or all of a chromosome. Balanced rearrangements include inversions (either paracentric or pericentric), and translocations.
•Balanced rearrangements usually result in no structural or functional disability in the affected individual, but they can result in unbalanced chromosome complements in offspring. Translocations and large pericentric inversions are particularly liable to result in aberrant reproductive outcome.
A [] is a pictorial representation of the chromosomes. Human chromosomes are classified according to 3 characteristics: (1) [] (2) [] and (3) [].
A karyotype is a pictorial representation of the chromosomes. Human chromosomes are classified according to 3 characteristics: (1) centromere position (2) size and (3) banding pattern
Metacentric…
…chromosomes have a central centromere.
Submetacentric…
…chromosomes have an off center centromere.
Acrocentric…
…chromosomes are recognizable by the centromere being near one end.
•The human acrocentric chromosomes (13, 14, 15, 21, and 22) have small masses of chromatin known as satellites attached to their short arms by narrow stalks. The stalks of these five chromosome pairs contain the genes for 18S and 28S ribosomal RNA (rRNA). (Since this genetic material is duplicated elsewhere in the genome, deletion of the satellites and stalks of one pair of acrocentric chromosomes has no effects on the phenotype).
Standard Banding
- initially treated with an agent such as trypsin, which partially “digests” the protein of the chromosome
- secondary stain is then applied which allows for differential banding to be visualized. The most commonly used banding technique is known as G-banding, so-called because Giemsa stain is applied following treatment of the chromosome with trypsin
- the chromosomes appear as a distinctive series of light and dark bands
- darkly staining bands tend to be condensed, AT-rich and nontranscribed heterochromatin, while the light staining bands tend to be elongated, GC-rich and transcribed euchromatin
High Resolution Banding
- A technique that allows identification of greater detail in the karyotype.
- With this technique, chromosomes that have been prepared at an earlier stage of mitosis (prophase or prometaphase), when they are still in a relatively uncondensed state, are stained and analyzed.
- Prometaphase and prophase chromosomes reveal 550-850 bands or even more in a haploid set in comparison to standard metaphase preparations which show only about 400 bands.
Fluorescence in-situ hybridization (FISH)
- Allows a focused search for the absence (deletion) or excess (duplication) of specific chromosomal regions.
- In this technique, specific DNA segments that are complementary to segments of interest in affected individuals are constructed and “tagged” with a fluorescent label. These specific segments are then hybridized to a standard metaphase chromosome preparation from an affected or potentially affected individual. If the segment of interest is present, then there will be hybridization of the fluorescent probe and it can be detected under the fluorescent microscope.
- Through this technique, clinical detection of both numerical and structural chromosome abnormalities can be accomplished for selected disorders. There are three classes of FISH probes used for these purposes:
- Locus-specific
- Alphoid or centromeric probes
- Chromosome-sepcific painting probes
Locus-specific FISH probes
- used to detect specific genetic disorders for which the disease-causing sequence is known. This is generally used to see if both copies of a particular sequence are present.
- There is a long list of disorders to which this technology can be applied, including the 22q11 deletion syndrome and Prader-Willi syndrome. In the normal individual there will be 2 fluorescent signals; whereas in the person with that segment deleted, only one signal will be present.
Alphoid or centromeric probes
- Used to detect abnormalities in chromosome number.
- These probes take advantage of the fact that most chromosome pairs have a unique sequence near the centromere that distinguishes this chromosome pair from the other 22 pairs.
-For example, a chromosome 21 specific centromere probe would detect 2 signals from a normal person with 2 copies of chromosome #21 and 3 copies from an individual with trisomy 21/Down syndrome.
- This type of FISH analysis can be used when there is a need for a rapid diagnosis of trisomies, which might include pregnancies in which one of the trisomies is likely because of characteristic prenatal ultrasound features; or, when there is a critical need for diagnosis of a newborn with life-threatening birth defects and a trisomy is suspected.
- For routine diagnosis of trisomies, however, standard chromosome analysis is adequate and will readily detect the extra chromosome.
Chromosome-specific painting probes
- Used to detect rearrangements suspected by standard cytogenetic techniques.
- In this technique the whole chromosome is “painted” with a cocktail of unique sequences to give fluorescence of essentially the whole chromosome. This may be used in the case when there is extra chromosomal material present but the material is of unknown origin. This FISH technique can identify the origin of the abnormal chromosomal material.
Array comparative genomic hybridization (aCGH)
•Also known as chromosome microarray (CMA) represents a second generation of molecular cytogenetic techniques that can detect chromosome deletions or duplications as small as a few hundred base pairs.
Massively parallel/Next generation DNA sequencing
•There are a few varieties of these next generation sequencing (NGS) technologies, but all rely on sequencing each base of DNA multiple (2-1000) times to overcome inherently high error rates. Essentially, the methods simultaneously perform millions of sequencing reactions and read the sequencing from each using a detector connected to a computer. Each DNA sequence read is typically 50-200 bases long and significant computational resources are then used to map these short reads back to a reference human genome.
Methylation Studies
•The methylation state of a specific region of DNA can be assessed with methylation-sensitive restriction enzymes. Essentially, these will only cut methylated (or unmethylated) sequences, while the alternative methylation state will remain uncut. After attempted cleavage, patient DNA can be separated by electrophoresis and the sizes of bands analyzed by Southern blotting. Whether the source DNA was or was not methylated can then be inferred by the sizes of the resulting bands. Methylation state can also be determined treating DNA with sodium bisulfite that converts cytosine to uracil, but leaves 5-methylcytosine residues unaffected. Amplifying the resulting sequencing with specific primers, direct sequencing, or methylationspecific microarray can then allow determination of the underlying methylation state of a sequence or a small region.
In general, people with autosomal chromosome abnormalities have the triad of [], [] and [].
In general, people with autosomal chromosome abnormalities have the triad of growth retardation, intellectual disability (previously referred to as mental retardation) and birth defects.
Sex chromosome abnormalities medical problems primarily involve [] and [].
Sex chromosome abnormalities medical problems primarily involve abnormalities of stature and sexual maturation.
Nondisjunction
- In the normal situation during meiosis, the 2 members of each homologous chromosome pair disjoin, with one chromosome going to each daughter cell. When these 2 chromosomes fail to disjoin, it is known as nondisjunction.
- Nondisjunction may produce either a trisomic or a monosomic line of cells for the involved chromosome (commonly 21, 18 or 13).
- An autosomal monosomic cell line (45 chromosomes) generally does not persist, but the trisomic cell line may survive. Trisomic (47 chromosomes) live births are not rare, although postnatal survival may be compromised by multiple malformations.
- The rate at which autosomal nondisjunction occurs increases with advancing maternal age.
- There are three clinically important autosomal trisomies: trisomy 21, 18 and 13. For the most part, these are the only autosomal trisomies that are seen in liveborn children, since trisomies for other autosomes result in very low viability and spontaneous abortion (miscarriage).
- Autosomal monosomies are uniformly lethal prenatally or in very rare circumstances, soon after birth.
Trisomy 21
- Down Syndrome
- About 95% of children with Down syndrome have what is known as free 21 trisomy, which is designated 47,XY,+21 or 47,XX,+21 and is the result of nondisjunction. There is about a 1% risk for having additional children with an autosomal trisomy in the parents of children with free 21 trisomy. Parent karyotypes are not recommended when the child has free 21 trisomy.
- However, about 4% of children have a Robertsonian translocation leading to Down syndrome. In these children, one of the parents may be a translocation carrier, so parent karyotypes are recommended. If one of the parents is found to carry a balanced translocation, then they have about a 10% risk of having another child with Down syndrome.
- About 1% of children with Down syndrome have mosaicism. Mosaicism means that there are two or more cell lines in one individual. In the case of Down syndrome the karyotype designation is 46,XY/47,XY,+21 in the case where one cell line is normal and the other contains a trisomy 21 cell line. Some children with mosaicism may have a milder phenotype as a result of the mitigating effect of the normal cell line.
Trisomy 18
- Children with trisomy 18 have low birthweight, small mouth, prominent occiput, short sternum, cardiac defects in 80% or more and decreased viability.
- While the median life expectancy is around one week, many patients can live much longer.
Trisomy 13
- Children with trisomy 13 are also characteristic, with frequent features being holoprosencephaly (a severe abnormality of brain development), cleft lip and palate, extra digits and decreased viability.
- Median life expectancy is similar to children with trisomy 18.
Deletions
•Deletions are losses of chromosome material and may be terminal deletions, occurring at the end of one of the chromosome arms; or interstitial deletions, occurring between the centromere and telomere and removing an intervening sequence.
