Chromosomal abnormalities Flashcards
What is the definition of diploidy?
There are two sets of chromosomes, one from each parent
Chromosomes form a number of homologous pairs
Describe the loci pattern on homologous chromosomes?
Homologous chromosomes have identical sets of loci
What is a locus?
A positioned occupied in a chromosome by a particular gene/allele
How many pairs of autosomes are there?
22
Which proteins are associated to DNA?
Histone proteins
What is the term used to describe the DNA-histone complex?
Chromatin
How many histone molecules are wound around DNA?
8
What is the characteristic description in regards to nucleosomes during the non-division of DNA?
Long chains of nucleosomes
Which type of staining is used to form a recognizable band pattern on chromosomes?
Giemsa staining
What is an ideogram?
Diagrammatic representation of the karyotype that represents the homologous pairs of chromosomes present in the nucleus; pairs ordered in size, centromeres are aligned (short arm uppermost )
What is giemsa staining?
G-banding
A cytogenetic technique that enables the production of a visible karyotype by staining condensed chromosomes
During which cell cycle stage is a giemsa stain taken and why?
Metaphase-Chromosomes are condensed and visible
What is the purpose of a trypsin solution during Giemsa staining?
Partially hydrolyzes the chromosomes, and subsequently stained with Giemsa
What do dark bands represent during Giemsa staining?
Adenine-thymine rich, therefore are gene poor
Which protein prevents chromosome unraveling?
Telomere
What is haplosufficiency?
This occurs whereby a single copy of a functional gene at a locus in a heterozygous combination with a variant allele is sufficient to maintain normal function/phenotype.
Heterozygote carriers of recessive alleles exhibit haplosufficiency.
Demonstrates dominant gene action within homologous pairs in diploids
What do imprinted genes mean?
Epigenetic dependent causes genes to be expressed in a parent-of-origin specific manner
Epigenetic modification on chromosomes inactivates expression of genes within the germline of the parents and are maintained through mitotic cell division in the somatic cells of an organism
How does histone modification result in the inactivation of genes?
Methylation of histones results in the gene present on the maternal and paternal homologous being inactivated -transcription is repressed
What is haploinsufficiency?
The circumstance whereby a single copy of a gene at a locus within the homologous pair is inactivated or deleted and the remaining functional copy of the gene is inadequate to produce the required gene product to preserve normal function
Both alleles are required for phenotypic expression
How much blood is collected as a sample for a Giemsa stain?
0.5ml of blood in a 5ml culture medium
What is added to a blood sample selected for a Giemsa stain?
Mitogen phytohemagglutinin
What is the purpose of mitogen phytohemagglutinin?
Stimulates lymphocyte b cells to divide
What is a mitogen?
A substance that induces or stimulates mitosis
Mitogen triggers signal transduction pathways involving mitogen-activated protein kinases (MAPK) leading to mitosis
How long is the culture suspended for during Giemsa staining?
48-72 hours
Why is the Giemsa culture cultured for 48-72 hours?
Enables subsequent divisions and establishes sufficient sample size.
What substance is added 48-72 hours of culturing?
Colcemid
What is the purpose of colcemid in a Giemsa stain?
Increases the proportion of cells at metaphase, through spindle fiber inactivation - induces cell arrest in metaphase, inhibiting the continuation of mitosis
How does colcemid work?
Induces cell arrest in metaphase, inhibiting the continuation of mitosis; depolymerizes microtubules, and limits microtubule formation, thus arresting cels and allowing cell harvest and karyotyping to be conducted
Why is hypotonic potassium chloride solution added to a Giemsa culture?
Establishes a water potential gradient, therefore, stimulating osmosis in the intracellular direction. The influx of water contributes towards cell swelling, increasing cytoplasmic volume to assist in the chromosomal movement
What ratios should methanol and acetic added be added to a Giemsa stain?
3:1
What enzyme is added in a Giemsa stain?
Trypsin, to establish recognizable banding pattern
What is a chromosome non-disjunction mutation?
This occurs when members of a homologous pair fail to separate during meiosis. This may occur when sister chromatids fail to separate during anaphase-II
Causes a gamete to be absent from a sex chromosome, associated with the age of the parent
What is Down’s syndrome?
Trisomy-21
Non-disjunction of the pair of chromosomes designated 21 leads to some gametes with 2 copies of 21, and some with none. Thus gamete fusion will result in zygote with 3 copies of chromosome-21 –> Individuals with Down’s syndrome have 47 chromosomes
What are the pathologies of Down’s syndrome?
Hypotonia, smallmouth, protruding tongue, and mental disability
Medical complications include vulnerability to infections, congenital heart defects, and hypothyroidism
What is Turner’s syndrome?
Monosomy X, therefore 45 chromosomes
Describe the fertility of triploid individuals?
Genetically infertile as pairing is incorrect
Where are diploid spermatogonia produced?
Seminiferous tubules in the testis
Which cells are produced at the end of meiosis 1 in spermatogenesis?
Secondary spermatocyte (haploid)
Primary spermatocytes are diploid or haploid?
Diploid
Which cells do secondary spermatocytes mature into?
Spermatids –> Spermatozoa
How many mitotic divisions do primary spermatocytes undergo?
23 mitotic divisions
Which genes are involved in point mutations resulting for Apert, Crouzon, and Pfeiffer syndrome?
FGFR2
FGFR3
RET
What is selfish spermatogonial selection?
Mutated spermatogonia undergo favoured mitosis in comparison to neighboring wild type cells.
Where do diploid oogonia derive from?
Foetal epithelial tissue
When is female meiosis paused?
Paused in utero as primary oocytes until hormone secretion from pituitary stimulates development into a secondary oocyte
How ova are produced from one primary oocyte?
One
Three polar bodies are formed through completed meiosis
What is caused by non-disjunction mutations during oogenesis?
Aneuploidy
Why does aneuploidy occur within oogenesis?
This is attributed to the degradation of factors responsible for holding homologous chromatids together; perturbing kinetochore-microtubule interactions & contributing towards spindle disintegration
What is the relationship between age and non-disjunction mutations?
The risk of maternal non-disjunction increases with age
Which stage of meiosis does 75% trisomy 21 non-disjunction occurs in?
Meiosis I
What is a Robertsonian translocation?
Robertsonian translocation is a chromosomal abnormality where the entire long arms of two different chromosomes become fused to each other. It is the most common form of chromosomal translocation in humans
What do unbalanced Robertsonian translocations cause?
Syndromes of multiple malformations and mental retardation.
21q14q (q arms of chromosome 21 associates with q arms of chromosomes 14)
3 copies of chromosome 21q causes down syndrome (trisomy 21)
What are the three stages of PCR?
Denaturation
Annealation
Elongation
What is PCR?
Procedure incorporated to amplify and produce significantly sample of DNA.
The automated method of making multiple copies of an extracted sample. DNA exposed to a repeating sequence of different temperatures, allowing different enzymes to work.
What temperature is DNA subjected due during PCR?
95 degrees
What occurs during the denaturation process of PCR?
The double-stranded of DNA molecules are separated into two strands by denaturation, thus exposing base sequences
What is added during annelation?
Primers
What are primers?
Required to initiate DNA synthesis, primers are short sections of DNA, consisting of a complementary base sequence to the initial segment of DNA
forms hydrogen bonds with the beginning of the DNA strand, process refers to annealing
What temperature does annelation occur?
55 degrees.
Which enzyme is added during elongation?
DNA polymerase
(Taq polymerase)
What bonds are formed by DNA polymerase?
Catalyzes the formation of phosphodiester bonds
What are exons?
Structural genes and regions of DNA that encode for polypeptides through the synthesis of mRNA
Base sequences are expressed
What are introns?
Non-coding regions
Introns are transcribed into mRNA primary transcript
What occurs during post-transcriptional modification?
The primary RNA transcript include sequences of introns & exons, the introns are removed, therefore exons are spliced together to form the length of messenger RNA; translated into an amino acid sequence
What are variable short sequence introns referred to as?
Variable number tandem repeats (VNTRs), these are known as microsatellites and minisatellites, number of variants comprised within a gene is uniquely specific toa in individual, enabling them to be used for personal or parietal identification
Which areas of DNA are used for DNA profiling?
Introns and VNTRs
Which enzymes are used to isolate VNTRs?
Restriction endonucleases
What happens when ddNTP is inserted into synthesized DNA?
Will cessation of strand elongation, considering the absence of the 3-hydroxy of group, the formation of a phosphodiester bond in the adjacent nucleotide is inhibited.
Chain termination labeled with fluorescent dye
Subjected to electrophoresis to identify the base sequence
What is quantitative fluorescence PCR?
Electrophoretohram derived from qualitative fluorescence PCR (QF-PCR)
Amplification of short polymorphic distinct tandem repeats (STRs) specific to chromosome 21.
primers are labeled with fluorescent dye
Trisomic pattern = 3 peaks in equal height
Two imbalanced peaks with 2:1 ratio
Diploidy = two peaks
Which peak patterns indicate trisomy?
Trisomic pattern = 3 peaks in equal height
Two imbalanced peaks with 1:2 or 2:1 ratio
Which trisomy cases lead to higher cases of miscarriage?
Trisomy 16 & 22
What is the purpose of non-coding RNA?
Non-coding RNA base complementary pairs to specific sequences that do not encode for essential proteins; therefore this prevents transcription, as hydrogen bonds cannot form, inhibiting mRNA synthesis of a particular region
What structure is formed from X-inactivation?
Barr body
X chromosome supercoils and condenses forming an inactive structure
One of the x chromosomes is coated by a length of non-coding RNA called X-inactive specific transcript.
How is polysomy X tolerated?
Epigenetically modified through RNA silencing and histone methylation to inhibit transcription and expression of chromosomes to polysomy X
what are the four types of chromosome mutations?
Deletions
Duplications
Inversions
Translocations
What are deletions?
Involves the loss of genes
What inversions, translocations, and duplications?
Lead to a reshuffling of genes, and hence can assist genetic variations between individuals, as can point gene mutations.
What are duplications?
Leads to a new segment with no functional constraints, the introns can, therefore, incur multiple mutations without having a deleterious effect
What is a reciprocal translocation?
Part of chromosome breaks and rejoins onto a non-homologous chromosome
What are the issues associated with carriers of reciprocal translocations?
Phenotypically normal, however during prophase 1, chromosome must contort into unusual configurations to attain synapsis
This presents a reproductive risk increasing the potential for aneuploidy to proceed and chance of unbalanced gametes.
What specific chromosomal synapsis structure is formed for reciprocal chromsome pairs?
Pachytene cross
What are pericentric inversions?
Contribute towards insufficient central synapse, and relatively larger inversions will direct synapsis in an alternative direction, whereby telomeric ends of chromosomes are not sufficiency synapsed.
Regions configuring with high gene content in unbalanced chromosomes will attribute ___ tolerance:
Less
Why are unbalanced regions of the low gene content of high tolerance?
Potential effects on individuals are reduced and restricted, phenotypic changes insignificant.
Which is more tolerated: Monosomy or trisomy?
Trisomy
What happens during terminal deletions?
Removal of telomeres – phenotype depends on imbalance size,
What is an interstitial deletion?
Associated with intermediate loss of chromosomal segments
What are ring chromosomes?
Ring is unstable, mitosis results in two interlinking rings
What happens during contiguous gene syndrome?
Deletion or duplication that removes multiple genes within close proximity to one another on the chromsome.
What is a phenotype first approach?
Grouping children with similar development delay or dysmorphism, thereon identify patterns in genetic abnormalities
Fish probe to screen a larger number of children
What is the disadvantage of a genotype first approach?
Cost of genomic analysis
What is the advantage of a genotype first approach?
Ability to detect minor genetic imbalances
What are the physical deformities with Williams syndrome (7q11.23) deletion?
Long philtrum, short upturned nose, arched eyebrows, supravalvular aortic stenosis
Absence of social anxiety
What is supravalvular aortic stenosis (SVAS)?
A congenital heart defect that results in stenosis of the large blood vessel that carries blood from the heart into the systemic circulation (aorta)
Which gene deletion contributes towards SVAS?
ELN (Elastin gene)
What is the purpose of Array comparative genome hybridization (ACGH)?
The technique that permits the detection of chromosomal copy number changes, an overview of chromosomal gains and losses throughout the whole genome of the tumor
What is tumour DNA labeled with?
Red fluorochrome
What is normal DNA labeled with?
Green fluorochrome; hybridized to norma, human metaphase preparations
What happens during ACGH?
The green and red labeled DNA fragments compete for DNA hybridization to their locus of origin on chromosomes.
What is measured during ACGH?
Respective green: red fluorescence ratio measured along chromosomal axis represents loss or gain of genetic material in the tumor at that specific locus
How is genetic variation within genes obtained during meiosis?
Genetic variation is introduced due to meiosis
During prophase 1, the pairs of chromosomes pair up through synapsis, resulting in bivalents, crossing over proceeds whereby chiasmata form between non-sister chromatids
Chiasmata exchange parts of non-sister chromatid introducing a combination of paternal and maternal genes
Recombinant chromosomes form
What is non-allelic homologous recombination (NAHR)?
NAHR is mediated by low copy repeats occurring between two DNA lengths that have high sequence similarity, however, are not alleles
Recombination does not proceed between identical loci, LCR recombination results in looping whereby one chromatid obtains a duplicate, whereas the other receives a deletion
polymorphic parental inversions of LCRs increases risk of nahr of meiosis
What is a read in terms of next-generation sequencing?
Single uninterrupted series of nucleotides representing the sequence of the template
How can point mutations be identified in next-generation sequencing?
Aligning reads to the reference genome can be used to identify type of point mutations
What are non-sense mutations?
Point mutations result in trinucleotide sequence that encodes for a stop codon, suppresses and creases elongation of polypeptide strand, truncating protein,
