L16: Structural Chromosomal Abnormalities Flashcards
What are examples of structural abnormalities?
→Translocations →Inversion →Deletion →Duplication →Rings →Isochromosomes
What are the two types of translocation abnormalities?
→Reciprocal
→Robertsonian
What is translocation?
→Exchange of two segments between non-homologous chromosomes
→two double strands breaks, each on a different chromosome
What is NHEJ?
→that instead of joining together the correct two bits, the DNA repair mechanism happens to stitch together the chromosome in incorrect pairs
→end of another chromosome attached and vice verse in this chromosome
What kind of translocations are NHEJ?
→balanced translocations
When do NHEJ occur?
→spontaneously during meiosis
What is the genetic material content like in NHEJ?
→no net gain or loss of genetic material
What is a Philadelphia chromosome?
→a reciprocal translocation involving chromosomes 9 and 22
→The break points of the translocation create a fusion
Which two genes create a fusion to form Philadelphia chromosome?
→ABL1 on chromosome 9
→ BCR on chromosome 22.
What is BCR on chromosomes like?
→prone to DNA double strand break
What is the Philadelphia chromosome implicated in?
→chronic myelogenous leukemia (CML)
What are the consequences of reciprocal translocations in meisosis?
→Tetravalent forms instead of bivalent
What does reciprocal translocation mean?
→no loss or gain of material
→little consequence to the cell of carrying a reciprocal translocation.
What is a pachytene quadrivalent?
→ balanced translocation ends up pairing in fours
What is pachytene?
→each tetrad shortens, thickens, and separates into four distinct chromatids joined at the centromere
What is the consequence of reciprocal translocations dependent on?
→the particular chromosomes involved
→size of the translocated material
What can the results be upon fertilisation in reciprocal translocations in meiosis?
→there could be trisomy to different regions of the chromosome
→monosomic for one chunk
What are the results of unbalanced reciprocal translocation?
→Many lead to miscarriage
→Learning difficulties, physical disabilities
→Tend to be specific to each individual so exact risks and clinical features vary
What is a Robertsonian translocations?
→two acrocentric chromosomes break at or near their centromeres,
→when the fragments are joined together again it’s possible for just the two sets of long arms to be brought together
→there’s loss of the satellites.
Which chromosomes are prone to Robertsonian translocations?
→chromosomes 13 and 14
→accounts for approximately 1/3 of all Robertsonian translocations
What is the nature of the long arms that are joined together in Robertsonian translocations?
→long arms of different chromosomes
→unusual to see, for example, the maternal and paternal long arms of chromosome 13 fused together
How many chromosome in balanced carrier after Robertsonian translocation?
→has 45 chromosomes
What does it mean if 46 chromosomes are present including Robertsonian?
→unbalanced- clinical problems
What do p-arms encode?
→rRNA
→multiple copies so not deleterious to lose some
Which chromosomes are common with Robertsonian translocations and what they lead to?
→13;14 and 14;21 relatively common.
→21;21 translocation
→leads to 100% risk of Down syndrome in foetus
Why do couples where one person has a Robertsonian translocation have miscarriages?
→because of the way the chromosomes segregate,
→loss of a chromosome or a trisomy
→is incompatible with life
What are the outcomes of translocations?
→Very difficult to predict
→Only have approximate probability of producing possible gametes
→Some unbalanced outcomes may lead to spontaneous abortion of conceptus so early that not seen as problem
→Some unbalanced outcomes may lead to miscarriage later on and present clinically
→Some may result in live-born baby with various problems
If the end of a chromosome is lost, how are they made stable
→if a new telomere is added
What are inversions?
→where there are two breakpoints within the same chromosome
→when these are repaired the middle section is “upside down”
What is a ring chromosome?
→two breaks in the same chromosome
→ non-homologous end joining mechanism joins the two ends of the large chunk together
What are other structural chromosome changes?
→terminal deletion →interstitial deletion →inversion →duplication →ring chromosome
How are microdeletions detected?
→arrayCGH
Are microdeletions visible on metaphase spread?
→no
How are microdeletions detected?
→High resolution banding,
→FISH
→CGH showed
What are examples of microdeletions?
→Wolf-Hirschhorn, 4p16
→Williams, 7q11
→Smith-Magenis, 17p11
What is unequal crossing over?
→chromosomes have not aligned properly- result is simultaneous deletions and duplications
What are the sources of sample for pre-natal testing?
→Amniocentesis
→Chorionic villus sampling
→Cell-free fetal DNA from maternal plasma
What are the sources of sample for postnatal testing?
→blood
→saliva
What stain is used for staining?
→Giemsa
Which bases are euchromatin rich in?
→GC-rich
Which bases are heterochromatin rich in?
→AT
What phase is required for banding?
→metaphase
What does G-banding look for?
→aneuploidies,
→translocations
→very large deletions
How is FISH used for detection?
→Fluorescent probe
→Denature probe and target DNA
→Mix probe and target DNA
→Probe binds to target
What is Cri-du-chat syndrome?
→5p minus syndrome- 5p deletion on Chromosome 5
What are the symptoms of Cri-du-chat syndrome?
→developmental delay
→microcephaly
What is arrayCGH used to detect?
→of sub-microscopic chromosomal abnormalities
→How many copies of a particular genomic region does the patient have
What does arrayCGH use?
→Uses extracted DNA
What does QF-PCR detect?
→microsatellites
What is the nature of flanking sequence of microsatellites?
→same in individuals
→used to generate probes for detection
What are the 4 steps in detecting microsatellites?
→Isolate DNA from individual
→Design primers specific to flanking sequences
→PCR amplification
→Gel electrophoresis
What are the components of PCR reaction?
→Template – DNA to amplify
→Primers – Short pieces of ssDNA (15-30bp)
→Polymerase – thermostable enzyme (Taq)
→Nucleotides – single base mixture (dNTPs)
→Buffer – To maintain pH
MgCl2 – Essential for polymerase activity
Explain the peaks of QF-PCR
→If homozygous, there will be a single peak of high signal
→If heterozygous, there will be two peaks of similar, lower signal
Explain 3 peaks in trisomy after QF-PCR
→three different repeat lengths
→one of each copy so all three peaks have the same intensity
Explain 3 peaks (one taller than the other) in trisomy
→Higher peak is two copies of one strand with same repeats
What is involved in non-invasive pre-natal testing?
→Cell free fetal DNA
→Trisomy testing
→Next-generation sequencing
