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omgmbd > L16: Structural Chromosomal Abnormalities > Flashcards

L16: Structural Chromosomal Abnormalities Flashcards

1
Q

What are examples of structural abnormalities?

A 
→Translocations
→Inversion
→Deletion
→Duplication
→Rings
→Isochromosomes
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2
Q

What are the two types of translocation abnormalities?

A

→Reciprocal

→Robertsonian

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3
Q

What is translocation?

A

→Exchange of two segments between non-homologous chromosomes

→two double strands breaks, each on a different chromosome

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4
Q

What is NHEJ?

A

→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

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5
Q

What kind of translocations are NHEJ?

A

→balanced translocations

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6
Q

When do NHEJ occur?

A

→spontaneously during meiosis

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7
Q

What is the genetic material content like in NHEJ?

A

→no net gain or loss of genetic material

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8
Q

What is a Philadelphia chromosome?

A

→a reciprocal translocation involving chromosomes 9 and 22

→The break points of the translocation create a fusion

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9
Q

Which two genes create a fusion to form Philadelphia chromosome?

A

→ABL1 on chromosome 9

→ BCR on chromosome 22.

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10
Q

What is BCR on chromosomes like?

A

→prone to DNA double strand break

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11
Q

What is the Philadelphia chromosome implicated in?

A

→chronic myelogenous leukemia (CML)

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12
Q

What are the consequences of reciprocal translocations in meisosis?

A

→Tetravalent forms instead of bivalent

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13
Q

What does reciprocal translocation mean?

A

→no loss or gain of material

→little consequence to the cell of carrying a reciprocal translocation.

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14
Q

What is a pachytene quadrivalent?

A

→ balanced translocation ends up pairing in fours

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15
Q

What is pachytene?

A

→each tetrad shortens, thickens, and separates into four distinct chromatids joined at the centromere

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16
Q

What is the consequence of reciprocal translocations dependent on?

A

→the particular chromosomes involved

→size of the translocated material

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17
Q

What can the results be upon fertilisation in reciprocal translocations in meiosis?

A

→there could be trisomy to different regions of the chromosome

→monosomic for one chunk

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18
Q

What are the results of unbalanced reciprocal translocation?

A

→Many lead to miscarriage
→Learning difficulties, physical disabilities
→Tend to be specific to each individual so exact risks and clinical features vary

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19
Q

What is a Robertsonian translocations?

A

→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.

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20
Q

Which chromosomes are prone to Robertsonian translocations?

A

→chromosomes 13 and 14

→accounts for approximately 1/3 of all Robertsonian translocations

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21
Q

What is the nature of the long arms that are joined together in Robertsonian translocations?

A

→long arms of different chromosomes

→unusual to see, for example, the maternal and paternal long arms of chromosome 13 fused together

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22
Q

How many chromosome in balanced carrier after Robertsonian translocation?

A

→has 45 chromosomes

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23
Q

What does it mean if 46 chromosomes are present including Robertsonian?

A

→unbalanced- clinical problems

24
Q

What do p-arms encode?

A

→rRNA

→multiple copies so not deleterious to lose some

25
Q

Which chromosomes are common with Robertsonian translocations and what they lead to?

A

→13;14 and 14;21 relatively common.
→21;21 translocation
→leads to 100% risk of Down syndrome in foetus

26
Q

Why do couples where one person has a Robertsonian translocation have miscarriages?

A

→because of the way the chromosomes segregate,

→loss of a chromosome or a trisomy

→is incompatible with life

27
Q

What are the outcomes of translocations?

A

→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

28
Q

If the end of a chromosome is lost, how are they made stable

A

→if a new telomere is added

29
Q

What are inversions?

A

→where there are two breakpoints within the same chromosome

→when these are repaired the middle section is “upside down”

30
Q

What is a ring chromosome?

A

→two breaks in the same chromosome

→ non-homologous end joining mechanism joins the two ends of the large chunk together

31
Q

What are other structural chromosome changes?

A 
→terminal deletion
→interstitial deletion
→inversion
→duplication
→ring chromosome
32
Q

How are microdeletions detected?

A

→arrayCGH

33
Q

Are microdeletions visible on metaphase spread?

A

→no

34
Q

How are microdeletions detected?

A

→High resolution banding,
→FISH
→CGH showed

35
Q

What are examples of microdeletions?

A

→Wolf-Hirschhorn, 4p16
→Williams, 7q11
→Smith-Magenis, 17p11

36
Q

What is unequal crossing over?

A

→chromosomes have not aligned properly- result is simultaneous deletions and duplications

37
Q

What are the sources of sample for pre-natal testing?

A

→Amniocentesis
→Chorionic villus sampling
→Cell-free fetal DNA from maternal plasma

38
Q

What are the sources of sample for postnatal testing?

A

→blood

→saliva

39
Q

What stain is used for staining?

A

→Giemsa

40
Q

Which bases are euchromatin rich in?

A

→GC-rich

41
Q

Which bases are heterochromatin rich in?

A

→AT

42
Q

What phase is required for banding?

A

→metaphase

43
Q

What does G-banding look for?

A

→aneuploidies,
→translocations
→very large deletions

44
Q

How is FISH used for detection?

A

→Fluorescent probe
→Denature probe and target DNA
→Mix probe and target DNA
→Probe binds to target

45
Q

What is Cri-du-chat syndrome?

A

→5p minus syndrome- 5p deletion on Chromosome 5

46
Q

What are the symptoms of Cri-du-chat syndrome?

A

→developmental delay

→microcephaly

47
Q

What is arrayCGH used to detect?

A

→of sub-microscopic chromosomal abnormalities

→How many copies of a particular genomic region does the patient have

48
Q

What does arrayCGH use?

A

→Uses extracted DNA

49
Q

What does QF-PCR detect?

A

→microsatellites

50
Q

What is the nature of flanking sequence of microsatellites?

A

→same in individuals

→used to generate probes for detection

51
Q

What are the 4 steps in detecting microsatellites?

A

→Isolate DNA from individual

→Design primers specific to flanking sequences

→PCR amplification

→Gel electrophoresis

52
Q

What are the components of PCR reaction?

A

→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

53
Q

Explain the peaks of QF-PCR

A

→If homozygous, there will be a single peak of high signal

→If heterozygous, there will be two peaks of similar, lower signal

54
Q

Explain 3 peaks in trisomy after QF-PCR

A

→three different repeat lengths

→one of each copy so all three peaks have the same intensity

55
Q

Explain 3 peaks (one taller than the other) in trisomy

A

→Higher peak is two copies of one strand with same repeats

56
Q

What is involved in non-invasive pre-natal testing?

A

→Cell free fetal DNA
→Trisomy testing

→Next-generation sequencing

omgmbd (29 decks)

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