L16: Structural Chromosomal Abnormalities

Question 1

What are examples of structural abnormalities?

Answer 1

→Translocations
→Inversion
→Deletion
→Duplication
→Rings
→Isochromosomes

Question 2

What are the two types of translocation abnormalities?

Answer 2

→Reciprocal

→Robertsonian

Question 3

What is translocation?

Answer 3

→Exchange of two segments between non-homologous chromosomes

→two double strands breaks, each on a different chromosome

Question 4

What is NHEJ?

Answer 4

→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

Question 5

What kind of translocations are NHEJ?

Answer 5

→balanced translocations

Question 6

When do NHEJ occur?

Answer 6

→spontaneously during meiosis

Question 7

What is the genetic material content like in NHEJ?

Answer 7

→no net gain or loss of genetic material

Question 8

What is a Philadelphia chromosome?

Answer 8

→a reciprocal translocation involving chromosomes 9 and 22

→The break points of the translocation create a fusion

Question 9

Which two genes create a fusion to form Philadelphia chromosome?

Answer 9

→ABL1 on chromosome 9

→ BCR on chromosome 22.

Question 10

What is BCR on chromosomes like?

Answer 10

→prone to DNA double strand break

Question 11

What is the Philadelphia chromosome implicated in?

Answer 11

→chronic myelogenous leukemia (CML)

Question 12

What are the consequences of reciprocal translocations in meisosis?

Answer 12

→Tetravalent forms instead of bivalent

Question 13

What does reciprocal translocation mean?

Answer 13

→no loss or gain of material

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

Question 14

What is a pachytene quadrivalent?

Answer 14

→ balanced translocation ends up pairing in fours

Question 15

What is pachytene?

Answer 15

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

Question 16

What is the consequence of reciprocal translocations dependent on?

Answer 16

→the particular chromosomes involved

→size of the translocated material

Question 17

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

Answer 17

→there could be trisomy to different regions of the chromosome

→monosomic for one chunk

Question 18

What are the results of unbalanced reciprocal translocation?

Answer 18

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

Question 19

What is a Robertsonian translocations?

Answer 19

→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.

Question 20

Which chromosomes are prone to Robertsonian translocations?

Answer 20

→chromosomes 13 and 14

→accounts for approximately 1/3 of all Robertsonian translocations

Question 21

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

Answer 21

→long arms of different chromosomes

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

Question 22

How many chromosome in balanced carrier after Robertsonian translocation?

Answer 22

→has 45 chromosomes

Question 23

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

Answer 23

→unbalanced- clinical problems

Question 24

What do p-arms encode?

Answer 24

→rRNA

→multiple copies so not deleterious to lose some

Question 25

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

Answer 25

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

Question 26

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

Answer 26

→because of the way the chromosomes segregate, →loss of a chromosome or a trisomy →is incompatible with life

Question 27

What are the outcomes of translocations?

Answer 27

→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

Question 28

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

Answer 28

→if a new telomere is added

Question 29

What are inversions?

Answer 29

→where there are two breakpoints within the same chromosome →when these are repaired the middle section is “upside down”

Question 30

What is a ring chromosome?

Answer 30

→two breaks in the same chromosome → non-homologous end joining mechanism joins the two ends of the large chunk together

Question 31

What are other structural chromosome changes?

Answer 31

``` →terminal deletion →interstitial deletion →inversion →duplication →ring chromosome ```

Question 32

How are microdeletions detected?

Answer 32

→arrayCGH

Question 33

Are microdeletions visible on metaphase spread?

Answer 33

→no

Question 34

How are microdeletions detected?

Answer 34

→High resolution banding, →FISH →CGH showed

Question 35

What are examples of microdeletions?

Answer 35

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

Question 36

What is unequal crossing over?

Answer 36

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

Question 37

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

Answer 37

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

Question 38

What are the sources of sample for postnatal testing?

Answer 38

→blood | →saliva

Question 39

What stain is used for staining?

Answer 39

→Giemsa

Question 40

Which bases are euchromatin rich in?

Answer 40

→GC-rich

Question 41

Which bases are heterochromatin rich in?

Answer 41

→AT

Question 42

What phase is required for banding?

Answer 42

→metaphase

Question 43

What does G-banding look for?

Answer 43

→aneuploidies, →translocations →very large deletions

Question 44

How is FISH used for detection?

Answer 44

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

Question 45

What is Cri-du-chat syndrome?

Answer 45

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

Question 46

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

Answer 46

→developmental delay | →microcephaly

Question 47

What is arrayCGH used to detect?

Answer 47

→of sub-microscopic chromosomal abnormalities | →How many copies of a particular genomic region does the patient have

Question 48

What does arrayCGH use?

Answer 48

→Uses extracted DNA

Question 49

What does QF-PCR detect?

Answer 49

→microsatellites

Question 50

What is the nature of flanking sequence of microsatellites?

Answer 50

→same in individuals | →used to generate probes for detection

Question 51

What are the 4 steps in detecting microsatellites?

Answer 51

→Isolate DNA from individual →Design primers specific to flanking sequences →PCR amplification →Gel electrophoresis

Question 52

What are the components of PCR reaction?

Answer 52

→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

Question 53

Explain the peaks of QF-PCR

Answer 53

→If homozygous, there will be a single peak of high signal →If heterozygous, there will be two peaks of similar, lower signal

Question 54

Explain 3 peaks in trisomy after QF-PCR

Answer 54

→three different repeat lengths | →one of each copy so all three peaks have the same intensity

Question 55

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

Answer 55

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

Question 56

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

Answer 56

→Cell free fetal DNA →Trisomy testing →Next-generation sequencing