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Clinical genetics > Chromosomal Abnormalities I > Flashcards

Chromosomal Abnormalities I Flashcards

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1
Q

What can a chromosome not hold together without?

A

➝ a centromere and intact telomeres

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2
Q

What are chromosomes divided into?

A

➝ P arm and Q arm

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3
Q

How do histones bind DNA?

A

➝ they are highly positively charged proteins that are attracted to the negative charge of DNA

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4
Q

How do chromosomes exist as?

A

➝ chromatin

➝ DNA double helix bound to histones

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5
Q

How many histones form a nucleosome?

A

➝ 8

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6
Q

What is euchromatin?

A

➝ ‘open’ DNA that allows gene expression

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7
Q

What is heterochromatin?

A

➝ highly condensed and genes are not expressed

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8
Q

How many copies of each chromosome do we have?

A

➝ 2

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9
Q

What do homologs contain?

A

➝ the same genes but in allelic forms

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10
Q

Why is it important for cells to replicate?

A

➝ so they have sufficient DNA copies for daughter cells

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11
Q

When new cells are needed what happens?

A

➝ the cell cycle

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12
Q

What are the phases of the cell cycle and what happens during them?

A

➝The cell starts with a pair of homologous chromosomes existing as two chromatids (2 lines)
➝They go through G1 where various proteins are produced
➝Then they enter S phase where the DNA duplicates and you get exact copies of the single chromatids (4 lines) but still two chromosomes
➝G2 - synthesis of proteins - microtubules

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13
Q

How many chromosomes do humans have?

A

➝ 22 pairs of autosomes

➝ 1 pair of sex chromosomes

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14
Q

What is a metacentric chromosome?

A

➝ P & Q arms are even in length

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15
Q

What are the metacentric chromosomes?

A

➝ 1,2,3

➝ 16,17,18

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16
Q

What is a submetacentric chromosome?

A

➝ P arm is shorter than the Q arm

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17
Q

What are the submetacentric chromosomes?

A

➝ 4,5,6,7,8,9,10,11,12

➝19,20, X

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18
Q

What is an acrocentric chromosome?

A

➝ long Q and small P

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19
Q

What do the satellite arms code for in acrocentric chromosomes?

A

➝ rRNA

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20
Q

Why doesn’t it matter if some acrocentric chromosomes are missing?

A

➝ the satellite arms all code for the same rRNA

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21
Q

What do the P arms contain in acrocentric chromosomes?

A

➝ no new DNA

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22
Q

What are the acrocentric chromosomes?

A

➝ 13,14,15

➝ 21,22,Y

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23
Q

When are chromosomes visible and why?

A

➝ metaphase

➝ they are condensed enough to see

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24
Q

What are the two classes of chromosomal changes?

A

➝ Structural

➝ Numerical

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25
Q

What are the 3 types of numerical abnormalities in chromosomes?

A

➝ Trisomy
➝ Monosomy
➝ Mosaicism

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26
Q

What is haploid?

A

➝ one set of chromosomes

➝ N=23 as in a normal gamete

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27
Q

What is diploid?

A

➝ contains two sets of chromosomes

➝ n=46

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28
Q

What is polyploid?

A

➝ multiple of the haploid number

➝ 4n = 92

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29
Q

What is aneuploid?

A

➝ chromosome number which is not an exact multiple of a haploid number due to extra or missing chromosomes
➝ 2n+1 = 47

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30
Q

What are the 3 types of trisomy?

A

➝ Edwards
➝ Downs
➝ Patau

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31
Q

Describe what happens during meiosis?

A

➝The chromatids duplicate to form chromosomes then they duplicate again
➝Prophase - nuclear membrane is breaking down
➝Metaphase - chromosomes are aligning across the equator
➝In meiosis the homologous pairs align together allowing recombination
➝Anaphase - one of the homologous chromosomes is pulled apart to the opposite pole - DISJUNCTION
➝Telophase - new nuclear membrane form around the chromosome

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32
Q

What is the difference between meiosis and mitosis?

A

➝ in meiosis you introduce genetic variation

➝ in mitosis you want to produce identical daughter cells

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33
Q

What kind of cells are needed in mitosis?

A

➝ diploid

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34
Q

How does aneuploidy arise in meiosis I ?

A

➝If nondisjunction occurs in meiosis I
Instead of the two homologues pulling apart, they both go into a single cell in the first division
➝ two copies of one chromosome in a single cell
➝ two daughter cells

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35
Q

How does aneuploidy arise in meiosis II?

A

➝ The first division happens normally
➝ non-disjunction occurs in the second division and one cell has two chromatids
➝ 3 daughter cells

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36
Q

What is mosaicism?

A

➝ the presence of two or more genetically different cell lines derived from a single zygote

37
Q

What happens if there is meiotic non-disjunction?

A

➝ every cell is trisomic

38
Q

What happens if there is mitotic non-disjunction?

A

➝ this occurs after the generation of the zygote

➝ some cells will be trisomic and most of the cells are disomic

39
Q

What are the cells like in post-zygotic non-disjunction?

A

➝ mixture of most 2n but some 2n+1

40
Q

What is anaphase lag?

A

➝ a delayed movement during anaphase where one homologous chromosome in meiosis or one chromatid in mitosis fails to connect to the spindle apparatus or is tardily drawn to its pole and fails to be included in the reforming nucleus
➝ the chromosome forms a micronucleus in the cytoplasm and is lost from the cell and degraded (this rescues it from trisomy III)

41
Q

What are the 3 mosaic phenotypes?

A

➝ Down
➝ Klinefelter
➝ Turner

42
Q

What is a relatively common sex chromosome monosomy?

A

➝ Turners

43
Q

How does full monosomy arise?

A

➝ non disjunction

44
Q

How does Turners arise?

A

➝ Nullisomic gametes fertilised with a sperm carrying an X chromosome will be XO (Turners)

45
Q

Why are autosomal monosomies not seen?

A

➝ they are inconsistent with life

46
Q

What are the nullisomic gamete combinations and what are the outcomes?

A

➝ O + X = XO turners

➝ O + Y = lethal

47
Q

What are the disomic gamete combinations and what are the outcomes?

A

➝ XX + X = Triple X syndrome
➝ XX + Y = XXY - Klinefelters
➝ XY + X = XXY Klinefelters
➝ XY + Y = XYY syndrome

48
Q

What are the autosomal numerical abnormalities?

A

➝ trisomy 13,18,21

49
Q

What are the sex chromosome numerical abnormalities?

A

➝ XO
➝ XXY
➝ XYY

50
Q

How do you do a karyotype?

A 
➝  take venous blood
➝  add phytohaemagglutinin and culture medium
➝  culture at 37 C for 3 days 
➝  add colchicine and hypotonic saline
➝  fix cells
➝  spread cells onto a slide by dropping
➝  digest with trypsin and stain with Giemsa
➝  analyze metaphase spread
51
Q

When do you do chorionic villus sampling?

A

➝ 11-14 weeks

52
Q

What is the miscarriage rate for chorionic villus sampling?

A

➝ 0.5 - 1%

53
Q

What is chorionic villus sampling for?

A

➝ to detect transverse limb defects

54
Q

When do you do amniocentesis?

A

➝ > 16 weeks

55
Q

What is the miscarriage risk for amniocentesis?

A

➝ 0.5-1%

56
Q

What stain is used for G banding?

A

➝ Giemsa stain

57
Q

What are chromosomes lines up based on?

A

➝ Size
➝ Banding
➝ centromere position

58
Q

What does Giemsa staining do?

A

➝ highlight heterochromatic regions which are less likely to contain genes

59
Q

What can chromosomal banding be used for?

A

➝ to differentiate chromosomes and to compare them

60
Q

What is the most common type of banding?

A

➝ G banding

61
Q

What are the G bandings due to?

A

➝ Chromatin

62
Q

What are euchromatic regions rich in, how is it packed and what kind of genes does it contain?

A

➝ GC rich
➝ loosely packed
➝ active genes

63
Q

What are heterochromatic regions rich in, how is it packed and what kind of genes does it contain?

A

➝ AT rich
➝ tightly packed
➝ inactive genes

64
Q

How do you do FISH?

A

1) Fluorescent probe
2) denature the probe and target DNA
3) mix probe and target DNA
4) Probe binds to target

65
Q

How do you do quantitative fluorescence PCR to find microsatellites?

A

➝ Design primers that are specific for a microsatellite on chromosome 21
➝ They are amplified using PCR
➝ You measure how big the microsatellite is and how many copies we have of the microsatellite

66
Q

What are two invasive methods of prenatal diagnosis ?

A

➝ Amniocentesis (14-20 weeks) amniotic fluid

➝ chorionic villus sampling (11-14 weeks) placental cells

67
Q

What is a non invasive way of prenatal testing?

A

➝ Cell free foetal DNA : DNA fragments in maternal plasma (10 weeks onwards)
➝ for trisomies you still need confirmation with amniocentesis

68
Q

How do you detect aneuploidy from cell free DNA?

A

➝ Collect cell free DNA in the placenta
➝ parallel sequencing of total DNA present in the maternal plasma
➝ alignment of sequencing reads to human genome sequence and determination of relative chromosome presentation

69
Q

How do you detect monogenic disorders from cell free DNA?

A

➝ Collect cell free DNA in placenta
➝ Conventional or real time PCR using primers to genes unique to the fetus and not present in the mother
➝ detection of PCR products corresponding to fetal specific genes such as RHD

70
Q

What are the 7 clinical features of Downs?

A

➝ Hypotonia particularly in newborn period
➝ developmental delay
➝ cardiac abnormalities
➝ GI abnormalities
➝ acute lymphocytic leukaemia/acute myeloid leukaemia
➝ conductive hearing loss
➝ features of Alzheimer’s > 40 years

71
Q

How common is Downs syndrome?

A

➝ 1 in 650-1000 live births

72
Q

How common is Patau syndrome?

A

➝ 1 in 10,000 live births

73
Q

What are the 4 clinical features of Pataus?

A 
➝  Hypotelorism - abnormally close eyes
➝  holoprosencephaly - forebrain doesn't divide properly 
➝  midline cleft lip/palate
➝  scalp defects 
➝  post axial polydactyly 
➝ heart defects/renal abnormalities
74
Q

What is survival like in Pataus syndrome?

A

➝ most die by 1 month

75
Q

What is the incidence of Edwards syndrome?

A

➝ 1 in 6000 live births

76
Q

What are the 8 clinical features of Edwards syndrome?

A 
➝  Intrauterine growth retardation
➝  Micrognathia
➝  cleft lip +/- palate
➝  short palpebral fissures 
➝  fixed flexion deformities of fingers
➝  heart defect > 95% 
➝  inguinal/diaphragmatic hernia
➝  renal malformations
77
Q

What is survival like in Edwards?

A

➝ 30% die by 1 months
➝ 50% die by 2 months
➝ 90% die by 1 year

78
Q

What is the incidence of Turners syndrome?

A

➝ 1 in 4000 female births

79
Q

What is a prenatal sign of Turners syndrome?

A

➝ raised nuchal translucency

80
Q

What are the 4 clinical features of Turners syndrome at birth?

A

➝ Oedema of hands and feet
➝ neck webbing
➝ coarctation of aorta
➝ renal malformation

81
Q

What are the 2 clinical features of Turners syndrome that present later?

A

➝ Short stature

➝ infertility secondary to gonadal dysgenesis

82
Q

What happens in XY mosaicism?

A

➝ usually normal male but with potential for gonadoblastoma

83
Q

How does Variant Turner syndrome arise?

A

➝ mosaicism with ring or isochrome Xq

84
Q

What is the incidence of Klinefelters?

A

➝ 1 in 1000 male births

85
Q

What is present in Klinefelters?

A

➝ Barr bodies

86
Q

How does Klinefelters arise?

A

➝ NDJ paternal meiosis I

➝ others NDJ maternal meiosis or zygotic mitotic error

87
Q

What are the clinical features of Klinefelters?

A

➝ present during childhood with behavioral problems
➝ adult infertility
➝ tall stature
➝ eunuchoid body habitus
➝ behavioral and minor learning difficultires

88
Q

How do you treat Klinefelters and why?

A

➝ testosterone

➝ lack of secondary sexual characteristics

Clinical genetics (5 decks)

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