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POM GENETICS > Chromosomal Abnormalities > Flashcards

Chromosomal Abnormalities Flashcards

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Biology 101 flashcards
1
Q

What is meant by the karyotype?

A

It is the chromosome set of an individual species in terms of number and structure of chromosomes in the cell nuclei

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2
Q

How do you prepare a karyotype?

A
  1. Collect roughly heparinized venous blood (can use amniotic cells, CVS)
  2. isolate white cells
  3. Culture in presence of phytohaemagglutinin (stimulates T-lymphocyte growth/ differentiation)
  4. After 48 hours add colchicine (causes mitotic arrest- metaphase)
  5. Place in hypotonic saline
  6. Place on slide
  7. Fix and stain
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3
Q

What are ideograms?

A

Graphical representations of chromosomes
Chromosomes have some common structural features

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4
Q

How many chromosomes do humans have?

A

22 pairs of normal chromosomes, 1 pair of sex chromosomes
46, XX

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5
Q

How are bands indentified?

A

Bands caused by differently stained
Bands originally identified with low level of resolution (only a few bands visible per chromosome e.g., 1,2,3)
Improved tech more bands visible
Named as sub-bands e.g., 11,12,21,22,23 etc
Further improvements leads to sub-sub-bands e.g., 11.1, 11.2 etc

Improved resolution helps identify smaller aberration

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6
Q

What does bphs stand for?

A

bands per haploid set (bands DO NOT represent genes or families of genes)

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7
Q

What stain do you use for karyotypes?

A

giemsa

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8
Q

Right now, for karyotypes what stage of mitosis is preferred?

A

prophase (over the usual metaphase)

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9
Q

What is giesma staining the basis of?

A

basis of nomenclature

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10
Q

What is another method of staining karyotypes?

A

Karyotype fluorescent stain

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11
Q

What is a nucleosome?

A

When the DNA is wrapped around the histone proteins

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12
Q

When nucleosomes condense, what is formed?

A

A chromatin fibre

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13
Q

What is formed when chromatin condenses?

A

Chromosome

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14
Q

How do very long DNA molecules fit into the cell?

A

The DNA is wrapped around histones, forming nucleosomes which are then coiled into the chromatin fibre which is then further condensed to fit into the chromosome

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15
Q

Which cells do not contain chromatin?

A

Red blood cells

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16
Q

During what phase of mitosis are chromsomes usually karyotypes and why?

A

During metaphase, as the chromosomes are more condensed

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17
Q

Which cells are commonly chosen to be karyotyped and why?

A

White blood cells as they easily enter into mitosis

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18
Q

What is the short arm of a chromosome called?

A

The p arm

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19
Q

What is the long arm of the chromosome called?

A

The q arm

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20
Q

What connects the p arm with the q arm?

A

A centromere

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21
Q

What is added when karotyping to prevent mitosis from progressing past the metaphase?

A

Colchicine

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22
Q

How are the chromosomes released from the cell?

A

During karyotyping, a hypotonic solution is added, which causes the cell to swell and burst, releasing all the chromosomes

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23
Q

What is used during karyotyping to fix the chromosomes in place on the glass slide?

A

Carnoys fluid

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24
Q

what is carnoys fluid?

A

A combination of acetic acid and ethanol used to fix the chromosomes in place

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25
Q

What are the tips of the chromosome called?

A

Telomere

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26
Q

What does giemsa staining produce?

A

Recognizable banding patterns on the chromosomes

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27
Q

How are the bands on chromosomes numbered?

A

The bands closest to the centromere is 1, and they increase in number the further you get from the centromere

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28
Q

What do dark bands on the chromosomes represent (G-dark)?

A

Heterochromatin - this is where the DNA is wrapped more tightly around the histone proteins and is more condensed

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29
Q

What do light bands on chromosomes represent (G-light)?

A

Euchromatin - where the DNA is not packed as tightly around the chromatin so it is more open

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30
Q

What is meant by metacentric chromosomes?

A

Where the p arm and the q arm are the same length

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31
Q

What are submetacentric chromosomes?

A

Where the p arm is shorter than the q arm

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32
Q

What is meant by acrocentric chromosomes?

A

Where the short p arms have been reduced to a stump and replaced by non-coding satellites

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33
Q

What type of translocation can acrocentric chromosomes undergo?

A

Robertsonian translocations

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34
Q

What is a robertsonian translocation?

A

Where there is translocation between two acrocentric chromosomes which results in the formation of one chromosome with normal lengthed arms, and one where the two satellite sections have combined = loss of chromosome

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35
Q

Which chromosomes do Robertsonian translocations usually occur in?

A

13, 14, 15, 21 and 22

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36
Q

What is meant by a de novo abnormality?

A

A chromosomal abnormality which is not inherited from the parents

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37
Q

During what stage does cross-over occur?

A

During prophase of meiosis 1

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38
Q

What is the purpose of meiosis?

A

To reduce the diploid number to haploid so that diploid zygotes can be formed

To ensure there is genetic variation amongst off spring

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39
Q

What is the purpose of mitosis?

A

For growth, repair and replace exhausted daughter cells - helps to maintain diploid number

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40
Q

Describe how crossing over occurs?

A

The pairs of homologous chromosomes align, chiasma form and sections of DNA are exchanged

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41
Q

What is a recombinant chromatid?

A

A chromatid in which sections of DNA have been swapped during cross over

42
Q

What is monosomy?

A

A loss of a chromosome - so now the chromosome only consists of one chromatid

43
Q

What is trisomy?

A

Where there is a gain of a chromosome, 3 copies of chromosomes

44
Q

What is the purpose of meiosis?

A

To achieve reduction from diploid (46->23)
To ensure genetic variation in the gametes
Enables random assortment of homologues and recombination

45
Q

What is the most common cause of numerical chromosomal abnormalities?

A

Non-dysjunction

46
Q

What is non-dysjunction?

A

The failure of homologous chromosomes to separate properly during cell division

47
Q

When does non-dysjunction occur?

A

Meiosis I= all daughter cells affected
Meiosis II= half affected

Always occurs in either +1 or -1 chromosome

48
Q

What does aneuploidy mean?

A

abnormal number of chromosomes

49
Q

What are the three stages in which chromosomal abnormalities can occur?

A

Mitosis, meiosis 1 or meiosis 2

50
Q

What is Trisomy 21?

A

Down syndrome

51
Q

What causes trisomy 21?

A

failure of the 21st chromosome to separate during egg or sperm development
(maternal non-disjunction)

52
Q

What can be used to rapidly detect pre-natal Trisomy?

A

Quantitative Flourescence PCR

53
Q

What is the cause of 85-90% of trisomy 21?

A

Maternal dysjunction

54
Q

What increases the risk of maternal dysjunction?

A

Increasing maternal age

55
Q

Why does maternal non-dysjunction increase with age?

A

Vulnerability of oogenesis
Paused in utero in prophase I until puberty
Secondary oocyte arrests in metaphase II
Only competes if fertilized
One primary oocyte yields only one ovum
Finite number of primary oocytes

56
Q

When does most aneuploidy caused by non-disjunction arise?

A

Oogenesis
Likely due to degradation of factors which hold homologous chromatids together

57
Q

What is the paternal age effect?

A

Vulnerability of male meiosis
No equivalent to oocyte mitotic arrest
Primary spermatocytes undergo roughly 23 mitotic divisions per year and potentially accumulate defects

Paternal age is not a risk factor for increased aneuploidy

58
Q

What is a risk factor for aneuploidy in males?

A

Smoking

59
Q

What is the karyotype of someone with trisomy 21?

A

47 + 21, or 47, XX+21

60
Q

What causes turners syndrome?

A

X monosomy

61
Q

What causes Kleinfelder’s disease?

A

XXY - trisomy where you have an extra X chromsome

62
Q

What is the most common form of aneuploidy?

A

Sex chromosome imbalance

63
Q

How does the body tolerate an imbalance in sex chromsomes?

A

(X-inactivation) The excess X chromosomes get inactivated
Low gene content of Y chromosomes

64
Q

How are excess X chromosomes silenced?

A

It is packed into a transcriptionally inactive heterochromatin

65
Q

What is a Barr body?

A

An inactive X chromosome

66
Q

If excess X chromosomes are silenced, why can some XXY and XXX have effects?

A

Some genes may be located in the PAR regions - genes in this region behave like autosomal genes and therefore are not inactivated like the rest of the X chromosome

67
Q

Are trisomies compatible with life?

A

Most trisomies are not compatible with life

68
Q

What is mosaicism?

A

Presence of 2 or more populations of cells with different genotypes
X-inactivation results in mosaic expression
Results in generally milder phenotype

69
Q

Is everyone thought to be mosaic?

A

yes

70
Q

Is there a way that aneuploidy is more survivable?

A

Yes, if they are mosaic

71
Q

What are PAR regions?

A

Pseudoautosomal regions - contain pseudoautosomal genes that behave like autosomal genes and therefore are not silenced when excess X or Y chromosomes are

72
Q

What causes single chromosome abnormalities?

A

Can be the result of unequal crossover

Breaks in chromosome

Can occur at ends of chromosomes (or around centromere)

73
Q

What do single chromosome abnormalities cause?

A

Can cause reproductive problems
Carriers are often unaffected

74
Q

What is it which allows crossing over between X and Y chromosomes?

A

Psuedoautosomal regions

75
Q

What translocation results in the formation of the Philadelphia gene?

A

t(9;22)(q34;q11)

76
Q

What condition can the fusion of the ABL and BCR gene lead to?

A

Chronic Myeloid Leukemia

77
Q

What is haploinsufficiency?

A

A dominant phenotype in diploid organisms that are heterozygous for a loss of function allele

aka an allele is inactivated so the gene overall does not function

78
Q

What is genomic imprinting?

A

Changes to the DNA or chromatin that are inherited but do not affect the DNA sequence

the process by which only one copy of a gene in an individual (either from their mother or their father) is expressed, while the other copy is suppressed.

79
Q

How is genomic imprinting achieved?

A

Through DNA methylation

80
Q

Carriers of balanced translocations have increased risk of what?

A

Creating gametes with unbalanced chromosome translocations, leading to infertility, miscarriages or children with abnormalities

81
Q

What is the severity of the phenotype dependant on?

A

The gene content of the affected segment

82
Q

Is trisomy or monosomy usually more tolerated?

A

Trisomy

83
Q

What are some signs and symptoms of patients with Williams syndrome?

A

Long philtrum - space between nose and lip

Short, upturned nose

Arched eyebrows

Funny, cocktail party personalities

84
Q

Why is Williams syndrome detected using targeted FISH (fluorescent in situ hybridisation)?

A

The deletion is too small to be detected using karyotyping

85
Q

What are the three types of structural chromosomal abnormalities that can occur?

A

Deletion, duplication and inversion

86
Q

What can be seen when higher levels of karotyping resolution is done?

A

Sub-bands and sub-sub bands

87
Q

Most aneuploidy caused by non-disjunction arises in what process?

A

Oogenesis

88
Q

Is paternal age a risk factor for aneuploidy?

A

No

89
Q

What three conditions are affected by the paternal age effect?

A

Apert syndrome
Crouzon syndrome
Pfeiffer syndrome

90
Q

How many times per chromosome per cycle of meiosis does cross over occur?

A

1-3 times

91
Q

When does crossing over occur and what does it do?

A

Prophase I
Increases genetic diversity
Pairs of chromosomes align
Chiasma form and crossover occurs

92
Q

What does unequal crossing over cause?

A

Duplication or deletion

93
Q

What syndrome is 7q11.23 deletion?

A

Williams syndrome

94
Q

What are some signs and symptoms of 7q11.23 duplication syndrome?

A 
Delayed speech development
Autistic behaviours that affect social interaction and communication
Dilatation of the aorta
Flat eyebrows
Broad nose and short philtrum
95
Q

what are the two causes of mosaicism?

A

Mitotic disjunction during embryonic development

Loss of extra chromosome in early development

96
Q

What is paracentric inversion?

A

Inversion not around the centromere

97
Q

What is pericentric inversion?

A

Inversion of a section of the chromosome, around the centromere

98
Q

What is an example of a disease of chromosomal deletions?

A

Cri-du-chat syndrome (46,XY,del(5p))
and
Velocardiofacial/ DiGeorge syndrome (22q11.2 del)

They are microscopic, easily detectable in microscope

99
Q

How do you see micro-deletions?

A

seen in very high resolution banding; molecular genetics

100
Q

What are the classes of chromosome?

A

Metacentric= short arm p, long arm q (same length)

Submetacentric= short arm is shorter

Acrocentric= stalk and satellite instead of short arm

101
Q

What is Robertsonian translocation?

A

Occurs between acrocentric chromosomes
Can be homologous or non homologous
Most common 13 and 14; 14 and 15; 14 and 21
Most people show no effects
Can cause problems in offspring

POM GENETICS (9 decks)

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