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Medical genetics > Chromosomes > Flashcards

Chromosomes Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

Describe two arms of chromosomes

A

Short, p arm

Long, q arm

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2
Q

How are types of chromosomes classified? Give examples

A

Where centromere is:

Acrocentric: centromere at end
Metacentric: centromere in the middle

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3
Q

What is at the tip of chromosomes?

A

Telomeres

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4
Q

What do telomeres consist of?

A

TTAGGG tandem repeats (highly conserved)

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5
Q

What purposes do telomeres serve?

A

Prevent chromosome shortening during replication

Differentiates chromosome ends from double strand breaks preventing end to end joining

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6
Q

What replicates telomeres?

A

Telomerase

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7
Q

How many homologous autosomes do humans have?

A

22

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8
Q

What is meant by karyotype?

A

Number and appearance of chromosomes in nucleus of eukaryotic cell

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9
Q

Describe method for G banding

A

1) Denature protein with trypsin

2) Stain with Giesma (DNA binding dye) -active/transcribed areas stain lightly due to less condensed chromatin

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10
Q

What are chromosomes arranged on after G banding?

A

Karyogram

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11
Q

What does FISH allow?

A

Used to identify a known mutation e.g. trisomy 21

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12
Q

What is the resolution of karyotyping?

A

6-8mb of NDA

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13
Q

What is the method for FISH?

A

Single stranded DNA (probe) to anneal to target sequence on metaphase spread

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14
Q

When is chromosome painting used?

A

When you don’t know what you’re looking for (i.e. a generic translocation)

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15
Q

Describe method for chromosome painting

A

Mixture of probes specific for each chromosome

These are colour labelled and anneal

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16
Q

What is comparative genome hybridisation array used for

A

Detect regions of gene amplification/gene loss

Allow smaller bits of chromosomes to be identified (not normally seen on metaphase plate, higher resolution)

Can identify unusual chromosome patterns in cancers

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17
Q

What is the method for Array CGH?

A

Test DNA labelled with red paint

Normal control DNA labelled with green paint

DNA hybridised

Gene amplification: DNA shows red

Gene loss: DNA shows green

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18
Q

What chromosome identification method is used for dysmorphic individuals (unusual appearance)?

A

CGH used to detect chromosome abnormalities

Should check parents for abnormality to see if these gene abnormality is responsible for phenotype

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19
Q

What is sex detrmining region of Y chromosome?

A

SRY gene

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20
Q

What does presence of SRY on Y chromosome mean?

A

Maleness regardless of how many X chromosomes present

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21
Q

How deos SRY lead to male differentiation (and how is female differentiation inhibited)?

A

Leads to testis production from Wolffian ducts. Testis sertoli cells make Mullerian inhibtory factor (inhibits female genitalia production)

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22
Q

What is the genotype of Turner’s syndrome?

A

45X (XO)

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23
Q

What are the symptoms of Turner’s syndrome?

A

Webbed neck, infertility, broad chest, no puberty, wide carrying angle

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24
Q

What is the genotype for Klinefelter’s syndrome?

A

47XXY (XXY)

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25
Q

What are symptoms of Klinefelter’s disease?

A

Breasts, infertile, slight ID and testis atrophy

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26
Q

What are effects of 47XXX genotype?

A

Female: Slight ID, tall

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27
Q

What is XYY genotype associated with?

A

Male: Aggression

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28
Q

Compare the amounts of products of X linked Gene products in males and females, why?

A

Same

Dosage compensation - inactivation of all but one X chromosome

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29
Q

What does the lyon hypothesis state about X inactivation in somatic cells of female?

A

Occurs in embryonic life

Is random (either X chromosome)

Permanent + complete

Clonally propagated through mitosis

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30
Q

Why is the Lyon hypothesis not always correct?

A

Inactivation not always random (structurally abnormal X preferentially inactivated)

Inactivation not complete

Inactivation not permanent (reversed in development of germ cells)

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31
Q

Where is inactivated X chromosome found?

A

Barr body

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32
Q

How many Barr bodies would 48 XXXX have, why?

A

3 Barr bodies

3 Inactive X chromosomes (one barr body for each inactive X)

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33
Q

How do Barr bodies appear histologically?

A

Dark nucleus-like structure on the cell periphery

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34
Q

What are structural chromosomal abnormalities (6)?

A 
Translocation
Deletion
Insertion
Inversion
Ring
Isochromosome
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35
Q

What are examples of numerical abnormalities (2)?

A

Aneuploidy

Polyploidy

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36
Q

What is the difference between aneuploidy and polyploidy?

A

Aneuploidy: Numerical change in cell’s usual chromosomes (i.e. loss/gain of one chromosome)

Polyploidy: Numerical change in cell’s usual chromosomal sets (i.e 3 chromosomes in every set)

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37
Q

How can trisomy caused by non disjunction in meiosis II?

A

Sister chromatids don’t separate thus gamete contains 2 copies of one homologue

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38
Q

How can trisomy be caused by non disjunction in meiosis I?

A

Pair of homologous chromosomes fail to separate thus gamete contains both homologoues of one chromosome pair

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39
Q

What is the difference between a trisomy non-disjunction at meiosis I and II?

A

I - Contains both homologoues of one chromosome pair

II - Contains 2 copies of one homologue

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40
Q

What is mosaicism?

A

Presence in cell line of two or more cell lines with different genetic constitution but from same zygote.

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41
Q

What is the trisomy for Down Syndrome?

A

21

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42
Q

What can cause down syndrome?

A

Robertsonian translocation

Non-disjunction

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43
Q

What are symptoms of down syndrome?

A

Alzheimers, ID, cardiac problem widended sandal gap, leukaemias

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44
Q

What is a risk factor for trisomy (based on mother)?

A

Old maternal age

Spindle less likely to form

Non-disjunction

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45
Q

What is the trisomy in Patau syndrome?

A

13

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46
Q

What is the trisomy in Edward’s Syndrome?

A

18

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47
Q

Symptoms of Pautau syndrome

A

Mental retardation
Clefting
Finger and toe abnormality
Cyclopia

48
Q

Symptoms of Edward’s syndrome

A

Rocker bottom feet, overlapping fingers, clenched fingers

49
Q

What causes monosomy?

A

Non disjunction

Anaphase lag (loss of chromosome as it moves to pole of cell in anaphase)

50
Q

What else does meiotic division produce to form monosomy cell?

A

Trisomic cell

51
Q

What is polyploidy?

A

Addition of one or more complete haploid complements e.g. trisomy - 69 chromosomes

52
Q

How does polyploidy arise?

A

Failure of maturation meiotic division in ovum/sperm gives diploid gamete

Fertilisation of ovum by 2 sperm

53
Q

What is a translocation?

A

Transfer of genetic material from one chromosome to another

54
Q

What are 2 types of translocation?

A

Balanced: no net gain/loss of material (generally harmless)

Unbalanced: incorrect amount of chromosome material (abnormalities arise)

55
Q

What happens in a reciprocal translocation? Give an example

A

Break in 2 separate chromosomes, segments exchanged between 2 chromosomes to form 2 derivative chromosomes.

i.e Philadelphia chromosome from 9 and 22 in CML

56
Q

What happens in a Roberstonian translocation?

A

Translocation where breakpoints are close to centromeres of 2 acrocentric chromosomes.

Followed by fusion of long arms

This results in the loss of short arms

Can occur between homologous or non-homologous chromosomes

57
Q

What sort of chromosomes are affected by robertsonian translocation?

A

Acrocentric chromosomes

58
Q

What are 2 types of robertsonian tranlocation?

A

Homologous acrocentrics or non homologous acrocentrics involved

59
Q

What happens to short arms in robersonian translocation?

A

Lost, (no clinical effects)

60
Q

What is the negative consequence of roberstonian translocation on gametes?

A

Can produce imbalanced chromosome complement

61
Q

Describe a balanced roberstonian translocation

A

Long arms of chromosome 14 and 21 joined, along with normal 14 and normal 21 (i.e. equivalent to having separate copy of chromosome 21 and 14) therefore 2 copies of each and normal

62
Q

Describe how a Robertsonian translocation can lead to trisomy 21

A

2 copies of 21
Long arms of 14 and 21 joined
1 other copy of 14

2 copies of 14 hence normal
But 3 copies of 21 hence Down syndrome

63
Q

What is an isochromosome?

A

One arm of chromosome lost and other arm duplicates to replace it

Trisomy of duplicated arm and monosomy of lost arm

64
Q

What is a ring chromosome?

A

Double deletion mutation (in both p and q arms) new ends join together to form ring , acentric fragments lost

65
Q

What does ring chromosome 14 lead to?

A

Epilepsy, MR and craniofacial abnormality

66
Q

What happens in an inversion?

A

2 breakages within chromosome and central segment reversed 180 degrees and reinsert into chromosome

67
Q

What Is the difference between a paracentric and pericentric inversion?

A

Pericentric: involve p and q arm (more severe)
Paracentric: only involves one arm

68
Q

What is an insertion (chromosome mutation)?

A

DNA inserted into chromosome e.g into regulatory element uncontrolled cell division.

69
Q

What are different types of chromosome deletions? Examples?

A

Large deletion e.g. Wolf Hirschhorn

Microdeletion: DiGeorge, WAGR, williams syndrome

70
Q

What normally causes mosaicism?

A

Non disjunction in early embryonic mitosis division

i.e. can have mosaic Down syndrome (some cells normal some cells have trisomy 21)

71
Q

What is chimerism?

A

Present in individual of 2 or more genetically distinct cell lines derived from more than one zygote (i.e. different genetic origin)

72
Q

What are causes of chimerism?

A

2 sperm, 2 eggs, one embryo

Blood chimeras: (exchange of cells between non identical twins via placenta in utero)

73
Q

What is a point mutation?

A

Substitution of a base

74
Q

How does point mutations arise?

A

Spontaneous errors in DNA replication

Repair/error due to mutagens

75
Q

What are 2 types of point mutation, what do they involve?

A

Transition: purine to purine or pyrimidine to pyrimidine

Transversion: purine to pyrimidine or vice versa

76
Q

What is a synonymous mutation?

A

Doesn’t change amino acid

77
Q

What is a non-synonymous mutation?

A

Changes amino acid

78
Q

What are two types of non synonymous mutation?

A

Missense (replace one AA with another)

Nonsense: codon specifying AA replaced by stop codon

79
Q

What is an insertion/deletion mutation?

A

When other than 3 bases - frame shift (disrupt coding sequence)
Often produces premature stop codon

80
Q

How can mutation (e.g. deletion, insertion, point mutation) in non coding region be bad?

A

Interfere with binding of TF to this region - reduce transcription of coding gene

81
Q

What happens to trinucleotide repeats in replication?

A

Become unstable during replication can lead to amplification of more repeats (expansion)

Disease manifest when you pass certain threshold

82
Q

How do trinucleotide repeat quantity change over generations , what’s the effect?

A

Expansion progressive down generations (amplification in gametogenesis) so more severe phenotype with successive generations and more early onset diseases = anticipation

83
Q

What is meant by anticipation?

A

Earlier onset + more severe phenotype going down generations

84
Q

What types of cells does expansion occur in?

A

Somatic AND germ line cells

85
Q

What causes expansion?

A

Strand slippage in DNA replication

86
Q

What are the effects of amplification in non coding region?

A

Interfere with transcription, induce methylation and non functional promoter

87
Q

Example of non coding region trinucleotide expansion

A

Fragile X syndrome

Myotonic dystrophy

88
Q

What causes Fragile X syndrome?

A

Expansion of CGG

ID and large head/testes in boys, and female: premature ovarian follicle

89
Q

What expansion causes myotonic dystrophy?

A

CTG repeat

90
Q

What disease caused by amplification in coding regions?

A

Huntington’s disease

91
Q

What is ineritance pattern of Huntingdon’s disease?

A

Autosomal dominant

92
Q

What is the expansion in Huntingdon’s disease?

A

CAG repeat

93
Q

After how many repeats does Huntingdon’s manifest?

A

> 60 repeats: onset before 25

> 35 repeats: positive predictive test

94
Q

Describe penetrance of Huntingdon’s disease?

A

100%

95
Q

What are symptoms of Huntingdon’s disease?

A

Progressive dementia, unsteady gait and jerky involuntary movement

96
Q

What sort of point mutations can occur in non coding regions?

A

mRNA regulatorty elements (affect gene expression)

Mutation in splice sites (intron retention or exon skipping)

97
Q

Most loss of function mutations are…

A

Recessive (cell can still function with 50% if gene product)

98
Q

What is haploinsufficiency?

A

When 50% gene product not sufficient so abnormal phenotype

99
Q

How can dominant negative effect arise from loss of function mutation?

A

Mutant protein lost function and interferes with protein of normal allele

100
Q

Example of loss of function mutation?

A

Waardenburg syndrome

101
Q

Most gain of function mutations are…

A

Dominant

102
Q

Example of gain of function mutations

A

Huntington, Myotonic dystrophy and achondroplasia

103
Q

What causes Prader Willi syndrome?

A

Failure of paternally inherited SNPRN gene

104
Q

What can cause imprinted gene diseases?

A

Mutation, methylation abnormality, uniparental disomy (i.e. 2 copies of gene from dad no maternal one)

105
Q

In down syndrome do all affected individuals have three separate copies of 21?

A

No

106
Q

What may increase risk for down syndrome (2)?

A

Maternal age

Family carries a balanced translocation of chromosome 21

107
Q

What is a non-transmissible disease, how can it occur?

A

Cannot be passed from one human to another. Somatic mutation

108
Q

Does mitochondrial DNA encode all the mitochondrial respiratory chain?

A

No

109
Q

Does mitochondrial DNA have a high mutation rate?

A

Yes

110
Q

Is mitochondrial DNA present in multiple copies in each cell?

A

Yes

111
Q

What is a chromosome abnormality incompatible with foetal life?

A

Autosomal monosomy

112
Q

What is the smallest chromosome?

A

Male Y chromosome

113
Q

Females are mosaics expressing either the maternal or paternal X-chromosome in different cells

A

True

114
Q

Part of the Y-chromosome genome is homologous with part of the X-chromosome

A

True

115
Q

Normal females inactivate the same X chromosome in the nucleus of all somatic cells.

A

False

Medical genetics (6 decks)

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