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Genetics > Principles of cytogenetics > Flashcards

Principles of cytogenetics Flashcards

1
Q

Cytogenetics

A

study of chromosomes, their structure and inheritance, as applied to the practice of medical genetics.

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2
Q

Chromosome sources

A

T-lymphocytes, chorionic villi, amniotic fluid. Arrest cells in metaphase since they are condensed and separated

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3
Q

Chromosome nomenclature

A

chromosome arms (p1, p2, q1,q2 etc) divided into regions counting outward from the centromere. Regions are divided into bands labeled p11, p12 . . . sub-bands: p11.1, p11.2 . . .sub-sub bands p11.21 p11.22 etc

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4
Q

Metacentric

A

centromere near central region of chromosome

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5
Q

Submetacentric

A

centromere is off center and arms are clearly of different lengths

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6
Q

Acrocentric*

A

centromere near one end (13, 14, 15, 21, 22, Y)-must know! Contain small distinctive masses of chromatin called satellites attached to their short arms by narrow stalks which contain hundreds of copies of ribosomal RNA genes. p arm consists only of rRNA; q arm will hold the important genes.

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7
Q

Telocentric

A

centromere at one end and only has a single arm ( does not occur in a normal human karyotype-unless mutation loss of p arm)

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8
Q

Giemsa banding (G banding)

A

Most common method;pro/metaphase cells treated with trypsin to digest proteins->Giemsa stain->produces light and dark bands. (400bands)

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9
Q

Quinacrine mustard (Q banding)

A

stain with fluorescent quinacrine mustard. Examine by fluorescence microscopy. Bright Q bands correspond exactly to dark G bands. Used to detect heteromorphisms/variants in chromosome morphology. reflect diffs in satellite DNA amts

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10
Q

R banding

A

“Reverse banding” chromosomes receive special treatment such heating before staining. Dark/light band pattern opposite of that observed by G or Q banding. Used to examine regions that stain poorly by G or Q banding. Common in Europe-abns clearly visualized.

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11
Q

T banding

A

“Telomere” IDs subset of R bands esp concentrated at telomeres. Exteme heat treatment before staining with Giemsa-combo of dyes and fluorochromes.

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12
Q

C banding

A

Staining CENTROMERIC REGIONS of each chrome and other regions containing CONSTITUTIVE HETEROCHROMATIN

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13
Q

High-Resolution banding (Prometaphase banding)

A

G or R banding of chromosomes from a prophase or pro metaphase cell. Less condensed chromatin useful to detect subtle structural abns in a chromosome. (850)

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14
Q

Fluorescence In Situ Hybridization (FISH)

A

CHROMOSOMAL PAINTING-probes for entire chromosome or arm. SPECTRAL KARYOTYPING-can analyze multiple targets simultaneously utilizing diff fluorochromes. Cell culture still req’d, but no need to lyse, just add fluorochromes. Locus specific, satellite DNA, Chromosome paint.

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15
Q

Spectral karyotyping

A

Use diff colors/fluorochromes for all 24 chromosomes-> can ID abns.

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16
Q

Comparative Genome Hybridization (CGH)

A

small changes in # of copies of a DNA segment can be identified using CGH, measures the difference b/w 2 diff DNA samples in copy, or dosage of a particular DNA segment.

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17
Q

Microarray analysis

A

CGH adds info to karyotype, but doesn’t replace. Assess relative copy #, complements karyotyping, more sensitive, analyzes whole genome, but does not give info about structural changes i.e. translocations, rearrangements

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18
Q

Chromosome Abnormalities

A

Numerical or Structural, may involve autosomes or sex chromes or both.

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19
Q

*Euploidy

A

chromosome # that is a multiple of n (23)
Haploid is n=23
Diploid is 2n=46; normal somatic cells

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20
Q

*Heteroploidy

A

any chromosome complement other than 46.
Triploid (3n) and Tetraploid (4n)
euploid and heteroploid at the same time-not compatible with life-seen in abortion.

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21
Q

Triploid

A 
3n, often due to fertilization by 2 sperm
-failure of meiotic division
-partial hydatidiform moles->
  extra paternal -> abnormal placenta
  extra maternal-> spontaneously aborted
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22
Q

Tetrapoid

A

4n; failure to complete an early cleavage division of the zygote.(meiosis I)

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23
Q

Aneuploidy

A

Not a multiple of 23 or haploid #. A result of MEIOIC NONDISJUNCTION.
Monosomy, trisomy
monosomies and trisomies of 6 chromosomes are more compatible with life.

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24
Q

Monosomy

A

autosomal cell with only 1 copy of a chromosome-lethal
monosomy of sex chromosome-can survive
monosomy of autosome-death of zygote

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25
Q

Trisomy

A

3 copies of a chromosome- potentially lethal

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26
Q

Nondisjunction

A

failure of a pair of chromosomes to disjoin properly during one of the 2 meiotic divisions. usually meiosis I.

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27
Q

Nondisjunction during meiosis I

A

gamete with 24 chromosomes has both maternal and paternal members of pair

28
Q

Nondisjunction in meiosis II

A

gamete with 24 chromosomes has either maternal or paternal copies of the pair

29
Q

Nondisjunction in mitosis

A

Mosaicism=2 DIFF GENOTYPES DERIVED FROM 1 CELL TYPE->zygote->cleavage divisions->morula->then cells divide and will always have the nondisjunction. So some cells will have trisomies and some will have normal.

30
Q

Abnormalities of chromosome structure: result from . . .

A

result from chromosome breakage followed by reconstitution in an abnormal combination.
Less common, spontaneous, induced by BREAKING AGENTS (clastogens, viral infections, ionizing rad, chems)
may be in all cells or mosaic
balanced-> chrome set has normal complement of chromosomal material
unbalanced-> additional or missing material

31
Q

Abnormalities of chromosome structure: stablel rearrangements

A

Stable: capable of passing unaltered through mitotic and meiotic divisions. STABILITY REQS NORMAL STRUCTURAL ELEMENTS-including a fxnl CENTROMERE and 2 TELOMERES.

32
Q

Abnormalities of chromosome structure: Unbalanced rearrangements

A

Phenotype likely to be abn due to deletions, duplications, or both. Important class->submicroscopic changes involving telomeres.
Idiopathic Mental retardation: may be diagnosed by targeted cytogenetic analysis of telomeric regions by FISH

33
Q

Unbalanced rearrangements:deletions

A

clinical consequences due to haploinsufficiency.
TERMINAL or INTERSTITIAL
due to chromosomal breakage and loss of acentric segment, unequal crossing over, abn segregation of a balanced translocation or inversion. detected by FISH or high-resolution banding

34
Q

Unbalanced rearrangements:duplications

A

results in partial trisomy

due to unequal crossing over, ban segregation of a translocation or inversion.

35
Q

Unbalanced rearrangements: marker and ring chromosomes

A

small unidentified chromosomes often seen in mosaicism. supernumerary chromosomes or extra structurally abn chromes. Dx with FISH. Commonly seen with chrome 15 and sex chromes.

36
Q

Unbalanced rearrangements: marker and ring chromosomes. NEOCENTROMERE

A

Neocentromere: subclass of marker chromosomes; small fragments of chromosomes that do not have centromeric seqs; mitotically stable, has acquired centromere activity

37
Q

Unbalanced rearrangements: Ring Chromosomes

A

marker chromes without telomeres; formed by loss of terminal ends of both arms and the ends join together; rare-mitotically stable.

38
Q

Unbalanced rearrangements: Isochromosomes

A

chromes in which one arm is missing and the other duplicated in a mirror image fashion.
misdivision through centromere in meiosis II; exchange involving one arm of a chrome and its homolog at the proximal edge of the arm adjacent to centromere. MOST COMMONLY OBSERVED: ISOCHROMOSOME X(involving q arm)

39
Q

Unbalanced rearrangements: Dicentric chromosomes

A

Rare: 2 chrome segs each with a centromere fused end to end.
Psuedocentric: can be mitotically stable if the 2 centromeres are coordinated in their movement during anaphase or one centromere is inactivated.

40
Q

Balanced rearrangements

A

usually no phenotypic effect in carriers; likely to produce unbalanced gametes -> increased risk of abn offspring.
Ex: X-linked diseases in female carriers of balanced X: autosome translocations

41
Q

Balanced rearrangements: Inversions

A

single chromosome undergoes 2 dbl strand breaks and is reconstituted with the segment b/w the breaks inverted.

42
Q

Balanced rearrangements: Inversions-Paracentric

A

Not involving centromere-> unbalanced gametes with a centric or dicentric recombinant chromes not viable-> risk of live born with karyotype is low

43
Q

Balanced rearrangements: Inversions-Pericentric

A

Involves centromere->unbalanced gametes with duplication and deficiency of segments distal to inversion. Increased risk of abn offspring

44
Q

Pericentric inversions of Chormosome 3

A

Carriers of inv(3) are normal, but offspring have characteristic abn phenotype.

45
Q

Pericentric inversion of chromosome 8

A

carriers of inv(8) have 6% chance of child with recombinant 8 syndrome->lethal disorder with severe cardiac abns and mental retardation.

46
Q

Most common Pericentric inversion

A

inv(9)(p11q12) . . . considered a normal variant

47
Q

Balanced rearrangements: Translocations

A

Most important type! Exchange of chromosome segments b/w non-homologous chromosomes (i.e.9 and 10)

48
Q

Reciprocal Translocation (balanced)

A

break in 2 diff chromes and material exchanged->unbalanced gametes and abn progeny.

49
Q

Robertsonian Translocation

A 
Long arms (q arm) of 2 homologous *acrocentric chromes- fuse at the centromere forming a single chromosome and p arm is lost. 
Balanced karyotype with 45 chromes.
Clinically->carriers for chromosome 21 have risk of producing child with Down syndrome-young mothers
50
Q

Balanced rearrangements: Insertions

A

nonreciprocal, segment removed from one chrome is inserted into a diff chorme. Reqs 3 breaks. 50% risk of abn offspring.

51
Q

Mosaicism

A

2 or more chromosome complements in an individual-> structural-> commonly nondisjunction in a post zygotic mitotic division.
Abns can be seen in all tissues, but only some of the cells exhibit the abnormal phenotype.

52
Q

Pseudomosaicism

A

mosaicism that arose in culture

53
Q

Spontaneous abortions

A

20% of cases 45,X . . . overall rate is 15%

54
Q

Genomic imprinting (unusual inheritance patterns)

A

diffs in gene expression b/w the allele inherited from the mother and the allele inherited from the father. Caused by changes in chromatin like methylation or cytosines, modification or substitution of specific histone types-> affects the expression of the gene, but not the DNA seq.
Characteristic feature: only 1 allele is expressed.

55
Q

Epigenetic mechanism

A

factors that affect gene fxn without changing the genotype; methylation, histone modifications, chromatin structue etc.
Affect gene expression without changing primary seq.

56
Q

Genomic imprinting: Prader-Willi and Angelman syndromes

A

Genomic imprinting diseases involving the SAME CHORMOSOMAL LOCATION:15q11q13.
Key diff: whether the abn chrome is maternally/paternally derived!!
If ABN CHROMOSOME 15 is inherited from:
Father-Prader-Willi; patients’ genetic info derived only from their mothers
Mother-Angelman; patients’ have genetic info derived only from their fathers
Usually 2 cytogenetically normal chromes both inherited from one parent

57
Q

Prader-Willi syndrome phenotype

A

Obesity, excessive and indiscriminate eating habits, small hands and feet, short stature, hypogonadism, and mental retardation

58
Q

Angelman syndrome phenotype

A

Unusual facial appearance, short stature, severe mental retardation, spasticity, and seizures

59
Q

Uniparental Disomy

A

Minority of PWS and AS cases have inherited both chromosomes from one parent:
PWS if both from mother
AS if both from father

60
Q

Isodisomy

A

if identical chromosome is present in duplicate

61
Q

Heterodisomy

A

if both homologs from one parent present

62
Q

Uniparental Disomy: Beckwith-Wiedemann syndrome

A

Infants large at birth, enlarged tongue, frequent protrusion of umbilicus, severe hypoglycemia, development of kidney, liver, and adrenal malignancies
Causes: excess of paternal or loss of maternal contribution of genes or both on chromosome 11q15, including Insulin-like GF2 gene.

63
Q

Hydatidiform mole

A

only paternal contribution; caused by abn growth of chorionic villi-> epithelium proliferates and stoma undergoes cystic cavitation-> placenta converted into a mass of tissues resembling a bunch of grapes.
Occurs when sperm fertilizes an ovum that lacks a nucleus-> duplication of haploid male chromosome set(sperm chromes); no maternal chromes so no fetal tissue at all!
Can lead to choriocarcinoma

64
Q

Hydatidiform mole patient presents with:

A

excessive vomitting, height of fundus does not correspond to the date-its higher than expected; enlarged uterus mean abn pregnancy

65
Q

Hydatidiform mole treatment

A

Fragile uterus easily ruptured so can’t do endometrial curettage which can cause death. So use suction curettage and slowly suck out the tissue. HCG levels must fall. If still high, not all of mole was removed so must go back and suck it out.

66
Q

Ovarian teratomas

A

benign, arise from 46,XX cells containing only maternal chromosomes.
Confined placental mosaicism-placenta has abnormality that is not apparent in fetus, but leads to abnormal fetus.

67
Q

Mendelian disorders with cytogenetic effects

A

Chromosome instability syndromes: rare single gene syndromes with characteristic cytogenetic abnormality.

Genetics (12 decks)

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