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MHG > Cytogenetics > Flashcards

Cytogenetics Flashcards

1
Q

Telomere Sequence

A

Repeating sequence TTAGGG

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2
Q

Heterochromatin

A

Highly condensed form of the chromosome; not available for replication or transcription.

  • Dark staining with gimsa stain
  • Later replicating
  • AT rich
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3
Q

Euctromatin

A

More relaxed version of chromosome and is capable of being replicated and transcribed.

  • Light staining with gimsa stain
  • Early replicating
  • GC rich
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4
Q

Centromere

A

Constriction near the center of the chromosome that has a repetitive DNA sequence ~171 bp long. The repetitive sequence is unique from one chromosome to another.
- Region of the chromosome where the spindle apparatus attaches during mitosis

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5
Q

Metacentric

A

Describes position of centromere on chromosome being in the middle, resulting in equal p and q arm lengths

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6
Q

Submetacentric

A

Describes position of centromere on chromosome being off-center, resulting in unequal p and q arm lengths

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7
Q

Acrocentric

A

Describes position of centromere being at the peak of the chromosome rendering the p arm insignificant. The p am does not have any affect on phenotype and contains many repeats of rRNAs and no euchromatin. Present in chromosomes 13,14,15,21,22,Y

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8
Q

Monosomy

A
  • 2n-1

- Monosomy for an entire chromosome is always lethal except for Monosomy X

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9
Q

Trisomy

A
  • 2n+1

- Can exist for any chromosome in the set but is hardly viable

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10
Q

What is the gold standard prenatal invasive diagnostic test?

A

Amniocentesis ~15-17 weeks

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11
Q

Aneupolid

A

Chromosome number is not the normal diploid number (i.e. in humans it is not 46). Can be a result of trisomy, monosomy, etc.

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12
Q

What are some possible prenatal diagnostic tissues and when can they be sampled?

A
  1. Amniotic fluid (15-17 wks)
  2. CVS (10-12 weeks)
  3. Percutaneous Umbilical Blood Sample (>18 wks)
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13
Q

PHA

A

Phytoheamgglutinin (mitogen). Administered to move lymphocytes from G0 phase to dividing phase so they may be sampled as a postnatal diagnostic tissue.

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14
Q

When would you use pheripheral blood as a diagnostic tissue?

A

Peripheral blood is a post-natal diagnostic tissue used to analyze the lymphocytes. Lymphocytes must be in mitotic phase so they need to be stimulated with PHA.
- You would not use peripheral blood as a diagnostic tissue sample if the patient acquired abnormalities such as leukemia.

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15
Q

What is significant about using skin as a diagnostic tissue?

A

Skin biopsy is used in cases where you cannot get a blood sample (rare) or if you want to see if there is a different karyotype in the skin than there is in the blood.

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16
Q

When are internal organs usually used as diagnostic tissues?

A

Following a miscarriage or abortion.

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17
Q

What is interphase? What state is the chromatin in?

A

G0, G1, S, G2. The chromatin is in a very highly decondensed state

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18
Q

How long does the cell cycle take in humans and how long is metaphase?

A

Cell cycle takes 18-24 hours in human and only about 1 hour of that is mitosis

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19
Q

Colcemid

A

A spindle fiber poison that allows chromosomes to condense but arrests them in the 2 chromatid phase of metaphase. Added during chromosome preparation for karyotyping.

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20
Q

Fixation. What is used to achieve fixation?

A

Step in chromosome preparation that makes cell membranes brittle to eliminate RBC. Methanol:acetic acid, 3:1 is used.

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21
Q

Why are RBC eliminated during chromosome preparation?

A

They do not have nuclei and thus no chromosomes to analyze.

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22
Q

What cells are used for karyotyping and what phase of the cell cycle are they naturally present in? What cell cycle phase must they be present in for karyotyping?

A

WBC are used to obtain chromosomes for karyotyping. They are usually present in G0 phase and must be in metaphase for karyotyping.

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23
Q

Why is a hypotonic solution used in chromosome preparation?

A

Causes the cells to swell for easier visibility of chromsomes.

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24
Q

How do aneuploidies arise?

A

Nondisjunction in Meiosis I of oogenesis.

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25
Q

Trisomy 21

A

Down Syndrome

  • Low nasal root, upslanting palpebral features, round cheeks, short neck, downturned mouth, 15-20 fold increase in leukemia, 40% with congenital heart defect, moderate to severe mental retardation, hypotonia
  • 75% are miscarried
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26
Q

Down Syndrome

A

Trisomy 21

  • Low nasal root, upslanting palpebral features, round cheeks, short neck, downturned mouth, 15-20 fold increase in leukemia, 40% with congenital heart defect, moderate to severe mental retardation, hypotonia
  • 75% are miscarried
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27
Q

Trisomy 18

A

Edward Syndrome

  • Prenatal grown deficiency
  • Congenital heart defects
  • Profound mental retardation
  • 95% miscarried, 50% die within first few weeks of life, 5% survive to 1 y.o.
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28
Q

Edward Syndrome

A

Trisomy 18

  • Prenatal grown deficiency
  • Congenital heart defects
  • Profound mental retardation
  • 95% miscarried, 50% die within first few weeks of life, 5% survive to 1 y.o.
29
Q

Trisomy 13

A

Patau Syndrome

  • Midline defects: oral clefts & spina bifida
  • Micropthalmia
  • Clenched hands, malformed ears, absent eyebrows
  • Congenital heart defects
  • Profound mental retardation
  • 95% miscarried, 5% survive to 1 y.o.
30
Q

Patau Syndrome

A

Trisomy 13

  • Midline defects: oral clefts & spina bifida
  • Micropthalmia
  • Congenital heart defects
  • Clenched hands, malformed ears, absent eyebrows
  • Profound mental retardation
  • 95% miscarried, 5% survive to 1 y.o.
31
Q

What are common phenotypic characteristics of autosomal abnormalities?

A
  • Developmental delay & mental retardation
  • Facial features of individuals are more similar to other people with the syndrome than to other family members
  • Growth delay
  • Congenital malformations
32
Q

What is the approximate number of genes on the Y chromosome and what is the reason for this number?

A

The Y chromosome has 50 genes on it. The reason for this is that most of the Y chromosome is in heterochromatin. Thus, if someone is born with an extra Y chromosome you do not get much more genetic info transcribed

33
Q

What is special about the X chromosome?

A

X-Chromosome inactivation/Lyonization. If someone has an extra X chromosome, the extra X will be largely inactivated.

34
Q

What is the only viable monosomy?

A

Monosomy X (Turner Syndrome)

35
Q

Turner Syndrome

A

45, X

  • Proportionate short stature
  • Infertile
  • Webbed neck
  • Low hairline
  • Broad shield-shaped chest
  • Congenital heart defects
  • Caused by loss of Xp
36
Q

Kleinfelter Syndrome

A

47, XXY

  • Taller than average
  • Disproportionately long arms and legs
  • Small testes
  • Sterile
  • 33% have gynecomastia
  • 10 to 15 IQ points lower than siblings
37
Q

What is unusual about Turner Syndrome?

A

It is the only sex chromosome abnormality that has phenotypic effects that comprimise health (i.e. congenital heart defects)

38
Q

What is one generalization of all sex chromosome abnormalities?

A

People affected by sex chromosome abnormalities have an IQ 10-15 points lower than their siblings

39
Q

Which sex chromosome anomalies are atypical?

A

47XXX and 47XYY
Both of these conditions are relatively benign with no external phenotypes. The only noticeable differences is that they are taller than average and have lower IQ than siblings.
47XXX experience premature ovarian failure.
47XYY experience hyperactivity, ADD and learning disabilities.

40
Q

What is the least common sex chromosome anomaly?

A

Turner Syndrome, 45X (1/4000 female births)

41
Q

Pseudoautosomal region

A

Area of X and Y chromosomes that are identical to allow for crossing over. Present in Xp and Yp and Xq and Yq (but nothing important happens in q arm)

42
Q

Barr Bodies

A

Inactivated X chromosomes. The number of barr bodies present is always (# of X chromosomes - 1)

43
Q

SRY

A

Gene present in Y chromosome near the pseudoautosomal region responsible for teste development. During crossing over in the sex chromosomes, the SRY gene can be involved in recombination along with the pseudoautosomal region. This results in abnormal phenotype for XX and XY individuals.

44
Q

Results of SRY recombination

A

XX + SRY: XX individual phenotypically male

XY - SRY: XY individual phenotypically female

45
Q

Robertsonian Translocation

A

Highly specialized transloacation that involves two acrocentric chromosomes (13,14,15,21,22,Y). There is whole long arm fusion of the two acrocentric chromosomes

46
Q

What is the most common Robertsonian translocation?

A

t(13,14)

47
Q

What is the balanced chromosome number for Robertsonian translocation?

A

45

48
Q

What is the unbalanced chromosome number for Robertsonian translocation?

A

46

49
Q

How many gametes can a woman with a balanced Robertsonian translocation make? How many of these gametes can make phenotypically normal children?

A

6; 2

50
Q

Reciprocal translocations

A

Non-Robertsonian translocations. Translocations that involve only small parts of the chromosomes, not whole arms. In order for the rearrangement to take place, the chromosomes must first be broken.

51
Q

How many gametes can a woman with a balanced reciprocal translocation make? How many of these gametes can make phenotypically normal children?

A

4; 2

52
Q

How can a karyotypically balanced child be formed from a mother with balanced reciprocal translocations?

A

If the mother passes on BOTH of the translocated chromosomes.

53
Q

Couples in which one person is a balanced translocation carrier have an increase of ________ for miscarriage.

A

5-35%

54
Q

In a fetus with a balance reciprocal translocation, why is there an increased risk for a phenotypic abnormality?

A

With a de novo mutation, there is no way of knowing if the deletion is within a gene or between genes. If the translocation is inherited, you can already see what the phenotype will be like with the parents.

55
Q

Microdeletion

A

<5Mb deletion which can be missed by giemsa banding studies.

56
Q

What are some syndromes cause by microdeletions?

A
  • Prader-Willi/Angelman
  • Retinoblastoma
  • William
  • DiGeorge
57
Q

Terminal Deletion

A

Involves a single break at the end of a chromosome and thus the loss of the end of the chromosome.

58
Q

Ring Deletion

A

Involves 2 breaks which result in loss of p and q termini. These raw ends then rejoin with the centromere to form a ring structure. Ring chromosomes do not procede through mitosis very well.

59
Q

Interstitial Deletion

A

Involves two breaks in a single chromosome arm that rejoin. Middle portion between the two breaks is lost.

60
Q

What is the most common microdeletion syndrome? Why?

A

DiGeorge. It is the most common because the region of deletion is tremendous.

61
Q

What is the difference in chromosome structure between prophase and anaphase? What is advantageous about this structural difference?

A

In anaphase, the chromosomes are much more condensed. In prophase, the chromosomes are less condensed and longer. Prophase chromosomes are used for high resolution analysis that allows for detection of small deletions

62
Q

Isochromosomes

A

Chromosomes containing two p or two 2 arms. They result from improper separation of sister chromatids during anaphase.

63
Q

What is the most common isochromosome?

A

Xq

64
Q

Pericentric Inversion

A

All genes are present, but now genes that were present on the q arm are in the p arm and genes that were on the p arm are in the q arm. Inversion is around the centromere. All genes are still present causing it to be a balanced structural rearrangement. Causes problems with homologous recombination during meiosis.

65
Q

Paracentric Inversion

A

Involves two breaks in a single arm and a chromosome segment flips and reinserts itself. Causes problems with homologous recombination during meiosis.

66
Q

What is a cytogenetic problem with CVS?

A

1-2% of the time there is confined placental mosaicism detected that does not accurately represent the genotype of the fetus.

67
Q

What is the difference between mitotic and meiotic nondisjunction?

A

Mitotic NDJ is independant of maternal age, which is not the case in meiotic NDJ. In mitotic NDJ, the cells start out as normal cells. In meiotic NDJ, the cells start out with certain aneuploidies.

68
Q

Which chromosomes are involved in genomic imprinting?

A

6,7,11,14,15. Thus, if you inherit two maternal or paternal chromosomes other than these, it will not matter because all other chromosomes do not take part in imprinting.

69
Q

What happens when there is a uniparental disomy for chromosomes other than 6, 7, 11, 14, 15?

A

Nothing! Since other chromosomes are not imprinted, the fact that there is uniparental disomy does not matter.

MHG (8 decks)

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