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Clinical Genetics > Chromosomal abnormalities 1 > Flashcards

Chromosomal abnormalities 1 Flashcards

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1
Q

What are histones?

A

Histones are highly positive charged proteins that are attracted to the negative charge of DNA

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2
Q

What is a chromosome and where is it found?

A

Chromosome is an organised package of DNA found in the nucleus of the cell

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3
Q

What can chromosomes not exist without?

A

Chromosome cannot exist without a centromere and intact telomeres

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4
Q

What are the 2 arms that make up chromoseomes?

A

P and q arms

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5
Q

What do chromosomes exist as?

A

Usually exist as chromatin
DNA double helix bounds to histones
Octamer of histones form nucleosome

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6
Q

What is euchromatin?

A

Less condensed

Gene rich

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7
Q

What is heterochromatin?

A

Highly condensed

Gene poor

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8
Q

What is the fundamental unit of DNA and whats it structure?

A

Nucleosome is fundamental unit of DNA

-Eight histones and two turns of DNA

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9
Q

What happens in the G1 phase of the cell cycle?

A

Cell makes a variety of proteins needed for DNA replication

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10
Q

What happens in the S phase of the cell cycle?

A

Chromosomes are replicated so that each chromosome now consists of two sister, identical chromatids

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11
Q

What happens in the G2 phase of the cell cycle?

A

synthesis of proteins especially microtubules

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12
Q

How many chromosomes does the human have?

A

Humans have 23 pairs of chromosomes

-22 pairs autosomes, 1 pair sex chromosomes XX or XY

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13
Q

What part of the cycle can we see chromosomes and why?

A

Can see chromosomes in the metaphase part of cell cycle

-They are condensed so visible

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14
Q

Lengths of the arms in metacentric chromsomes?

A

p & q arms even length

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15
Q

Where is the centromere in metacentric chromosomes?

A

centromere in the middle

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16
Q

What chromosomes are metacentric?

A

1-3, 16-18

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17
Q

Length of arms in submetacentric chromosomes?

A

p arm shorter than q

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18
Q

What chromosomes are submetacentric?

A

4-12, 19-20, X

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19
Q

Length of arms in acrocentric chromosomes?

A

Long q, small p

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20
Q

What do genes on the satellite arms code for?

A

Satellite arms

-Genes on these code for rRNA

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21
Q

What chromosomes are acrocentric?

A

13-15, 21-22, Y

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22
Q

What can you detect numerical chromosomal and changes through and examples?

A

Numerical

  • Can detect through traditional karyotyping, FISH, QF-PCR, NGS
  • E.g. Down’s syndrome
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23
Q

What can you detect structural chromosomal changes through and examples?

A

Structural

  • Can detect through traditional karyotyping, FISH
  • Trisomy 21
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24
Q

Definition of haploid

A

One set of chromosomes (n=23) as in a normal gamete

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25
Q

Definition of diploid

A

Cell contains two sets of chromosomes (2n=46; normal in human)

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26
Q

Definition of polypoid and when can it occur?

A
  • Multiple of the haploid number (e.g. 4n=92)
  • Rare
  • Can sometimes occur at conception
27
Q

Definition of aneuploid

A

Chromosome number which is not an exact multiple of haploid number - due to extra or missing chromosome(s

28
Q

What is mosaicism?

A

Presence of two or more genetically different cell lines derived from a single zygote

29
Q

What is disjuntion?

A

Pulling apart of chromosomes at anaphase

30
Q

What happens to genetic material in meiosis?

A

Double the amount of genetic material in meiosis

-2n–>4n–>2n–>n

31
Q

Aneupoloidy in meiosis 1

A

Non-disjunction arises

-Both homologues go into the same daughter cell

32
Q

Aneupoloidy in meiosis 2

A
  • Disjunction happens correctly and pulls chromatids apart and distributes them
  • Results in one cell being completely empty
  • Known as being disomic – two copies of one chromosome
33
Q

What does trisomy 3 lead to?

A

Trisomy 3 leads to early miscarriage so are rarely seen

34
Q

What are the 2 mechanisms in mosaicism?

A
  • Post-zygotic nondisjunction

- Anaphase lag

35
Q

What happens in anaphase lag?

A
  • Zygote is completely trisomic
  • As they’re going through mitosis there is a pause
  • Due to this anaphase delay, instead of there being 3 chromosomes in each cell, one is kicked out
36
Q

Steps involved in anaphase lag

A
  • Describes 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
  • Instead, the chromosome forms a micronucleus in the cytoplasm and is lost from the cell
  • The lagging chromosome is not incorporated into the nucleus of one of the daughter cells, resulting in one normal daughter cell and one with monosomy
37
Q

What can anaphase lag cause the rescue of?

A

Anaphase lag can also cause a rescue of the daughter cell if the cell was originally trisomy

38
Q

Is monosomy rare?

A

Autosomal monosomy is very rare

39
Q

What is the relatively common sex chromosome monosomy?

A

Relatively common sex chromosome monosomy = Turner’s

40
Q

What does fully monosomy arise by?

A

Full monosomy arises by NDJ

41
Q

What does meiotic NDJ affect?

A

all cells affected

42
Q

What individuals does mitotic NDJ affect?

A

mosaic individuals

43
Q

What is partial monosomy?

A

‘chunk’ of X chromosome is missing

44
Q

How does turners arise?

A
  • Nullisomic gametes fertilised with a sperm carrying an X chromosome will be XO (Turners)
  • Nullisomic gametes fertilised with a sperm carrying a Y chromosome will be YO
45
Q

Possible combinations in nullisomic gametes

A

$+ X chr = XO = Turner’s (physically female)

$+ Y chr = lethal

46
Q

Possible combination of disomic gametes(XX)

A

+ X chr = XXX = triple X syndrome

+ Y chr = XXY = Klinefelter’s (physically male)

47
Q

Possible combination of disomic gametes(XY)

A

+ X chr = XXY = Klinefelter’s

+Y chr = XYY = XYY syndrome

48
Q

When do we carry out chorionic villus sampling?

A

11-14 weeks

49
Q

What is the miscarriage rate of chorionic villus sampling?

A

Miscarriage rate 0.5% to 1%

50
Q

When do we carry out amniocentesis?

A

> 16 weeks

51
Q

What do we extract in amniocentesis?

A

extraction of amniotic fluid

52
Q

What is the miscarriage risk in amniocentesis?

A

Miscarriage risk (0.5-1%)

53
Q

What stain do we use in G banding?

A

Giemsa stain

54
Q

What is the line up based on in G banding?

A
  • Size
  • Banding
  • Centromere position
55
Q

What does Giesma highlight in G banding?

A

Giemsa highlights heterochromatic regions which are less likely to contain genes

56
Q

What can the banding in G banding be used to differentiate?

A

The banding can be used to differentiate between chromosomes and to compare chromosome banding patterns

57
Q

What bases are rich in euchromatin?

A

GC rich

58
Q

What bases are rich in heterochromatin?

A

AT rich

59
Q

What is FISH?

A

Fluorescent in situ hybridisation

60
Q

Steps involved in FISH

A
  1. Fluorescent probe
  2. Denature probe and target DNA
  3. Mix probe and target DNA
  4. Probe binds to target
61
Q

Steps involved in quantitative fluorescence PCR

A
  • Design primers for specific microsatellites on Chr 21
  • Amplify region using PCR
  • Look to see how big the particular microsatellite region are and how many copies of the microsatellite there is
62
Q

What are examples of invasive fetal testing?

A
  • Chorionic villus sampling

- Amniocentesis

63
Q

What are examples of non-invasive fetal testing?

A

-Cell free fetal DNA

64
Q

How do we use NGS in non-invasive diagnosis?

A
  • Expect to get same amount of DNA releative to each chromosome
  • If fetus is trisomic, you will have extra copies of the chromosome
  • The copies chromosome will be over-represented in NGS allowing you to see which chromosomes are copied

Clinical Genetics (4 decks)

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