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Fundamentals of P > Chromosomal Abnormalities I > Flashcards

Chromosomal Abnormalities I Flashcards

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1
Q

Describe packaging of chromosomes

A

Chromosomes usually exists as chromatin
DNA double helix bounds to histones
Octamer of histones form nucleosome

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2
Q

Describe structure of euchromatin and its function

A

Euchromatin
Extended state, dispersed through nucleus
Allows gene expression

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3
Q

Describe structure of euchromatin and its function

A

Heterochormatin

Highly condensed, genes not expressed

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4
Q

G1 - describe what happens

A

G1 = Cell makes a variety of proteins needed for DNA replication

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5
Q

S - describe what happens

A

S = synthesis; chromosomes are replicated so that each chromosome now consists of two sister, identical chromatids

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6
Q

G2 - describe what happens

A

G2 – synthesis of proteins especially microtubules

Some cells don’t replicate; some are senescent.

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7
Q

Metacentric - define and describe which ones

A

Metacentric
p & q arms even length
1-3, 16-18

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8
Q

Submetacentric - define and describe which ones

A

Submetacentric
p arm shorter than q
4-12, 19-20, X

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9
Q

Acrocentric - define and describe which ones

A

Acrocentric
Long q, small p
p contains no unique DNA
13-15, 21-22, Y

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10
Q

Detecting chromosomal changes

A

Numerical = Can detect through traditional karyotyping, FISH, QF-PCR, NGS

Structural = Can detect through traditional karyotyping, FISH

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11
Q

Haploid - define

A

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

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12
Q

Diploid - define

A

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

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13
Q

Polyploid - define

A

multiple of the haploid number (e.g. 4n=92)

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14
Q

Aneuploid - define

A

chromosome number which is not an exact multiple of haploid number - due to extra or missing chromosome(s) (e.g. 2n+1=47)

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15
Q

Numerical Abnormalities - list

A

Trisomy

Monosomy

Mosaicism

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16
Q

Disjunction - define

A

Pulling apart at anaphase = disjunction

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17
Q

How does aneuploidy arise?

A

Primary mechanism = Non-disjunction

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18
Q

Mosaicism - define

A

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

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19
Q

Mosaicism - describe mechanisms

A

2 mechanisms:

Post-zygotic nondisjunction, i.e. mitotic non-disjunction = All 2n to mixture of 2n and 2n+1

Anaphase lag, i.e. trisomic rescue = All 2n+1 to mixture of 2n+1 and 2n

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20
Q

Monosomy - cause

A

Full monosomy arise by NDJ

Partial monosomy (microdeletion syndromes) far more common – mechanism different

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21
Q

Monosomy - which type is R.V. common

A

Relatively common sex chromosome monosomy = Turner’s

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22
Q

How does Turner’s (45,X) 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

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23
Q

Nullisomic gametes - effects of possible combinations

Study These Flashcards
A

Nullisomic gametes
+ X chr = XO = Turner’s (physically female)
+ Y chr = lethal

24
Q

Disomic gametes - effects of possible combinations for XX

Study These Flashcards
A

XX:

+ X chr = XXX = triple X syndrome
+ Y chr = XXY = Klinefelter’s (physically male)

25
Disomic gametes - effects of possible combinations for XY
XY: + X chr = XXY = Klinefelter’s +Y chr = XYY = XYY syndrome
26
Numerical abnormalities - types
Types (all can be mosaic) Autosomal Trisomy 13, 18, 21 Sex chromosomes XO, XXY, XYY
27
Numerical abnormalities - mechanism
Mechanism: Nondisjunction Anaphase lag
28
Anaphase lag - define
Sister chromatids do not properly separate from each other because of improper spindle formation The chromosome or chromatid does not properly migrate during anaphase and the daughter cells will lose some genetic information.
29
Forming a karyotype
``` Take 5 mL venous blood Add phytohemagglutinin + culture medium Culture at 37 degrees celsius for 3 days Add colchicine and hypotonic saline Cells fixed Spread cells onto slide by dropping Digest with trypsin and stain with giemsa Analyze metaphase spread = karyotype ```
30
Chorionic villus sampling (CVS) - define
a prenatal test that is used to detect birth defects, genetic diseases A small sample of cells (called chorionic villi) is taken from the placenta where it attaches to the wall of the uterus.
31
Chorionic villus sampling (CVS) - +ves/-ves
11-14 weeks Miscarriage rate 0.5% to 1% maternal contamination transverse limb defects
32
Amniocentesis - +ves/-ves
>16 weeks extraction of amniotic fluid Biochemical diagnosis possible miscarriage risk (0.5-1%)
33
Giemsa - use
Giemsa highlights heterochromatic regions which are less likely to contain genes. But the crucial thing is that the banding can be used to differentiate between chromosomes ant to compare chromosomes.
34
Giemsa - Line-up based on
Size Banding Centromere position
35
Why chromosome bands?
Chromatin: 2 different sorts: euchromatin & heterochromatin Euchromatin = GC-rich; loosely packed; genes active Heterochromatin = AT-rich; tightly packed; genes inactive Stain differently
36
Most common bands?
Giemsa bands
37
Foetal testing - invasive vs non-invasive
Invasive Amniocentesis (14-20 wks, amniotic fluid) Chorionic villus sampling (CVS) (11-14 wks, placental cells) Non-invasive Cell free foetal DNA (cffDNA): DNA fragments in maternal plasma (10 wks onwards) Actually for trisomies still need confirmation with amnio/CVS
38
Trisomies - advantages
SAFE TEST – Trisomies 13, 18, 21 Available privately ~ £400 But now free at St George’s!
39
Down syndrome – clinical features
``` 1 in 650-1000 live births Most common cause of mental retardation Hypotonia, particularly in newborn period Developmental delay Cardiac abnormalities GI abnormalities Acute Lymphocytic Leukaemia/Acute Myeloid Leukaemia – 10-20 x relative risk Conductive hearing loss Features of Alzheimer’s >40 years ```
40
Down Syndrome - causes
95% patients non-disjunction (usually maternal meiosis I) 5% Robertsonian translocation involving chromosome 21 ~2% mosaic (ie only some cells contain an extra chromosome) ‘Older Egg Model’ – maternal age effect
41
Patau syndrome (risomy 13) - clinical features
~1 in 10 000 live births Midline defects - Hypotelorism - Holoprosencephaly - Midline cleft lip/palate - Scalp defects Post axial polydactyly Heart defects/renal abnormalities Survival – most die by 1 month
42
Hypotelorism define
Hypotelorism=abnormally close eyes
43
Holoprosencephaly - define
Holoprosencephaly – forebrain doesn’t divide properly-causes facial malformations as well
44
Post-axial - define
Post-axial=lateral side of foot or hand (little finger side)
45
Edwards syndrome (trisomy 18) - clinical features
``` Incidence ~1 in 6000 live births Intrauterine growth retardation Micrognathia Cleft lip +/- palate Short palpebral fissures Fixed flexion deformities of fingers Heart defect >95% Inguinal/diaphragmatic herniae Renal malformations ```
46
Micrognathia - define
Micrognathia = small lower jaw
47
Palpebral fissures - define
Palpebral fissures - gap between eye lids
48
Survival in Edwards syndrome
30% die by 1 month 50% die by 2 months 90% by 1 year
49
Sex chromosome aneuploidies - incidence
Incidence: male 1/400 female 1/600 all autosomal 1/700
50
Sex chromosome aneuploidies vs autosomal aneuploidy
Phenotype often less severe than autosomal aneuploidy
51
Turner syndrome - clinical features
``` Incidence 1/4000 female births 45X0 or mosaic (45X0/46XX) Raised nuchal translucency At birth: - oedema of hands and feet - Neck webbing - coarctation of aorta - renal malformation Short stature Infertility secondary to gonadal dysgenesis Intellectually normal ```
52
45X/46XY mosaicism - define
usually normal male but with potential for gonadoblastoma
53
Isochromosome Xq effect
Mosaicism with ring or isochromosome Xq = variant turner syndrome
54
Klinefelter (47,XXY) - clinical features
Incidence 1/1000 male live births Phenotype mild and variable – some cases undetected Barr body present NDJ paternal meiosis I (50%), others NDJ maternal meiosis or zygotic mitotic error (mosaic) Variants: 48,XXYY, 48,XXXY etc
55
Klinefelter (47,XXY) - effects
May present prenatally, during childhood with behavioural problems, or adulthood with infertility Tall stature Eunachoid body habitus Some behavioural and minor learning difficulties Lack of secondary sexual characteristics – treat with testosterone Infertility

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