Cytogenetics I & II

Question 1

Euchromatin & Heterochromatin

Answer 1

Euchromatin: light bands; CG rich, early replication
Heterochromatin: dark bands; AT rich, late replication

Question 2

Centromere positions (3)

Answer 2

1. Metacentric
2. Submetacentric
3. Acrocentric (p arm contains no unique sequence; same for: 13, 14, 15, 21, 22)

Question 3

prenatal diagnostic tissues for chromosome analysis (3)

Answer 3

1. amniotic fluid
2. chorionic villus sample
3. percutaneous umbilical blood sample

Question 4

Adult tissues for chromosome analysis (4)

Answer 4

Any viable cell with nucleus

1. peripheral blood: lymphocytes are in G0 (stimulated by mitogen pytohemagglutinin PHA)
2. Bone marrow: typically for acquired abnormalities (leukemia, lymphoma)
3. Skin: fibroblast culture from skin biopsy 1-3wks
4. Internal organs: fetal death, autopsy

Question 5

Chromosome Preparation procedure (6)

Answer 5

1. “harvest” anc culture actively dividing cells (phytohemagglutinin ~3 days)
2. Add COLCEMID, spindle fiber poison
3. Hypotonic solution (low KCl molarity) –> swell
4. Fixation: cell membrane brittle & fragile
5. Slide preparation; proteolytic enzymes cleave bound proteins
6. G-banding w/ Giemsa stain

Question 6

Microscope analysis of chromosomes

Answer 6

• Need 15-50 metaphases

- ID: individual chromosomes, missing/extra chromosomes, structural rearrangements

Question 7

Recombination during meiosis

Answer 7

1. At least one cross over (chiasmata) per chromosome arm
2. number of cross overs correlates to length of arm
3. decreased cross overs is associated with increased risk of nondisjunction

Question 8

Male meiosis (3)

Answer 8

1. 4 division products develop to sperm
2. Begins during puberty
3. After many mitotic divisions, develop into mature sperm

Question 9

Female meiosis (3)

Answer 9

1. 1 egg, 3 polar bodies
2. cytoplasm unequally divided
3. begins in fetal life (3-9 mos) –> prophase I at birth
   -ovulation: –> metaphase II
   stops until fertilized

Question 10

Constitutional cytogenetic abnormalities

Answer 10

• present at conception

- associated with birth defects and miscarriages

Question 11

Aneuploidy

Answer 11

• most common type of human chromosome disorder (>3-4%)
  2. All monosomies lethal except X
  3. Caused by nondisjunction (most often in meiosis I)

Question 12

Autosomal abnormalities

Answer 12

1. Developmental delay/mental retardation
2. facial features more characteristic of syndrome than of family members
3. Growth delay
4. Congenital malformations

Question 13

Sex reversal

Answer 13

Results from aberrant cross-over btwn X and Y chromosomes

• pseudo-autosomal region of homology can cross over
• SRY (sex determination in males) is nearby and if moved to X chromosome –> XX male

Question 14

Robertsonian translocation

Answer 14

Whole arms ( q) of two acrocentric chromosomes
-break on p arm near centromere & two long arms join –> loss of P arm –> insignificant
-two centromeres in hybrid
Balanced: 45 (t13:14); unbalanced: 46 (t14:21)

Question 15

Reciprocal translocation

Answer 15

Do not involve entire arm and centromere
-usually btwn two nonhomologous chromosomes
Balanced: 46, Unbalanced: 46 rarely 47
-Rearrangement of chromosomes (change sizes)

Question 16

Terminal deletions

Answer 16

Due to a break and loss of chromosome end

Question 17

Interstitial deletions

Answer 17

Two breaks; loss of middle portion

Question 18

Ring deletions

Answer 18

Two breaks; loss of P and Q terminus –> ends rejoin –> circular
-often lost or lead to abnormal phenotype

Question 19

Microdeletion syndromes

Answer 19

usually <5 megabases lost which can be missed by G-band studies
-Can be detected using high resolution chromosome analysis, FISH, array CGH
Ex: Prader-Willi/Angelman, Rb, William, DiGeorge/velocardiofacial

Question 20

Isochromosomes

Answer 20

Trasnverse division during meiosis at centromeres instead of longitudinal

• results in chromosome with two P arms and one with 2 q arms
• Turner syndrome phenotype: 46, X,i(Xq) 20%

Question 21

Pericentric inversion

Answer 21

Cut in both arms and middle region (containing centromere) is flipped

• duplication of one, deletion of another
• produce viable, abnormal offspring

Question 22

Peracentric inversion

Answer 22

-two breaks in same arm –> region in same arm is flipped, changing order of genes
-Results in chromosome with either two or no centromeres
Abnormal recombinant is almost never viable
-Viable offspring have normal offspring

Question 23

Complications with inversions (2)

Answer 23

1. Carriers are at risk of having offspring with duplication AND deletion parts = recombinant chromosome
2. problems in homologous alignment in meiosis –> abnormal chromosomes

Question 24

Indications for chromosome analysis (8)

Answer 24

1. recognized cytogenic syndrome
2. unrecognized syndrome with 2+ malformations
3. ambiguous genitalia
4. MR or developmental delay in children who are dysmorphic or have MCA
5. 1st deg relative with structural chromosomal abnormalities
6. Stillborn infants w/ malformations or no recognizable reason for fetal death
7. Females with proportionate short stature and primary amenorrhea
8. Males with small testes or significant gynecomastia

Question 25

Mosaicism (3)

Answer 25

1. Presence in a tissue or individual with at least two cell lines 2. Differ karyotypically, but are derived from a single zygote 3. Severity of phenotype is dependent upon frequency of abnormal cell line and tissue distribution

Question 26

Chromosomal mosaicism

Answer 26

Most common way to generate mosaicism is via nondisjunction - trisomy can result --> anaphase lag can cause extra chromosome to be lost --> restore normal 46 in some cells - therefore, some cells will have 47, others 46

Question 27

Confined Placental mosaicism

Answer 27

Discrepancy btwn placental and fetal karyotype - more widely recognized after chorionic villus sampling (CVS) - 20% of pregnancies with idiopathic intrauterine growth retardation (IUGR) have confined placental mosaicism - potential for false positives (if placenta has trisomy, but fetus doesn't) and negatives (if placenta doesn't have trisomy, fetus does)

Question 28

Uniparental disomy (UPD)

Answer 28

Two chromosomes present, but both from one parent | -1/3 of corrected trisomies (as a result of nondisjunction) via trisomy rescue results in UPD

Question 29

Genomic Imprinting

Answer 29

Differential expression depending on parental source (maternal and paternal fxn differently and can influence expression of disorders) 2. Modification of DNA during critical period; Reversible change in the genetic material 3. Contrary to basic Mendelian principles Occurs in certain parts and chromosomes (6 chromosomes where imprinting matters)