1-14 Meiosis Cytogenetics

Question 1

Levels of chromosome twirling

Answer 1

histone, nucleosome, solenoid, chromatin loop

Question 2

Cell cycle order overview

Answer 2

G0 (resting) S (synthesis) G2 (errors repaired from synthesis) M (mitosis- PMAT) *split* G1 (cell growth)

Question 3

Process of cytogenetics

Answer 3

1. peripheral blood (or any tissue with dividing cells) cultured for 1-3 days 2. Centrifuge, pellet, add Colcemid (spindle fiber inhibitor, pauses at metaphase) 3. Resuspended in hypotonic solution 4. Fix/harden 5. drop on slide 6. banding/staining/microscope

Question 4

Colcemid

Answer 4

spindle fiber inhibitor added to cells to pause them at metaphase for chromosome analysis

Question 5

Metacentric Submetacentric Acrocentric

Answer 5

equal length p and q arms unequal length no p arm mostly stalk and satellite

Question 6

Acrocentric chromosomes

Answer 6

13, 14, 15, 21, 22 very small p arm comprised of large tandem arrays of rDNA genes They associate in interphase to form the nucleolus At end of stalks are “satellites” polymorphic, same DNA on every acrocentric chromosomes so we can delete these regions without clinical consequence

Question 7

stalks satellites

Answer 7

tandem arrays of rDNA genes highly repetitive “junk’ DNA and no known coding sequences

Question 8

Ideogram

Answer 8

skematic of a chromosome banding

Question 9

Euploid Aneuploid

Answer 9

exact multiple of haploid set loss or gain of whole chromosomes (not a haploid set)

Question 10

Structural chromosome abnormalities

Answer 10

Terminal deletion Interstitial deletion duplication ring isochromosome pracentric inversion pericentric inversion translocation

Question 11

The Lyon Hypothesis

Answer 11

In females: one X chrom is “active” one X is “inactive” (remains condensed, barr body in interphase cells) x-inactivation occurs early in embryonic life (~2 weeks after fertilization) Random, x inactivation is clonal

Question 12

X inactivation

Answer 12

• mediated by XIST gene (x-inactivation specific transcript) - located in X inactivation center of Xq13 and is transcribed only from the inactive X chromosome - RNA transcripts remain in the nucleus and coats the inactive X affecting replication and condensation - Hypermethylation involved in inactivation X - DNA of active X is largely unmethylated - Incomplete, some genes still transcribed from “barr body” and give twice as much protein in females than males

Question 13

Meiosis overview

Answer 13

Specialized for of cell division occurs only during gametogenesis Meiosis 1: reduction divisions (goes from 46->23, diploid->haploid), occurs only in meiosis Meiosis 2: identical to mitosis in somatic cells except only 23 chrom are present

Question 14

Meiosis 1

Answer 14

Reduction division Prophase 1: + Leptotene: chrom begin to condense +zygotene: homologs align (synapse) and held togethery by synaptonemal complexes +pachytene: each pair of homologs (bivalent) coils tightly, crossing over occurs +diplotene: homologs begin to separate, but remain attached at points of crossing-over (chiasmata) +diakinesis: separation of homolog pairs, chrom are maximally condensed Metaphase 1: Anaphase 1: Telophase 1:

Question 15

Meiosis 2

Answer 15

Same as mitosis with 23 chromosomes instead of 46 PMAT

Question 16

Recombination of chromosomes

Answer 16

during prophase 1 of meiosis Number of chiasmata per chrom correlates with chrom size 1 chiasma per chrom arm required for normal segregation Only 1 sister chromatid involved in each cross over event female recom > male recomb recomb high near telomeres, low near centromeres

Question 17

Pseudoautosomal regions of chromosomes

Answer 17

X and Y share two regions of homology which undergo high levels of genetic recombination PAR1 (pseudoautosomal region 1): distal Xp and Yp PAR2: distal Xq and Yq Allows pairing in meiosis 1 Genes equally expressed from X and Y chroms

Question 18

Male vs. Female meiosis overview -commences -duration -# mitoses in gamete formation -# gamete per meiosis -# gamete production

Answer 18

Commences: puberty, early embryonic life Duration: 60 days, 10-50 years # mitoses in gamete formation: 30-500, 25 Gamete produced per meiosis: 4 spermatid, 1 ovum 2-3 polar bodies Gamete production: 150 million per ejaculate, 1 ovum per menstrual cycle

Question 19

Female meiosis overview

Answer 19

1. 3 month in womb: first meioses 2. At birth: arrested at dictyotene 3. Puberty: during every cycle, oocytes develop, meiosis continues in these oocytes 4. Ovulation: ovulates a metaphase 2 arrested egg 5. Fertilization: meiosis 2 completes upon fertilization

Question 20

Constitutional chromosome analysis

Answer 20

Prenatal: advance maternal age, FH, US abnormal, Abnormal screening tests Post natal: congenital heart defect, mental delay, ambiguous genitalia, primary amenorrhea, multiple unexplained miscarriages

Question 21

Nondisjunction

Answer 21

Failure of homologous chroms (Meiosis 1) or sister chromatids (M2) to separate Parental origin of extra chrom in trisomy is most often maternal and M1 error Increase in frequency with maternal age

Question 22

Parental origin of aneuploidy

Answer 22

21: 92% maternal, M1 18: 97% maternal, M2 16: 100% maternal, M1 XXX: 90% maternal, M1 XXY: 54% maternal, M1 X: 30% maternal

Question 23

Balanced translocations

Answer 23

Most balanced translocations have a normal phenotype unless breaks at important gene point if balanced de novo translocation 5% risk of dev delay of other abnormality, usually parents carry the translocation de novo rare Carriers at increased risk for miscarriages Chrom pair in a tetravalent

Question 24

Viable trisomies

Answer 24

13, 18, 21

Question 25

Robertsonian Translocation

Answer 25

translocation between 2 acrocentric chromosomes (13, 14, 15, 21, 22) results in loss of short arms of both chrom but does not affect the DNA content of the long arms Counts as 45, XY, rob(14;21)(q10:q10) Most commonly 13;14 Trivalent formed during meiosis 1

Question 26

Paracentric Pericentric inversion

Answer 26

does NOT include centromere, results in acentric or dicentric products (unviable) encapsulates centromere, duplication deletion products (potentially viable and pathogenic), damage depends on length of chrom involved (longer more damage) chromatin loop

Question 27

Microdeletion syndromes

Answer 27

phenotypically and genetically characterized syndromes involved complex but RECOGNIZABLE phenotypes deletion small region 10-100 genes, usually less than 5Mb low incidence sporadically occur, although can be inherited in dominant fashion clinical syndromes were defined before deletions identified Mechanism: direct repeats line up and lose some or gain some chromosome, or inverted repeats

Question 28

Karyotype resolution

Answer 28

Dictates how sensitive our analysis is Reported as “band-level”, 400-850, subjective but should be reported

Question 29

FISH (fluorescence in situ hybridization)

Answer 29

Gene / locus of interest probe 100kb-1Mb with flourescent molecule heat denature DNA and probe hybridizes Can ID numerical abnormalities using non-dividing cells (interphase) therefore quicker to run Higher resolution than karyotype FISh standard method for detecting microdeltions but must know which one looking for

Question 30

DiGeorge Syndrome

Answer 30

small del(22)(q11.2), intrinsic deletion

Question 31

Array based comparative genomic hybridization (microarray)

Answer 31

method to detect copy number imbalances cannot ID translocations objective (computer synth) Chip with clones which span most parts of the genome that are mized with control DNA at 1:1, varyations in ther ead out indicate extra or low patient DNA aCGH is same as thousands of FISH probes 50-500kb Many things we can ID have unknown clinical significance