1-14 Meiosis Cytogenetics Flashcards
Levels of chromosome twirling
histone, nucleosome, solenoid, chromatin loop
Cell cycle order overview
G0 (resting) S (synthesis) G2 (errors repaired from synthesis) M (mitosis- PMAT) *split* G1 (cell growth)
Process of cytogenetics
- 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
Colcemid
spindle fiber inhibitor added to cells to pause them at metaphase for chromosome analysis
Metacentric Submetacentric Acrocentric
equal length p and q arms unequal length no p arm mostly stalk and satellite
Acrocentric chromosomes
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
stalks satellites
tandem arrays of rDNA genes highly repetitive “junk’ DNA and no known coding sequences
Ideogram
skematic of a chromosome banding
Euploid Aneuploid
exact multiple of haploid set loss or gain of whole chromosomes (not a haploid set)
Structural chromosome abnormalities
Terminal deletion Interstitial deletion duplication ring isochromosome pracentric inversion pericentric inversion translocation
The Lyon Hypothesis
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
X inactivation
- 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
Meiosis overview
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
Meiosis 1
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:
Meiosis 2
Same as mitosis with 23 chromosomes instead of 46 PMAT
Recombination of chromosomes
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
Pseudoautosomal regions of chromosomes
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
Male vs. Female meiosis overview -commences -duration -# mitoses in gamete formation -# gamete per meiosis -# gamete production
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
Female meiosis overview
- 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
Constitutional chromosome analysis
Prenatal: advance maternal age, FH, US abnormal, Abnormal screening tests Post natal: congenital heart defect, mental delay, ambiguous genitalia, primary amenorrhea, multiple unexplained miscarriages
Nondisjunction
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
Parental origin of aneuploidy
21: 92% maternal, M1 18: 97% maternal, M2 16: 100% maternal, M1 XXX: 90% maternal, M1 XXY: 54% maternal, M1 X: 30% maternal
Balanced translocations
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
Viable trisomies
13, 18, 21
Robertsonian Translocation
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
Paracentric Pericentric inversion
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
Microdeletion syndromes
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
Karyotype resolution
Dictates how sensitive our analysis is Reported as “band-level”, 400-850, subjective but should be reported
FISH (fluorescence in situ hybridization)
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
DiGeorge Syndrome
small del(22)(q11.2), intrinsic deletion
Array based comparative genomic hybridization (microarray)
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
