Meiosis and Chromosome Structure Flashcards
Cytogenetics
The study of chromosomes and cell division
What are the elements of chromosome structure (small -> big)?
- Histones = proteins wrapped with 150 bp DNA; 50 bp linkers between histones
- Nucleosomes = segments of DNA wrapped around 8 histone cores
- Solenoid
- Chromatin loops
- Chromosomes = complete package
Metaphase spread
Method for viewing chromosomes in cytogenetics:
- spindle fiber inhibitor inhibits anaphase, arresting cells in metaphase
- hypotonic solution causes cell expansion
- fixation hardens the membrane to stabilize
What are the different regions of a chromosome?
- Telomere = TTAAGGG repeats on the end
- Short (p) arm = “top” arm
- Centromere = spindle fiber binding site in middle
- Long (q) arm = “bottom” arm
- Telomere
Metacentric chromosome
Centromere is in the center; p and q arms of similar length
Submetacentric chromosome
Centromere towards one end; p arm much shorter than q
Acrocentric chromosome
- 13, 14, 15, 21, 22
- Very small p arm (stalks and satellites)
- All 5 chromosomes have the same stalk DNA => duplications and deletions w/o clinical consequence means little selective pressure to maintain => polymorphism
- Stalks = tandem arrays of rRNA genes
- Satellites = highly repetitive “junk” DNA
- Associate in interphase to form the nucleolus
Ideograms
- Banding pattern from enzyme digest of chromosomes
- Identical pattern for all people
- Numbering: 24.3 = region 2, band 4, sub-band 3. Numbers out from centromere
Euploid
- Chromosome number is an exact multiple of the haploid set (23, 69, 92)
Aneuploid
- Loss or gain of whole chromosomes (45, 47, 49)
What are the types of structural chromosome abnormalities?
- Terminal deletion
- Interstitial deletion
- Duplication
- Ring (telomere deletion => circularization to maintain integrity)
- Isochromosome (2 copies of 1 arm)
- Paracentric inversion (inversion of region, not including centromere)
- Pericentric inversion (inversion of region including centromere)
- Translocation (swapping DNA between chromosomes)
Lyon hypothesis of X-inactivation
- One X chromosome is “active”, meaning genes are transcribed and translated
- One X is “inactive”, remaining condensed and staining darkly as a Barr body in interphase cells
When does X-inactivation occur?
Early in embryonic life (2 wks post-fertilization)
Is there a pattern to X-inactivation?
- Random (either paternal or maternal)
- Clonal: all of a cells descendants will have the same inactive X
How is X-inactivation mediated genetically>
- XIST (X-inactivation specific transcript) gene located in the X-inactivation center of Xq13
- Transcribed only from inactive X and never translated
- Transcript remains in the nucleus and coats the inactive X affecting replication and condensation
- Methylation: inactive X is hypermethylated; active X is mainly unmethylated
What are the effects of the incompleteness of X inactivation?
- For genes that are incompletely inactivated, females (XX) will have 2x the [protein] as males (XY)
What is meiosis?
- Specialized cell division that occurs during gametogenesis
- Shuffles genetic material through recombination and divides genetic material in half
What is the purpose of meiosis I?
- Reduction division (46 -> 23, 2n -> 1n, diploid -> haploid)
What are the stages of prophase I?
- leptotene = chromosome condensation after interphase
- zygotene = 2 homologs (maternal and paternal) align forming a synapse and are held together by synaptonemal complexes
- pachytene = each homolog pair (bivalent) coils tightly and crossing over occurs
- diplotene = homologs begin to separate but remain attached at chiasmata (crossing-over points)
- diakinesis = separation of homolog pairs; chromosomes maximally condensed
What are some characteristics of recombination?
- Number of chiasmata correlates with chromosome size
- > 1 chiasmata/chromosome arm required for normal segregation
- Only one sister chromatid involved in each cross-over event
- Female recombination > male recombination
- Recombination varies by location: less near centromeres and more near telomeres
Pseudoautosomal regions
- Two regions of homology on the X and Y chromosomes that undergo very high levels of recombination
- Same genes are on both X and Y so females and males have the same dosage of these proteins
What are the odds of 2 gametes from 1 individual having the same chromosomes?
Random segregation => 1 in 2^23 (1 in 8 million)
Compare start of meiosis, duration or meiosis, number of mitoses before meiosis, gametes produced per meiosis, and total gamete production for males and females.
Start: puberty (M) and early embryonic life (F)
Duration: 60-65 days (M) and 10-50 years (F)
# mitoses: 30-500 (M) and 20-30 (F)
gametes per meiosis: 4 spermatids and 1 ovum + 2-3 polar bodies
gamete production: 100-200 mil per ejaculate and 1 ovum per cycle
When do meiosis I and meiosis II occur in females?
- First meiosis begins in utero and arrests partway at birth
- With each menstrual cycle, 1 oocyte completes meiosis I and begins meiosis II
- Meiosis II is completed after fertilization
What are the clinical indications for prenatal constitutional cytogenetic testing?
- Advanced maternal age
- Family history of chromosome abnormality
- Ultrasound or screening test anomalies
What are the clinical indications for postnatal constitutional cytogenetic testing?
For babies born with:
- congenital heart defect
- multiple congenital anomalies
- mental retardation of unknown origin or associated with malformations
- ambiguous genitalia
- primary amenorrhea
- 3+ unexplained spontaneous miscarriages
How frequent are chromosome abnormalities in live births?
0.6%; often not compatible with life
What aneuploidy is associated with Turner syndrome?
- Only one sex chromosome (X)
Nondisjunction
- Mechanism of aneuploidy
- Failure of homologous chromosomes (MI) or sister chromatids (MII) to separate
- If homologous don’t separate: fertilized cells either trisomy or monosomy
- If sisters don’t separate: fertilized cells normal, trisomy, or monosomy
How is amniocentesis connected to maternal age?
- Amniocentesis = collection of amniotic fluid to screen for abnormalities; 0.5-1% risk of miscarriage
- Risk of miscarriage due to chromosomal abnormalities increases with maternal age
- At age > 35: risk of abnormality > risk of procedure
Why is most aneuploidy of maternal origin?
- NDJ sperm are less fit and thus less likely to fertilize an egg
- Most aneuploidy is of maternal MI origin
- Exceptions: +18 is parental MII; X is paternal
Balanced chromosomal translocation
- Swapping of genetic material between two chromosomes but no net gain or loss
- Usually no phenotype in self as breaks are usually in a noncoding sequence (only 5% of de novo translocations disrupt a gene)
- Increased risk of abnormal offspring and multiple miscarriages
How is meiosis I abnormal when a balanced translocation is present?
- Chromosome homologs don’t align perfectly and form a tetravalent (two chromosomes with the translocation + their original homologs)
What are the ways in which homologs can separate after forming a tetravalent?
- normal: normal and translocation carrier
- likely abnormal: slight monosomy A/slight trisomy B and slight monosomy B/slight trisomyA
- likely nonviable: almost monosomy A/almost trisomy B and almost monosomy B/almost trisomy A
Which trisomies are viable?
13, 18, 21
Robertsonian Translocation
- Translocation between two acrocentric chromosomes, resulting in the loss of both short arms but doesn’t affect the DNA content of the long arms
- Either occurs by centromere fusion (stalk and satellite DNA lost) or by translocation (stalk and satellite chromosome lost)
Nucleolus organizer regions
Acrocentric chromosomes; associate during interphase to form the nucleolus
How frequent are Robertsonian translocations
der(13;14) = 1 in 1500
How do chromosomes associate in meiosis after Robertsonian translocation?
Trivalent forms: two normal chromosomes + translocated combo chromosome
What are the possibilities for fertilized gametes with a Robertsonian translocation der(14;21)(q10;q10)?
- normal + balanced translocation (normal, balanced)
- unbalanced translocation + monosomy (unbalanced, unviable)
- trisomy + monosomy (both unviable)
Theoretical risk of Down’s is 33% but in reality: 10-15% for female carriers and 0-2% for male carriers
Paracentric inversion
Break and inversion on one chromosome arm
Pericentric inversion
Break on both chromosome arms and inversion (includes centromere)
What are the clinical consequences of inversion?
- Usually no clinical consequence for carrier because no gain/loss and breaks usually occur in noncoding regions
- Abnormal meiosis
Paracentric inversions and meiosis
- Inverted chromosome will stretch to align with normal homolog, forming an inversion loop
- If recombination occurs within the loop => acentric and dicentric products => miscarriage
- If recombination does not occur within the loop => normal
What are acentric and dicentric chromosomes?
- Products of meiosis after paracentric inversion
- Acentric = no centromere; dicentric = 2 centromeres
- Not stable/non-viable
Pericentric inversions and meiosis
- Inverted chromosome will stretch to align with normal homolog, forming an inversion loop
- If recombination occurs within loop => both homologs will have a centromere, but each will have a deletion and a duplication => viable or nonviable gametes depending
- If recombination doesn’t occur within loop => normal
Microdeletion syndromes
- phenotypically and genetically characterized
- usually not inherited but can be inherited in a dominant fashion
- deletion of < 5Mb (10-100) genes; not visible by cytogenetics
- low incidence: each individual syndrome is rare but all together, same frequency as Down’s
How do microdeletions occur during recombination?
- Misalignment between direct repeats => duplication and deletion
- Alignment between inverted repeats => non homologous end joining or U-type exchange => deletion of region between repeats
How does karyotype resolution vary?
- Depends on exact stage of condensation in metaphase
- Band level = level of condensation (400-850)
- More bands = less condensed = greater sensitivity
What is Fluorescent In-Situ Hybridization (FISH)?
- Fluorescently labeled probes target 100kb-1Mb regions of interest
- Probes added in excess to sample DNA
- Sample DNA is heat denatured and then probes allowed to anneal to targets on ss sample
What are the clinical uses of FISH?
- detect numerical abnormalities using dividing or nondividing cells (interphase)
- detect abnormalities that are not visible via karyotype (microdeletions)
- detect trisomies prenatally
Array based comparative genomic hybridization (aCGH)
- FISH clones (probes) arrayed on a chip
- fluorescently labelled patient and control DNA added
- patient and control DNA should compete equally for probe binding (color ratio of 1:1 or 0.8-1.2)
- look for gain or loss in color ratio
What can aCGH detect and with what sensitivity?
- can detect: copy number imbalances at a genome wide level (not all CNVs are pathogenic; 2-15 per patient)
- cannot detect: mechanism of abnormality (trisomy, translocation)
- microarray analysis more sensitive than chromosome analysis
- 108,000+ probes at ~10-50kb intervals along the genome
