Meiosis and Chromosome Structure Flashcards
1
Q
Cytogenetics
A
The study of chromosomes and cell division
2
Q
What are the elements of chromosome structure (small -> big)?
A
- 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
3
Q
Metaphase spread
A
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
4
Q
What are the different regions of a chromosome?
A
- Telomere = TTAAGGG repeats on the end
- Short (p) arm = “top” arm
- Centromere = spindle fiber binding site in middle
- Long (q) arm = “bottom” arm
- Telomere
5
Q
Metacentric chromosome
A
Centromere is in the center; p and q arms of similar length
6
Q
Submetacentric chromosome
A
Centromere towards one end; p arm much shorter than q
7
Q
Acrocentric chromosome
A
- 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
8
Q
Ideograms
A
- 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
9
Q
Euploid
A
- Chromosome number is an exact multiple of the haploid set (23, 69, 92)
10
Q
Aneuploid
A
- Loss or gain of whole chromosomes (45, 47, 49)
11
Q
What are the types of structural chromosome abnormalities?
A
- 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)
12
Q
Lyon hypothesis of X-inactivation
A
- 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
13
Q
When does X-inactivation occur?
A
Early in embryonic life (2 wks post-fertilization)
14
Q
Is there a pattern to X-inactivation?
A
- Random (either paternal or maternal)
- Clonal: all of a cells descendants will have the same inactive X
15
Q
How is X-inactivation mediated genetically>
A
- 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
16
Q
What are the effects of the incompleteness of X inactivation?
A
- For genes that are incompletely inactivated, females (XX) will have 2x the [protein] as males (XY)
17
Q
What is meiosis?
A
- Specialized cell division that occurs during gametogenesis
- Shuffles genetic material through recombination and divides genetic material in half
18
Q
What is the purpose of meiosis I?
A
- Reduction division (46 -> 23, 2n -> 1n, diploid -> haploid)
19
Q
What are the stages of prophase I?
A
- 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
20
Q
What are some characteristics of recombination?
A
- 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
21
Q
Pseudoautosomal regions
A
- 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
22
Q
What are the odds of 2 gametes from 1 individual having the same chromosomes?
A
Random segregation => 1 in 2^23 (1 in 8 million)
23
Q
Compare start of meiosis, duration or meiosis, number of mitoses before meiosis, gametes produced per meiosis, and total gamete production for males and females.
A
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
24
Q
When do meiosis I and meiosis II occur in females?
A
- 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
25
What are the clinical indications for prenatal constitutional cytogenetic testing?
- Advanced maternal age
- Family history of chromosome abnormality
- Ultrasound or screening test anomalies
26
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
27
How frequent are chromosome abnormalities in live births?
0.6%; often not compatible with life
28
What aneuploidy is associated with Turner syndrome?
- Only one sex chromosome (X)
29
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
30
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
31
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
32
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
33
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)
34
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
35
Which trisomies are viable?
13, 18, 21
36
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)
37
Nucleolus organizer regions
Acrocentric chromosomes; associate during interphase to form the nucleolus
38
How frequent are Robertsonian translocations
der(13;14) = 1 in 1500
39
How do chromosomes associate in meiosis after Robertsonian translocation?
Trivalent forms: two normal chromosomes + translocated combo chromosome
40
What are the possibilities for fertilized gametes with a Robertsonian translocation der(14;21)(q10;q10)?
1. normal + balanced translocation (normal, balanced)
2. unbalanced translocation + monosomy (unbalanced, unviable)
3. 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
41
Paracentric inversion
Break and inversion on one chromosome arm
42
Pericentric inversion
Break on both chromosome arms and inversion (includes centromere)
43
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
44
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
45
What are acentric and dicentric chromosomes?
- Products of meiosis after paracentric inversion
- Acentric = no centromere; dicentric = 2 centromeres
- Not stable/non-viable
46
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
47
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
48
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
49
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
50
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
51
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
52
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
53
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
