Cytogenetics Flashcards
What is the best stage for karyotyping and why?
Metaphase, because DNA is the most tightly packed.
G-banding patterns
Light bands = higher GC and lower AT content, early replicating, housekeeping genes
Dark bands = lower GC and higher AT content, late replicating, tissue specific genes
Metacentric chromosomes
Two arms of equal length
Submetacentric chromosomes
Two arms not of equal length (long arm q longer than short arm p)
Acrocentric chromosomes
Only has coding DNA on long arm
Band designation nomenclature
Chromsome # / p or q / Region / Band /Sub-band (e.g. 14q32.3)
Why do telomeres cap chromosomes?
To maintain structural integrity, ensure complete replication, and help establish chromosome pairing
Why are subtelomeric domains subject to translocations?
Because they have low copy repeats that are homologous across chromosomes
Numerical chromosome abnormality: Euploid - Haploid - Diploid - Polyploidy
Exact multiple of haploid set
- Haploid: normal number in gametes (N)
- Diploid: normal number in zygote and somatic cells (2N)
- Polyploidy: complete extra set(s) of chromosomes (#N)
Numerical chromosome abnormality
Aneuploid
- Monosomy
- Trisomy
Loss or gain of single chromosome
- Monosomy = 45
- Trisomy = 47
Recombination occurs during ____.
Why is recombination important? What does lack of recombination lead to? Which gender has more recombination? Where does it occur more closely to?
Meiosis I.
Recombination is important for proper alignment and segregation of chromosomes and for increasing genetic diversity. Lack of recombination leads to nondisjunction. Females have more recombination and it occurs more closely to telomeres.
Male v. female meiosis timing
Male meiosis begins at puberty and is ongoing through life. Female meiosis being prior to birth and arrests in M1 prophase until puberty. During ovulation, M1 completed and M2 is completed after fertilization
What is nondisjunction? What does it lead to? What is it correlated with and why?
Nondisjunction is a failure of paired chromosomes in meiosis I or sister chromatids in meiosis II to separate. It results in aneuploidy (monosomy or trisomy) It is correlated with advanced maternal age (particulalry trisomy) (but not paternal age) and likely because likelihood of recombination error goes up over time.
Trisomy 21
Trisomy 18
Trisomy 13
Other trisomies?
21 = Down's syndrome 18 = Edward's syndrome; early lethality 13 = Patau syndrome; early lethality
Other trisomies results in spontaneous abortion. Most common Trisomy 16 (results in miscarriage)
Where do most translocations occur and what do they involve?
Most translocations occur at the distal end of chromosomes and involve homologous parts of subtelomeric regions. They can be balanced or unbalanced.
What is a balanced rearrangement? Does it result in altered gene expression? Will karyotyping show it? What can it result in?
A balanced rearrangement is a type of reciprocal translocation that has no loss or gain in genetic material. It can still result in altered gene expression even though no material is lost due to small changes at insertion or breakage point. A balanced translocation can become unbalanced when passed on. Karyotyping will not show small deletions since we can’t see chromosomes at a resolution
What is a Robertsonian translocation? What does it result in?
A translocation occurring between 2 acrocentric chromosomes. It results in loss of short arms (which do not have coding DNA) but does not affect DNA content of long arms. It can result in improper alignment during meiosis leading to improper segregation and aneuploidy. Note: 14/21 Robertsonian translocation is the source of familial Down syndrome.
Intrachromosomal abnormalities (4)
(1) Deletions (terminal or interstitial: at end or within chrom.)
(2) Duplications: duplication of region of DNA in chrom.
(3) Isochromosome: chrom. with two identical arms
(4) Ring formation: ends of chromosomes fuse to form ring.
FISH technique (fluorescence in situ hybridization)
Can probe ~200kb segments using specific DNA probe (but have to know what sequence you’re looking, not exactly but closely). Useful for ID’s microdeletions
CGH arrays
Used to detect unbalanced chromosome abnormalities (copy number variations in particular). Resolution of about 100kb. Not great for doing entire genome because lots of noise. Can’t detect polyploidy.
Sequencing (two types)
- Has 1bp resolution; good for learning the DNA sequence, but can’t detect copy number variations
(1) Sanger (dideoxy) sequencing: high fidelity, low error rate, best for ID’ing small sequence changes in limited region
(2) Next-generation sequencing: can do whole genomes or specific subset; have to run multiple times bc of sporadic errors; expensive
The ____ of X and Y chromosomes allow them to pair during ___.
The pseudoautosomal regions of X and Y chromosomes allow them to pair during meiosis.
What happens to make a 46, XX individual phenotypically male?
SRY gene, which initiates development of male characteristics is translocated onto an X chromosome
X-inactivation
Prevents overexpression of X-linked genes in poly-X individuals. Relies on XIST protein.
