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25 Cards in this Set
- Front
- Back
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What percentage of spontaneous abortions have a chromosomal abnormality?
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54% of spontaneous abortions during early gestation have a chromosomal abnormality –
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What are most of the chromosomal abnormalities due to?
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autosomal trisomies (16 28%, 22 14%) most common followed by 45,X (20%), polypoidy usually triploidy next common
Balanced translocations also common in recurrent spontaneous abortions |
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What is a mutation?
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Permanent change in DNA (permanent alteration
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What is a point mutation?
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Point mutation – change in a single nucleotide base within a gene, may be silent if altered DNA codes for same amino acid, or missense, or nonsense
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What is a missense ?
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Missense – codes for different amino acid with different phenotypic effect, e.g., sickle cell disease where adenine replaces thymidine causing valine to replace glutamic acid in 6th position of beta-globin chain
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What is a nonsense mutation?
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Nonsense – codes for a stop codon leading to premature termination of protein synthesis, e.g., beta-thalassemia with premature termination of DNA transcription of beta-globin chain so marked decrease in synthesis of hemoglobin A & microcytic anemia
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What staining is used to match bands on homologous chromosomes?
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Giemsa stain G banding: light and dark bands matched up (A-G)
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How are chromosomes grouped on a karyotype
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--Banding pattern
--location of centromere – constriction where chromatids are joined & where chromosome attached to spindle during cell division ---according to length |
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Group A (1-3), B & C (4-12,X) and D-G (13-22, Y) has what type of centromere?
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Centrally located centromere: metacentric (A, 1-3)
Off center/eccentric: submetacentric (B & C, 4-12, X) Near very short end: acrocentric (D-G, 13-22, Y) |
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What is FISH?
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FLUORESCENCE IN SITU HYBRIDIZATION
-Sequences on specific chromosomes marked by specific DNA probes |
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How is FISH Done? What are its requirements?
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-No requirement for dividing cells
-FISH done on formalin fixed tissue in paraffin blocks -Probes labeled with fluorescent dyes then added to interphase nuclei Binding between complementary sequences so DNA sequence mapped to specific site |
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What can FISH tell us?
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Reveals subtle microdeletions and complex translocations
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What is Aneupoloidy?
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--chromosome number not a multiple of 23
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What are the common cause of Aneupoloidy?
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-most common cause meiotic nondisjunction, e.g. Trisomy 21 in Down syndrome associated with advanced maternal age
-anaphase lag resulting in loss of a chromosome and mosaicism with two cell lines monosomy, e.g., 45, X in Turner syndrome |
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What is polypoidy?
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-number greater than two of the haploid number
e.g., triploidy three times, tetraploidy four times observed in spontaneous abortions so rarely compatible with life acquired in malignancies (Hodgkin Lymphoma) |
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What is a Deletion?
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Deletion
absence of part of a chromosome designated by a minus sign e.g., cri-du-chat syndrome with 46,XX 5p- or 46,XY 5p- less often, loss of an entire chromosome |
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What is an Inversion?
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reunion/reattachment of a chromosome broken at two points
internal fragment reinserted in an inverted position |
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Isochromosome?
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transverse/horizontal rather than longitudinal division
formation of two new each consisting of either two long arms (usually) or two short arms (often lost), e.g., Turner syndrome due to 45,Xi(q) |
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What is a general translocation?
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exchange of chromosomal segments between nonhomologous chromosomes
denoted by t followed by involved chromosomes in numeric order |
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What is a reciprocal or balanced?
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break in two chromosomes leading to exchange of chromosomal material
without loss of genetic material often clinically silent |
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What robertsonian translocation?
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Variant in which long arms of two chromosomes joined with a common centromere
-Loss of short arms e.g. familial translocation form of Down syndrome most often t(14q;21q) or translocation involving 21 and 13, 15, 21, or 22 |
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Name some characteristics of Down Syndrome (DS)?
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Most common chromosomal disorder adn most common cause of mental retardation in US
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What is the common chromosomal abnormality in DS?
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trisomy 21: 47, XX +21 due to meiotic nondisjxn and assoc. with assoc. advanced maternal age
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What is the robertosonian translocation NOT associated with advanced maternal age?
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46 XX, t(14:21)(q:10;q10), +21
there is also mosaicism... 46, XX/47, XX +21 |
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What are the clinical features of DS?
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-Mental Retardation
- Centrally Located features -Large Forehead, Broad Nasal Bridge - epicanthal folds - large protruding tongue - Simian crease |
