Chromosome Structure and Method of Study Flashcards
cytogenetics
the study of chromosomes and cell division
cell culture, culture preparation, slide preparation
media mimics cell environment, add colcemid to prevent sindle fiber formation and prevents the cell from entering anaphase
cells accuulate in metaphase
not machine operated
homologues
chromosome pairs, one from mom and one from dad
base pairs are not identical
banding pattern
generated based on the used of a protease during processing
the protease digests scaffolding proteins at different places in the chromosomes
chromosome structure
Short arm is called the p arm for “petite”
The long arm is called the q arm because it is the next letter of the alphabet
Chromosomes are always oriented with the short arm up
chromosome types
metacentric - two distinct arms
submetacentric - two short arms
arcocentric - satellite arms
arcocentic chromosomes
a special class of chromosomes with very small p arms, coprised of a large, tandm array of rDNA genes
satellites at the end of the stalks are “junk” DNA
length varies from person to person because loss of some satellites is not significant due to the highly repetitive nature
consists of chromosomes 13, 14, 15, 21, and 22
ideogram
banding pattern for all chromosomes
every band given a number, starts from the centromere and moves outwards
divided into regions, bands, and sub-bands
numerical chromosome abnormalities
euploid - exact multiple of haploid set
aneuploid - loss or gain of whole chromosomes (not a haploid set)
structural chromosome abnormalities
terminal deletion, interstitial deletion, duplication, ring, isochromosome, paracentric inversion, pericentric inversion, translocation
terminal deletion
a break in the chromosome and the rest is missing
interstitial deletion
a piece in the middle gets deleted out
duplicated
longer chromosome because of duplication
ring chromosome
loss of ends of chromosomes, produce a circle to preserve itself fromdegradation
isochromosome
q or p class, two of the same arms stuck together
paracentric inversions
segment fliipped around, does not include centromere
pericentric inversion
flipped segment, including centromere
translocation
break on two chromosomes and swapping or material
cytogenetic nomenclature
Lyon hypothesis
in order to compensate for the extra dose of X in females, one gets inactivated
inactive chromosome is called the Barr body
X inactivation is random and occurs early in embryonic life
mediated by the XIST gene (X-inactivation specific transcript)
XIST gene on the X inactivation center (Xq13)
stages of meiosis I (name the stages of prophase I)
prophase I, metaphase, I, anaphase I, telophase I
prophase I - leptotene, zygotene, pachytene, diplotene, diakinesis
leptotene
chromosomes begin to condense
zygotene
homologs align (synapse) and held together 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, chromosomes are maximally condensed
recombination
chiasmata are the points of crossing over
at least one corssing over per arm is required for proper segregation of chromosomes
only one sister chromatid is involved in each cross-over event
females have greater recombination than males
decreases near centromeres and increases near telomeres
pseudoautosomal regions
X and Y chromosomes share two regions of homology, which undergo very high levels of genetic recombination
PAR1 is at distal and Xp and Yp
Par2 is in distal Xq and Yq
Compare and contrast female and male meiosis.
one of the biggest differences is timing
females only have about 20 mitotic events before meiosis starts
males have hundreds of mitotic events before meiosis starts
dictyotene
a stage in the first meiotic prohase where the egg gets arrested after the first female meiosis begins prenatally (3rd month)
polyploidy
complete sets of extra chromosomes
ex. triploidy = 3N = 69/cell, tetraploidy = 4N = 92/cell, etc.
Turner Syndrome
45, X - lack of second sex chromosome
nondisjunction
failure of chromosomes or sister chromatids to separate
trisomy and monosomy can originate from meiotic or mitotic nondisjunction
most often maternal due to a MI error
errors increas in frequenct with maternal age
balanced translocation pairing at meiosis
forms t structure, tetravalent orientation
alternate segregation will lead to a normal child
adjacent 1 and 2 will produce loss of part of one chromosome and the addition of another
robertsonian translocation
a translocation between two acrocentric chromosomes, which results in the loss of the short arms of both chromosomes, but does not affect the DNA content of the long arms
individuals have normal karyotypes, but only 45 chromosomes
meisosis segregation events for a robertsonian translocation
normal, balanced translocation, Down syndrome, monosomy 21, trisomy 14, monosomy 14
depends on the chromosomes involved, the only viable trisomies are 13, 18, and 21
the only monosomy that is viable is 45X
der(13;14) is the most common, 1 in 1500
recombinations of inverted chromosomes
paracentric - unstable chromosomes due to acentric and dicentric chromosomes, can have recombination events that create on viable offspring
pericentric - results in stable chromosomes with blanced DNA content, can have recombination events that create on viable offspring
karyotype resolution
the degree of banding that is visible dependingo n the condensation state of the DNA
band level is the measurement (typically 400-850)
the higher the number the greater the sensitivity
FISH
fluorescence in situ hybridization
use a probe that ranges from 100kb to 1Mb with imbedded fluorescent molecule
probe can bind to target sequence and identify the presence of a particular DNA sequence
can use cells in interphase
downside is that you have to know what you’re looking for
microdeletion syndroms
deletion of small region less than 5Mb, cannot be seen in normal karyotypes
only the resolution of FISH can detect these deletions
DiGeorge syndrom is one of the most common microdeletion diseases
DiGeorge Syndrome
thymus hypo/aplasia
parathyroid hypo/aplasia
cardiovascular defects
other developmental defects
caused by a microdeletion on chromosome 22 (q11.2)
array based comparative genomic hybridization
molecular cytogenetic method used to detect copy number imbalances
capable of genome-wide interrogation, objective method
two different colored probes hybridized to a plate, normal ratio is 0.8-1.2
control DNA made by combining the DNA of several different people
high resolution tiling oligo array
array of probes centered around telomeres, centromeres, disease genes, and ~100kb interval coverage for each chromosome
pitfalls of high resolution aCGH
cannot detect balanced rearangements and cannot distinguish mechanism
copy number variants (CNVs) - pathogenic, benign, unknown clinical significance
may misinterpret CNVs with this method because mechanism is unclear
