Download chromosome disorders.

DNA is packaged into
chromosomes..
• Unlike DNA chromosomes can be visualized during cell
division (light microscope)
• The word chromosome is derived from Greek chroma (color)
and soma (body)
• Chromosomes are the factors that distinguish one species
from another.
• Transmission of genetic info from one generation to the next.
• The study of chromosomes and cell division is referred to as
cytogenetics
• Before 1950 48 chromosomes ??? Sex chromosomes “X”
• After 1956 correct chromosomal number 46 Sex
chromosomes are “X” and “Y”
Human Chromosomes
Morphology
• At the submicroscopic level,
chromosomes made up of
supercoils of DNA, which has
been linked to the tightly
coiled network of wiring seen
in a solenoid.
• Special stains selectively taken
up by DNA have enabled each
individual chromosome
identified.
•
• Each chromosome consist of two
identical strands (CHROMATIDS or
SISTER CHROMATIDS)
• These sister chromatids joined at a
primary constriction (CENTROMERE)
• Centromeres consist of several
hundred kilobases of repetitive DNA
and responsible for the movement
of chromosomes at cell division
• Each centromere divides chromosome into short and long
arms;
• Short arm : P (petite)
• Long arm : Q (Grande)
Short
arm (p)
Long
arm (q)
• The tip of each chromosome arm is known as the TELOMERE.
• Telomeres play a crucial role in sealing the ends of
chromosomes and maintaining their structural energy
• Telomeres have been highly conserved throughout evolution
and in humans they consist of many tandem repeats DNA
Human chromosomes
Classification
• Morphologically chromosomes are classified according to the
position of the centromere
• Centrally localized:
METACENTRIC
• Terminally localized: ACROCENTRIC
• Intermediate position: SUBMETACENTRIC
• Chromosomes are classified not only to the position of the
centromere, but also in their overall length A-G
Group A:
Group B:
Group C:
Group D:
Group E:
Group F:
Group G:
1, 2, 3
4, 5
6, 7, 8, 9, 10, 11, 12, X
13, 14, 15
16, 17, 18
19, 20
21, 22, Y
• Today…
Chromosomes as seen at metaphase
during cell division
Telomere
DNA and protein cap
Ensures replication to tip
Tether to nuclear
membrane
Short arm
p (petit)
Long arm
q
Telomere
Light bands
Replicate early in S phase
Less condensed chromatin
Transcriptionally active
Gene and GC rich
Centromere
Joins sister chromatids
Essential for chromosome segregation at cell
division
100s of kilobases of repetitive DNA: some nonspecific, some chromosome specific
Dark (G) bands
Replicate late
Contain condensed chromatin
AT rich
Human
chromosome
banding patterns
seen on light
microscopy
Chromosome 1
Different chromosome banding resolutions can resolve bands, sub-bands and sub-sub-bands
A pair of homologous chromosomes (number 1) as
seen at metaphase
Locus (position of a gene or
DNA marker)
Allele (alternative form of a
gene/marker)
Methods of chromosome
analysis
• Conventional chromosome analysis
• High-resolution banding techniques
• Molecular chromosome analysis
• Florescent in-situ hybridization (FISH)
• Comparative genomic hybridization (CGH)
• Array based comparative genomic hybridization (array CGH)
Chromosome Preparation
• Any tissue with living nucleated cells that undergo division can
be used for studying human chromosomes.
• Most commonly circulating lymphocytes from peripheral
blood are used.
• Skin, bone marrow, chorionic villi, cells from amniotic fluid,
tumor tissue etc.
Cell culture
Peripheral blood sample is added to a small volume of nutrient
medium
The cells are cultured under sterile conditions at 37C for 3 days,
during which they divide
Colchicine is added each culture
Colchicine has extremely useful property of preventing
formation of the spindle, thereby arresting cell division during
metaphase
Metaphase is the time when the chromosomes are maximally
condensed and therefore most visible
Karyotype Analysis
1. Counting the number of chromosomes
2. Analysis of the banding pattern of each individual
chromosome in selected cells
Ideogram
FISH
• This diagnostic tool combines
conventional cytogenetics
with molecular genetic
technology.
• The DNA probe s labeled
with a fluorochrome which,
after hybridization with the
patient’s sample allows the
region where hybridization
occurred to be visualized
using fluorescence
microscope.
CGH
• This technique enabled the detection of regions of allele loss
and gene amplification.
• Patient DNA (green labeled) and reference DNA (red labeled)
samples are mixed and hybridized competitively to normal
metaphase chromosomes and an image captured.
• If the patient sample contained more DNA from a particular
chromosome region than the reference sample that region
was identified by an increase in the green to red fluorescence
ratio
Chromosomal
Abnormalites
• Clinical cytogenetics is the study of chromosomes, their
structure and their inheritance, as applied to the practice of
medical genetics.
• It has been apparent for nearly 50 years that chromosome
abnormalities—microscopically visible changes in the number
or structure of chromosomes—could account for a number of
clinical conditions that are thus referred to as chromosome
disorders.
• Chromosome disorders form a major category of genetic
disease. They account for a large proportion of all
reproductive wastage, congenital malformations, and mental
retardation and play an important role in the pathogenesis of
malignant disease.
• Cytogenetic disorders are present in nearly
• 1% of live births,
• in about 2% of pregnancies in women older than 35 years who
undergo prenatal diagnosis, and
• in fully half of all spontaneous first-trimester abortions.
Clinical indications of
chromosome analysis
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Problems of early growth and development.
Stillbirth and neonatal death.
Fertility problems.
Family history.
Neoplasia
Pregnancy in a woman of advanced age.
Abnormalities of Chromosome
Number
• A chromosome complement with any chromosome number
other than 46 is said to be heteroploid.
• An exact multiple of the haploid chromosome number (n) is
called euploid,
• and any other chromosome number is aneuploid.
• In addition to the diploid (2n) number characteristic of normal
somatic cells, two other euploid chromosome complements,
triploid (3n) and tetraploid (4n), are occasionally observed in
clinical material.
Aneuploidy
• Aneuploidy is the most common and clinically significant type
of human chromosome disorder, occurring in at least 5% of all
clinically recognized pregnancies.
• Most aneuploid patients have either trisomy or, less often,
monosomy
• Trisomy can exist for any part of the genome, but trisomy for a
whole chromosome is rarely compatible with life.
• Monosomy for an entire chromosome is almost always lethal;
an important exception is monosomy for the X chromosome,
as seen in Turner syndrome.
• Although the causes of aneuploidy are not well understood, it is
known that the most common chromosomal mechanism is
meiotic nondisjunction
• Nondisjunction can also occur in a mitotic division after
formation of the zygote. If this happens at an early cleavage
division, clinically significant mosaicism may result
Abnormalities of Chromosome
Structure
• Structural rearrangements result from chromosome breakage,
followed by reconstitution in an abnormal combination.
• overall, structural abnormalities are present in about 1 in 375
newborns.
• Structural rearrangements are defined as balanced, if the
chromosome set has the normal complement of chromosomal
material,
• or unbalanced, if there is additional or missing material.
Unbalanced Rearrangements
• In unbalanced rearrangements, the phenotype is likely to be
abnormal because of deletion, duplication, or both.
• Duplication of part of a chromosome leads to partial trisomy;
deletion leads to partial monosomy.
• Any change that disturbs the normal balance of functional
genes can result in abnormal development.
• Large deletions or duplications can be detected at the level of
routine chromosome banding
• Detection of smaller deletions or duplications generally
requires more sophisticated analysis, involving FISH or
microarray analysis
Deletions
• Deletions involve loss of a
chromosome segment,
resulting in chromosome
imbalance
• Cytogenetically visible
autosomal deletions have an
incidence of approximately 1
in 7000 live births.
• Smaller, submicroscopic
deletions detected by
microarray analysis are much
more common, but the clinical
significance of many such
variants has yet to be fully
determined !!!
Duplications
• Duplications involve gain of a chromosome segment, resulting
in chromosome imbalance.
Marker and Ring
Chromosomes
• Very small, unidentified chromosomes, called marker
chromosomes, are occasionally seen in chromosome
preparations.
• They are usually in addition to the normal chromosome
complement and are thus also referred to as supernumerary
chromosomes or extra structurally abnormal chromosomes.
• Cytogeneticists find it difficult to characterize marker
chromosomes specifically by banding, because they are
usually so small that the banding pattern is ambiguous or not
apparent.
• Ring chromosomes are quite rare but have been detected for
every human chromosome.
Isochromosomes
• An isochromosome is a chromosome in which one arm is
missing and the other duplicated in a mirror-image fashion.
• The most common isochromosome is an isochromosome of
the long arm of the X chromosome, i(Xq), in some individuals
with Turner syndrome
Balanced Rearrangements
• Chromosomal rearrangements do not usually have a
phenotypic effect if they are balanced because all the
chromosomal material is present even though it is packaged
differently.
• It is important to distinguish here between truly balanced
rearrangements and those that appear balanced
cytogenetically but are really unbalanced at the molecular
level.
• Even when structural rearrangements are truly balanced, they
can pose a threat to the subsequent generation because
carriers are likely to produce a high frequency of unbalanced
gametes and therefore have an increased risk of having
abnormal offspring with unbalanced karyotypes; depending
on the specific rearrangement, the risk can range from 1% to
as high as 20%.
Inversions
• An inversion occurs when a single chromosome undergoes
two breaks and is reconstituted with the segment between
the breaks inverted.
• Inversions are of two types:
• paracentric (not including the centromere), in which both
breaks occur in one arm;
• pericentric (including the centromere), in which there is a
break in each arm.
• Pericentric inversions are easier
to identify cytogenetically because
they may change the proportion of
the chromosome arms as well as
the banding pattern.
Translocations
• Translocation involves the exchange of chromosome segments
between two, usually nonhomologous, chromosomes.
• There are two main types:
• reciprocal
• Robertsonian.
Reciprocal Translocations
• This type of rearrangement results from breakage of
nonhomologous chromosomes, with reciprocal exchange of
the broken-off segments.
• Usually only two chromosomes are involved, and because the
exchange is reciprocal, the total chromosome number is
unchanged.
• Reciprocal translocations are relatively common and are found
in approximately 1 in 600 newborns.
• Balanced translocations are more commonly found in couples
that have had two or more spontaneous abortions and in
infertile males than in the general population.
Robertsonian Translocations
• This type of rearrangement involves two acrocentric
chromosomes that fuse near the centromere region with loss
of the short arms
• The resulting balanced karyotype has only 45 chromosomes,
including the translocation chromosome, which in effect is
made up of the long arms of two chromosomes.
• Although Robertsonian translocations involving all
combinations of the acrocentric chromosomes have been
detected, two (13q14q and 14q21q) are relatively common.
• Although a carrier of a Robertsonian translocation is
phenotypically normal, there is a risk of unbalanced gametes
and therefore of unbalanced offspring.
Insertions
• An insertion is a nonreciprocal type of
translocation that occurs when a
segment removed from one
chromosome is inserted into a different
chromosome, either in its usual
orientation or inverted
• Because they require three
chromosome breaks, insertions are
relatively rare.
• The average risk of producing an
abnormal child is high, up to 50%, and
prenatal diagnosis is indicated.
Incidence of
Chromosome
Anomalies