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Dr. Attya Bhatti
Chromosomal Abnormalities
Chromosomal Abnormalities
Any change in the normal structure or
number of chromosomes; often results in
physical or mental
Basic categories :Chromosome rearrangements:
Chromosome rearrangements alter the structure of chromosomes; for example, a piece of a chromosome might be duplicated, deleted, or inverted.
Aneuploids: the number of chromosomes is altered: one or more individual
chromosomes are added or deleted. Polyploids:
one or more complete sets of chromosomes are added.
Chromosome Abnormalities
Table: Types chromosome abnormality
NumericalAneuploidy
Monosomy
Trisomy
Tetrasomy
Polylpoidy Tripolody
Tetrapoildy
Different cell lines (mixoploidy)
Mosaicism
Chimerism
StructuralTranslication
Reciprocal
Robertsonian
Deletions
Insertions
InversionsParacenyric
Pericentric
Rings
Isochromosomes
HETEROPLOIDY ABERRATIONSChromosomal aberrations that changes the ploidy
(one set of chromosomes) of an organisms is called heteroploidy.
Euploid AberrationsAneuploid Aberrations
Numerical AbnormalitiesInvolves the loss or gain of one or more
chromosomes, referred as Aneuploidy
Euploid/ Polyploidy
EUPLOID ABERRATIONSEuploid mutations produces organisms
possessing multiple sets of chromosomes.
These are the changes in the number of
chromosomes.
MonoploidsOne set of chromosomes (n=ABC) is
present, mostly, in the nuclei of haplotonic organisms.
E.g. ChlamydomonasNeurosoporaAlso in diploid organisms, is usually in sex
cells (male bees and wasps)
TriploidsOrganisms may receives three sets of
chromosomes(3n= AAA-BBB-CCC).Results due to union of haploid gamete
with diploid gametes.These organisms are sterile and not
common in Natural populations.
TetraploidsTetraploid organisms have four sets of
chromosomes(4n= AAAA-BBBB-CCCC).Arises in body cells by the somatic
doubling of chromosomes number.Produced by the union of diploid gametes.
Two groupsAutotetraploids (auto-self): Produced by either somatic doubling of
homologous chromosomes, or by the union of diploid gametes of the same species.
Parental genotype AABBCC X AABBCC AAAABBBBCCCC
Allotetraploids (Allo: non-homologous)Produced by fusion of diploid gametes of
different species, Reproduce true and behave as a new
species.P.Genotyope: AABBCC X DDEEFF
AADDBBEECCFF
Found only in plants and called Amphi-di-ploids
PolyploidsPolyploid organisms have more than 2n
chromosomes.Wheat, e.g. hexaploidMany commercial fruits , ornamentals
plants and human liver cells are polyploidy.
Polyploidy provide for studying dosage effect (How many alleles interact to form phenotypes)
Polyploidy
Changes in the number of chromosome sets (polyploidy).
Polyploids include triploids (3n) ; Major cause is two sperm fertilization by single
egg (dispermy) tetraploids (4n), rare and lethal, due to failiur to complete the
first zygotic divisionpentaploids (5n), andhigher numbers of chromosome sets.
Polyploid organisms have more than 2n chromosomes. Many commercial fruits and ornamentals plants are polyploidy.
Polyploid cells contain multiples of the haploid number of chromosomes such as 69, triploidy, 92, tetraploidy, Wheat, e.g. hexaploid
PolyploidyPolyploid cells contain multiples of the
haploid number of chromosomes such as 69, triploidy
92 tetraploidy
PolyploidyPolyploid cells contain multiples of the
haploid number of chromosomes such as 69, triploidy
92 tetraploidy
CausesFailure of a maturation meiotic division in
an ovum or sperm.By fertilization of an ovum bt two sperms,
called dispermy.When triploidy results from the presence
of an additional set of paternal chromosome, the placenta ia usually swollen known as Hydatidiform changes.
Mixoploidy
1. Mosaicism; An individual possesses two or more genetically
different cell lines all derived from a single zygote.
2. Chimerism: An individual has two or more genetically
different cell lines originating from different zygotes.
(Organism derived from more than one zygote).
ANEUPLOID ABBERATIONSOrganism is that, which bears an
irregular number of a particular chromosomes
( addition and deletion of whole sets of chromosomes).
Usually caused by failure of chromosomes to separate during meiosis (non-disjunction).
Abnormal male (XO) and female(XXY).
Non-disjunctionWhen a member of synaped homologous pair of
chromosomes, at anaphase, fail to separate and the gametes thus formed become abnormal.
Some gametes receives both members of homologues while other gamete none.
Fertilization of such abnormal gametes from zygotes that either have an additional chromosomes(2n+1) or lack chrmosomes(2n-1).
Origin of non-disjunctionAn error in meiosis I leads to the gamete
containing both homologs of one chromosomes pair.
In meiosis II results in the gamete receiving two copies of one of the homologs of the chromosomes pair.
Can also occur during an early mitotic division in the developing zygote, which results in mosaicism
PARENTAL ORIGIN OF MEIOTIC ERROR LEADING TO ANEUPLOIDYChromosomes abnormality Parental (%) Maternal (%)
Trisomy 13 15 85
Trisomy 18 10 90
Trisomy 21 5 95
45,X 80 20
47XXX 5 95
47,XXY 45 55
47,XYY 100 0
Causes of Non-disjunctionAn aging effect on the primary oocyte,
which can remain in a state of suspended inactivity for upto 50 years.
Association b/w advancing maternal age and increased incidence of down Syndrome.
Factors causing Non-disjunction
An absence of recombination b/w homologous chromosomes in
foetal ovary
An abnormality in spindle formation
Radiation
Delayed fertilization after ovulation.
Monosomy
The absence of a single chromosome is refered to as monosomy.
Diploid organisms that has one chromosomes less than its normal
diploid number(2n-1= AABBC).
Monosomies on meiosis produces two types of gametes with (n)
and (n-1) chromosomes.
In animals , loss of one chromosomes often results in genetic
imbalance which is associated with high mortality or reduce
fertility.
Human Syndromic disease like Turners syndrome (XO) is an
example of monosomic mutations.
Results due to non-disjunction in meiosis
Monosomy
If one gamete receive two copies of a homologous
chromosomes, (Disomy)
While other corresponding daughter gamete will have no
copy of the same chromosome (nullisomy)
Also due to loss of a chromosomes as it moves to the
pole of the cell during anaphase, known as Anaphase
lag.
Trisomy
Presence of an extra chromosome is refered to as trisomy.
Down Syndrome (trisomy 21)
Patau Syndrome (trisomy 13)
Edward Syndrome (trisomy 18)
Trisomy
Caused by Failure of separation of one of the pairs of
homologous chromosomes during anaphase of meiosis I.
Can be caused by non-disjunction occurring during
meiosis II when a pair of a sister chromatids fails to
separate.
Trisomy
Diploid organisms that have one chromosome extra
(2n+1= AABBCCC) are called Trisomics.
Tetrasomics
Diploid organisms that have one chromosome in
quarduplicate (2n+ 2= AABBCCCC).
Nullisomics An diploid organism, that has lost one chromosome pair
from its genotype is called nullisomic.
It is lethal in diploids
Some polyploids, however, can lose one homologous
pair without serious effects (AAAABB)
Nullisomics of hexaploid wheat(6n-2) show reduced vigor
and fertility but can survive to maturity.
Double Trisomics
If in a diploid organism, two different chromosomes are
present in triplicate , called double trisomic and
presented as (2n+1+1) AABBCCC
In humans Klinefelter syndrome (XXYY)
Type No. of chromosomes Example
Normal diploid 2n AABBCC
Monosomic 2n-1 AABBC
Nullisomic 2n-2 AABB
Polysomic Extra chromosomes
a) Trisomic 2n+1 AABBCCC
b) Double trisomic
2n+1+1 AABBBCCC
c) Tetra somic 2n+2 AABBCCCC
d) Pentasomic 2n+3 AABBCCCCC
Sexual Aneuploids in human and their phenotypes
Sex chromosomes Sexual phenotype No. of bar bodies
Female
XX, 46 Normal 1
XO, monosomic, 45 Turner syndrome 0
XXX, trisomic, 47 MR female 2
Male
XY, 46 Normal 0
XXY, trisomic, 47 Klinefelter syndrome 1
XXYY, double trisomic, 48
Klinefelter syndrome 1
XXXY, tetrasomic, 48 Klinefelter syndrome 2
Chromosome Morphology
Under the microscope chromosomes appear as thin, thread-like structures.
They all have a short arm and long arm separated by a primary constriction called the centromere. The short arm is designated as p and the long arm as q.
The centromere is the location of spindle attachment and is an integral part of the chromosome. It is essential for the normal movement and segregation of chromosomes during cell division.
Human metaphase chromosomes can be categorized according to the length of the short and long arms and also the centromere location. • Metacentric chromosomes have short and long arms of roughly equal length with the
centromere in the middle. • Submetacentric chromosomes have short and long arms of unequal length with the
centromere more towards one end. • Acrocentric chromosomes have a centromere very near to one end and have very small
short arms. They frequently have secondary constrictions on the short arms that connect very small pieces of DNA, called stalks and satellites, to the centromere.
The stalks contain genes which code for ribosomal RNA. The diagrams showing region on chromosomes, called ideograms.
Metacentric(Chromosome 1)
Submetacentric(Chromosome 9)
Acrocentric(Chromosome 14)
•The ideogram is basically a "chromosome map" showing the relationship between the short and long arms, centromere (cen), and in the case of acrocentric chromosomes the stalks (st) and satellites (sa). Each band is numbered to aid in describing rearrangements.
Chromosomes are identified by their size, centromere position and banding pattern
Autosomes are numbered from largest to smallest, except that chromosome 21 is smaller than chromosome 22.
Group
Chromosomes
Description
A 1–3 Largest; 1 and 3 are metacentric but 2 is submetacentric
B 4,5 Large; submetacentric with two arms very different in size
C 6–12,X Medium size; submetacentric
D 13–15 Medium size; acrocentric with satellites
E 16–18 Small; 16 is metacentric but 17 and 18 are submetacentric
F 19,20 Small; metacentric
G 21,22,Y Small; acrocentric, with satellites on 21 and 22 but not on the Y
Cytogenetics
Is the study of the structure and properties of
chromosomes, chromosomal behaviour during mitosis
and meiosis, chromosomal influence on the phenotype
and the factors that cause chromosomal changes.
Related to disease status caused by abnormal
chromosome number and/or structure.
Methods for chromosomal analysis: Karyotyping and banding
The collection of all the chromosomes is referred to as a
Karyotype.
The method used to analyze the chromosome constitution of
an individual, known as chromosome banding.
Chromosomes are displayed as a karyogram.
Cell source:
Blood cells
Skin fibroblasts
Amniotic cells / chorionic villi
Increasing the mitotic index
- proportion of cells in mitosis using colcemid
Synchronizing cells to analyze prometaphase
chromosomes
Obtaining and preparing cells forchromosome analysis
Key procedure
In the case of peripheral (venous) blood
A sample is added to a small volume of nutrient medium containing
phytoheamagglutinin, which stimulates T lymphocytes to divide.
The cells are cultured under sterile conditions at 37C for about 3
days, during which they divide, and colchicine is then added to
each culture.
This drug has the extremely useful property of preventing formation
of the spindle, thereby arresting cell division during metaphase, the
time when the chromosomes are maximally condensed and
therefore most visible.
Hypotonic saline is then added, which causes the red blood cells
to lyze and results in spreading of the chromosomes, which are then
fixed , mounted on a slide and stained ready for analysis
PREPARATION OF CHROMOSOMES
Following Steps are involved; Counting the number of cells, sometimes referred as
metaphase spread Analysis of the banding pattern of each individual
chromosome in selected cells. Total chr. Count is determined in 10-15 cells, but if
mosaicism is suspected then 30 or more cell count will be undertaken.
Detailed analysis of the banding pattern of the individual chromosomes is carried out in approx. 3-5 metaphase spread, which shows high quality banding.
The banding pattern of each chromosome is specific and shown in the form of Idiogram.
Karyotype Analysis
MITOTIC CHROMOSOMAL SPREAD
Chromosome Banding
Chromosome banding is developed based on the
presence of heterochromatin and euchromatin.
Heterochromatin is darkly stained whereas
euchromatin is lightly stained during chromosome
staining.
oEuchromatin, which undergoes the normal process of
condensation and decondensation in the cell cycle, and
oHeterochromatin, which remains in a highly condensed state
throughout the cell cycle, even during interphase.
Euchromatin Exist in extended state, dispersed through the nucleus and staining diffusely.
Early-replicating and GC rich region.
In prokaryotes, euchromatin is the only form of chromatin present.
Genes may oy may not expressed
Heterochromatindarkly stained two types
1.Constitutive ; always inactive an condensed.
Consist of repetitive DNA
Late replicating and AT rich region
Present at identical positions on all chromosomes in all cell types of an organism.
Genes poorly expressed.
Human chromosomes 1, 9, 16, and the Y chromosome contain large regions of constitutive heterochromatin.
Occurs around the centromere and near telomeres.
2. Facultative;
Genetically active(decondensed) and inactive (condensed)
Variable in its expression. It varies with the cell type and may be manifested as condensed, or heavily stained.
Types of chromosome banding
G-banding
C-banding
Q-banding
R-banding T-banding
Chromosomal Banding G-banding, gives dark bands
C-banding: C-banding stains the constitutive heterochromatin, which usually lies near the centromere.
Q-banding: Q-banding is a fluorescent pattern obtained using quinacrine for staining. The pattern of bands is very similar to that seen in G-banding.
R-banding: reverse of G-banding (the R stands for "reverse").
Dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions).
T-banding: Identifies a subset of the R bands which are especially concentrated at the telomeres.
G-Banding
۩- G-banding is obtained with Giemsa stain following digestion of
chromosomes with enzyme trypsin.
۩- Giemsa stain, named after Gustav Giemsa, an early malariologist, is
used for the histopathological diagnosis of malaria and other parasites.
۩- It is a mixture of methylene blue and eosin.
۩- It is specific for the phosphate groups of DNA and attaches itself to
regions of DNA where there are high amounts of adenine-thymine
bonding.
۩- it yields a series of lightly and darkly stained bands – the dark regions
tend to be heterochromatic, late-replicating and AT rich.
۩- The light regions tend to be euchromatic, early-replicating
and GC rich .
G- Banding
Molecular Methods for chromosomal analysis Molecular Cytogenetics
Fluorescent in situ Hybridization (FISH)
Chromosome painting
Comparative Genomic Hybridization
(CGH)
Molecular karyotyping and Multiplex
FISH(M-FISH)
Spectral Karyotyping
Array CGH
Key points Cytogenetic analysis usually focuses on chromosomes in
dividing cells.
Dyes such as Quinacrine and Giemsa create banding patterns that’s are useful in identifying individual chromosomes within a cell.
A karyotype shows the photographed chromosomes of a cell arranged for cytogenetic analysis.
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