Chromosomal Aberrations

(Structural changes of
Chromosomes)
CC13
UNIT-3

DR. ARINDAM MANDAL

Assistant Professor
Bejoy Narayan Mahavidyalaya
Itachuna, Hooghly
West Bengal
 Chromosomes
• Chromosomes are thread-like structures located
inside the nucleus of animal and plant cells.
• Each chromosome is made of protein and a single
molecule of deoxyribonucleic acid (DNA). Thus,
chromosomes are actually a nucleoprotein complex.
• They carry the genetic information form generation to
generation.
• The DNA molecule contain genes, which are the basic
physical and functional unit of heredity
 Chromosomes
During cell division, the chromosomes can be seen to consist
of 2 parallel strands, which is known as chromatids. Two
strands are called as sister chromatids.
Two sister chromatids are held together at one point that is
known as centromere. Chromosomes are found in pairs in
diploid organisms, termed as homologous chromosomes.
Centromere contains a complex structure of proteins to which
microtubules attach during cell division, which is called as
kinetochore.
 Karyotype
• It is the set of chromosomes of an individual.
• It is the systematized arrangement of the
chromosomes of a single cell.
• In the human cell, there are 46 chromosomes or 23
pairs (diploid number); of these 23 pairs, 22 are
similar in both sexes and are called the autosomes.
The remaining pair is called sex chromosomes : XX
in the female cells and XY in the male cells.
Karyogram: A display of the chromosomes of a cell,
sorted into pairs.
Karyogram
Karyogram
 Karyotype
 Asymmetric karyotype is defined as the huge
difference between the largest and smallest
chromosome as well as less number of metacentric
chromosomes in a chromosome complement.
 Similarly, symmetric karyotype is defined as the
small difference between the largest and smallest
chromosome as well as more number of
metacentric chromosomes in a chromosome
complement.
 Chromosomal aberration
Two types:
1. Structural chromosomal aberration: Structural
chromosome abnormalities occur when part of a
chromosome is missing, a part of a chromosome is
extra, or a part has switched places with another
part leading to too much or too little genetic
material. It may be of following types
Deletion
Duplication
Inversion
Translocation
 Chromosomal aberration
2. Numerical chromosomal aberration: Numerical
chromosomal abnormalities occur when there is a
different number of chromosomes in the cells of the
body from what is usually found. So, instead of
normal number of chromosome, there may be extra
or less number of chromosomes.
Polyploidy
Aneuploidy
 Introduction
Also called chromosome rearrangements
Deletion: loss of segment
Duplication: gain of segment
Inversion: reversal of region
Translocation: movement of segment to
another chromosome
Origin
Origin
 Duplication types
(i) Direct tandem duplication in which the duplicated
gene sequence lies just next to normal
corresponding section.
(ii) Reverse tandem duplication in which the
duplicated section with reverse gene sequence lies
adjacent to normal sequence.
 Duplication types
(iii) Displaced direct duplication in which the duplicated
section is not adjacent or contiguous with the normal section
(i.e., separated by other segment).
(iv) Displaced Reverse duplication in which the duplicated
section with reverse gene sequence is separated from normal
segment by other segment
(v) Transposed duplication in which the duplicated gene
sequence is attached to another position owing to inter-
chromosomal duplication.
Duplication
 Deletion
(i) Terminal deletion: A single
break near the end of a
chromosome would be
expected to result in a
terminal deficiency.
(ii) Intercalary deletion: If
two breaks occur, a section
may be deleted and an
intercalary deficiency is
created.
 Meiotic behavior
 In individuals heterozygous for deletions, the normal
chromosome must loop during the pairing of
homologs in prophase I of meiosis to allow the
homologous regions of the two chromosomes to align
and undergo synapsis.
 This looping out generates a structure that looks very
much like that seen for individuals heterozygous for
duplications.
 Effect of deletion
Hemizygous: Gene is present in a single dose.
Psuedodominance: Hemizygous genes are expressed.
 Inversion
An inversion is a chromosome rearrangement
which occurs when a chromosome breaks at two
points and the segment bounded by the
breakpoints is reinserted in the reversed
orientation.
An inversion occurs when a single chromosome
undergoes breakage and rearrangement within
itself.
 Origin of Inversion
Inversions are of two types-
paracentric and pericentric.
 Pericentric Inversion
A chromosomal imbalance
is produced as a result of a
crossover event between a
chromatid bearing a
pericentric inversion and its
noninverted homolog.

The recombinant
chromatids that are directly
involved in the exchange
have duplications and
deletions.
 Pericentric Inversion
In plants, gametes receiving
such aberrant chromatids fail
to develop normally, leading
to aborted pollen or ovules.
Thus, lethality occurs prior to
fertilization, and inviable
seeds result.

In animals, the gametes have

developed prior to the meiotic
error, so fertilization is more
likely to occur in spite of the
chromosome error. However,
the end result is the
production of inviable
embryos following
fertilization.
 Paracentric Inversion
In prophase I of meiosis, an
inversion loop forms,
allowing the homologous
sequences to pair up.
If a single crossover takes
place in the inverted region ,
an unusual structure results .
The two outer chromatids,
which did not participate in
crossing over, contain
original, non-recombinant
gene sequences.
 Paracentric Inversion
The two inner chromatids, which
did cross over, are highly
abnormal: each has two copies
of some genes and no copies of
others.
One of the four chromatids now
has two centromeres and is said
to be a dicentric chromatid; the
other lacks a centromere and is
an acentric chromatid.
In anaphase I of meiosis, the
centromeres are pulled toward
opposite poles and the two
homologous chromosomes
separate.
 Paracentric Inversion
This action stretches the dicentric chromatid across the
center of the nucleus, forming a structure called a
dicentric bridge.
Eventually, the dicentric bridge breaks, as the two
centromeres are pulled farther apart.
Spindle fibers do not attach to the acentric fragment,
and so this fragment does not segregate into a nucleus
in meiosis and is usually lost.
 Paracentric Inversion
In the second division of meiosis,
the sister chromatids separate and
four gametes are produced.
Two of the gametes contain the
original, non-recombinant
chromosomes . The other two
gametes contain recombinant
chromosomes that are missing
some genes; these gametes will not
produce viable offspring.
Thus, no recombinant progeny
result when crossing over takes
place within a paracentric inversion.
 Translocation
Translocations (movement of a chromosomal
segment from one location to another)
Types
1. Nonreciprocal (1 segment moves to a new location
without an exchange)
2. Reciprocal (exchange of segments)
Reciprocal translocations require two breaks in two
different chromosomes followed by rejoining of the
ends.
 Origin of translocations
1. Mechanical shear: Breaks occur frequently due to
mechanical shear because of chromosome
entanglement at interphase or prophase. The
broken chromosome segments then reunite with
non-homologous chromosome to produce
translocations.
2. Formation of interlocked bivalents: Interlocking
during prophase of meiosis when a non-
homologous chromosome passes through a loop
of two homologous chromosomes that are in the
process of pairing. The interlocked bivalents
subsequently separate at anaphase-I, but during
this process breakage and reunion takes place.
 Origin of translocations
3. Physical and chemical agents: Physical mutagens (like
X-rays) and various chemical agents induce breaks in the
chromosomes. If, two or more breaks occur
simultaneously in the non-homologous chromosomes,
may result in translocations. Most of the naturally
occurring translocations are due to these mutagenic
agents because the organisms are continuously exposed
to these agents, in the environment.

4). Crossing over in homologous regions: Some times,

some duplicated segments are found between non-
homologous chromosomes. These duplicated segments
are homologous to each other and crossing over
between these segments may lead to translocations
 Cytology of Translocated Heterozygotes
In meiosis I cells
heterozygous for the
Translocation, cross
conformation forms to get
proper alignment of the
homologous chromosomes.
1.Alternate: In this case,
alternate chromosomes will
be oriented towards the
same pole, or the adjacent
chromosomes will orient
towards opposite poles.

2.Adjacent-I: In this type of

orientation adjacent
chromosomes having non-
homologous centromeres are
oriented towards the same
pole.

3.Adjacent-II: In this type of

orientation, adjacent
chromosomes with
homologous centromeres
will move towards the same
pole at anaphase.
 Robertsonian translocations
 The fusion of two acrocentric
chromosomes with the subsequent
loss of the two short arms is termed
Robertsonian translocation or centric
fusion.
 Although this translocation causes
loss of the short arms, it is maintained
as a balanced translocation. This is
explained by the fact that the genes
on the short arms are most rRNA
genes that are present in many copies
on other chromosomes; thus deletion
of these copies doesn’t have much
phenotypic manifestation.
 One of the commonly seen such
translocation is between chromosome
14 and 21.
Effect of translocations
1. Semisterility: due to adjacent segregation in meiosis
2. Position effects: altered expression of a gene when it is
moved to a new location
Detection
1. Genetic
a) Semisterility
b) Apparent linkage of genes on separate chromosomes
c) Position effects
2. Cytological
a) Can change the location of the centromere
b) Change in the size of the chromosome
c) Cross formation in meiosis I