3. WHAT IS A CHROMOSOME?
In the nucleus of each cell, the DNA molecule is packaged into thread-like
structures called chromosomes.
Each chromosome is made up of DNA tightly coiled many times around
proteins called histones that support its structure.
Each chromosome has a constriction point called the centromere into two
sections, or “arms”.
4.
5. CHROMOSOMAL MUTATION….
An unpredictable change that occur in a chromosome.
These changes are most often brought by problems
that occur during cell division or by mutagens like
chemicals, radiations, etc.
Chromosome mutations can result in change in the
number of chromosomes in a cell or changes in the
structure of a chromosome.
6. VARIATION IN CHROMOSOME
STRUCTURE
There are four common types of chromosomal mutations involving
changes in chromosome structure:
1. deletion
2. duplication
3. inversion
4. translocation
7. Deletion and duplication involve change in the amount of
DNA on a chromosome
Inversion involve a change in the orientation of a
chromosomal segment
Translocation involve a change in the location of a
chromosomal segment
8. Deletion
It is a chromosomal mutation in
which a part of a chromosome is
missing or deleted.
Since, a segment of chromosome is
missing so deletion mutations
cannot be revert to the wild type.
ACENTRIC CHROMOSOME: If the
deletion involves the loss of the
centromere
9. cont…..
Some human disorders are caused by deletions of chromosome segments.
Cri-du-chat is one of the human disorder caused by a heterozygous
deletion.
A segmental deletion in chromosome number 5 causes this syndrome.
Children with this syndrome are severely-
mentally retarded,
many physical abnormalities,
cry with a sound like the mew of a cat.
About 1 out of 50,000 live births has a cri-du-chat syndrome.
10. cont….
Another example of chromosomal deletion is Prader-Willi syndrome.
Results due to deletion of long arm of chromosome number 15.
Sucking reflex of the effected infants are weak which results in poor
growth.
But, at the age of around 5 to 6, children with this syndrome become
compulsive eaters, producing obesity and related health problems.
Reason still unknown.
About 1 in 25,000 suffers from this syndrome, predominantly males.
12. Duplication
A duplication is a chromosomal
mutation that results when a
segment of DNA breaks off and
attaches onto the homologous
chromosome.
There are different forms of
chromosomal duplications or
different tandem configuration
1. tandem
2. reverse tandem
3. terminal tandem
13. When the order of genes in the duplicated segment is the opposite of the
order of the original, the mutation is a reverse tandem duplication.
If the duplicated segments are arranged in the tandem at the end of a
chromosome , the mutation is then called terminal tandem duplication.
14. Duplications can cause phenotypic variation that might at first
appear to be caused by a simple gene mutation. The Bar-eye
phenotype in Drosophila is a classic example. Instead of the normal
oval-eye shape, Bar-eyed flies have narrow, slitlike eyes.
15. Inversion
It is a chromosomal mutation that results when a segment of a
chromosome is excised and then reintegrated at an orientation
180 degree from the original orientation.
There are two types of inversions:
1. Paracentric inversion
2. Pericentric inversion
Typically genetic material is not lost when and inversion takes
place.
16. If the
centromere is
not part of the
rearranged
chromosome
segment, it is a
paracentric
inversion.
If the centromere
is part of the
inverted segment,
it is described as a
pericentric
inversion.
17. Translocation
If a segment of chromosome breaks off and attaches to a non-
homologous chromosome then the situation is called translocation.
Translocation can be of three different types:
1. Nonreciprocal intrachromosomal translocation :
when a chromosome segment changes position within the the
same. chromosome.
2. Nonreciprocal interchromosomal translocation :
when a chromosome segment is transferred from one chromosome
to another and it was a one way transfer.
3. Reciprocal interchromosomal translocation : when
a chromosome exchange occurs between both the chromosomes.
18.
19. Robertsonian translocation
One common type of translocation
involves breaks at the extreme ends of
the short arms of two nonhomologous
acrocentric chromosomes. These small
segments are lost, and the larger
segments fuse at their centromeric
region. This type of translocation
produces a new, large submetacentric
or metacentric chromosome, often
called a Robertsonian translocation.
20. One such translocation accounts for cases in
which Down syndrome is familial (inherited).
21. Other types of chromosome
structural aberrations…..
Isochromosomes :
An isochromosome is a chromosome with two
identical arms. Instead of one long (q) arm and one short
(p) arm, an isochromosome has two long arms or two
short arms. As a result, these abnormal chromosomes
have an extra copy of some genes and are missing copies
of other genes.
22. Cont…..
Dicentric chromosomes :
Unlike normal chromosomes,
which have a single constriction
point (centromere), a dicentric
chromosome contains two
centromeres. Dicentric chromosomes
result from the abnormal fusion of
two chromosome pieces, each of
which includes a centromere. These
structures are unstable and often
involve a loss of some genetic
material.
23. Ring chromosomes:
Ring chromosomes usually occur when a chromosome breaks in two
places and the ends of the chromosome arms fuse together to form a circular
structure. The ring may or may not include the chromosome’s constriction
point (centromere). In many cases, genetic material near the ends of the
chromosome is lost.
Cont…..
25. Aneuploidy
Chromosomal mutation which results in the variation
of individual chromosome number are examples of
aneuploidy.
Changes in chromosome number can occur in both
diploid and haploid organisms.
The nondisjunction* of one or more chromosome
during meiosis I or meiosis II typically is responsible
for generating gametes with abnormal numbers of
chromosomes.
27. Nullisomy
A Nullisomic cell involves a loss of one
homologous chromosome pair-
the cell is 2N-2.
Monosomy
loss of a single chromosome :
(2n - 1)
Human monosomies are conceived but none survive embryonic and
fetal development. With the exception of Turner syndrome (45, X),
these are NOT tolerated in animals.
28. Trisomy
It involves a single extra chromosome: 2N+1
In general, the effects of trisomy parallel
monosomy, but the addition of an extra
chromosome produces somewhat more
variable individuals.
Tetrasomy
A tetrasomic cell involves an extra pair of
chromosome, i.e. there are four copies of one
particular chromosome:
the cell is 2N+2
32. Euploidy
Euploid organisms have varying numbers of complete haploid
chromosomal sets.
MONOPLOIDY and POLYPLOIDY involve variation from the normal
state in the number of complete sets of chromosomes.
Monoploidy and Polyploidy are lethal in most animal species, but are less
consequential in plants.
33. Monoploidy
A monoploid individual has only one set of chromosomes instead of the
usual two sets.
Some male wasps, ants and bees, for example, are monoploid because they
develop from unfertilized eggs.
Cells of a monoploid individual are very useful for producing mutants,
because there is only one dose of each of the genes. Thus, mutants can be
isolated.
34. Polyploidy
Polyploidy is the chromosomal constitution of a
cell or an organism that has three or more sets
of chromosomes.
Almost all animals and plants probably have
some polyploidy tissues. For example, the
endosperm of plants is triploid, the liver of
mammals and perhaps other vertebrates is
polyploidy.
Plants that are completely polyploidy include
wheat, which is hexaploid (6N), and the
strawberry, which is octaploid (8N). Some
animal species, such as the North American
sucker (a freshwater fish), salmon, and some
salamanders are polyploid.
Two types of polyploidy are encountered in
plants:
35. Autopolyploidy
In this, all the sets of chromosomes originate in the
same species. The condition probably results from a
defect in meiosis that leads to diploid or triploid gamets.
The cultivated banana is an example of triploid
autopolyploid plant.
In general,the development of “seed-less” fruits such as
grapes and watermelons relies on odd-number
polyploidy.
Allopolyplody
In this, the sets of chromosomes involved come from
different, though usually related, species.
This situation can arise if two different species interbreed
to produce an organism with one haploid set of each
parents chromosomes and then both chromosome sets
double.
A classic example of allopolyploidy resulted from crosses
made between cabbages and radishes by Karpechenko in
1928.
