Karyotypic Differentioation between Drosophila and Mammals
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Transcript of Karyotypic Differentioation between Drosophila and Mammals
INTRODUCTION
• Chromosomes in a cell are visible only when
the cell is dividing
• At metaphase stage chromosomes are fully
condensed and easy to see.
• Metaphase spreads are selected and
photographed in order to analyze
The chromosomes are then arranged in
homologous pairs.
• The homologous pairs are then
placed in order of descending size.
The sex chromosomes are placed at
the end.
• A picture of chromosomes arranged
in this way is known as a karyotype.
HOW INDIVIDUAL
CHROMOSOMES ARE IDENTIFIED
• .
CENTROMERE POSITION
KARYOTYPIC
DIFFERENTIATION IN
DROSOPHILA
DROSOPHILA MELANOGASTER
• Drosophila melanogaster is a species
of fly (the taxonomic order diptera).
The species is known generally as
the common fruit fly or vinegar fly.
• D. Melanogaster continues to be
widely used for biological research in
studies of genetics, physiology,
microbial pathogenesis, and life history
evolution. It is typically used because it
is an animal species that is easy to
care for, has four pairs
of chromosomes, breeds quickly, and
lays many eggs.
• Thus it is also refered to as a model
organism.
GENOME
• The genome of D. melanogaster contains four pairs
of chromosomes: an X/Y pair, and
three autosomes labeled 2, 3, and 4. The fourth
chromosome is so tiny, it is often ignored.
• The D. melanogaster sequenced genome of 139.5
million base pairs has been annotated and contains
around 15,682 genes.
• Determination of sex in Drosophila occurs by the X:A
ratio of X chromosomes to autosomes, not because
of the presence of a Y chromosome as in human sex
determination. Although the Y chromosome is
entirely heterochromatic, it contains at least 16
genes, many of which are thought to have male-
related functions
POLYTENE CHROMOSOMES
• Polytene chromosomes are giant
chromosomes common to many
dipteran (two-winged) flies.
• They begin as normal chromosomes,
but through repeated rounds of DNA
replication without any cell division
(called endoreplication), they become
large, banded chromosomes.
• For unknown reasons, the centromeric
regions of the chromosomes do not
endoreplicate very well. As a result,
the centromeres of all the
chromosomes bundle together in a
mass called the chromocenter.
KARYOTYPIC DIFFERENTIATION AMONG
THE SPECIES
The melanogaster species subgroup of Drosophila comprises six sibling species. The
interrelationship between these species has been studied by analysis of the banding
patterns of their polytene chromosomes. The species fall into two groups.
First group.
D.melanogaster D. simulans D.mauritiana
SECOND GROUP
D.Erecta D.teissieri D.yakuba
OBSERVATIONS FOR GROUP-1
• The former group are chromosomally closely related, indeed simulans and
mauritiana are homosequential.
• These species are now shown to have identical heterochromatin
distributions in mitotic metaphase chromosomes.
OBSERVATIONS FOR GROUP-2
The latter group (all African endemic species) are less closely related although
they all share eight autosomal inversions of the standard (i.e. melanogaster)
sequence.
• From this shared sequence the chromosomes of the three African endemic
species have diverged considerably by many paracentric inversions. Both D.
teissieri and D. yakuba are polymorphic
HUMANS AND CHIMPANZEE
• When one looks at the chromosomes of humans and the chimpanzee, it is
immediately apparent that there is a great deal of similarity between the
number and overall appearance of the chromosomes . The apes all having 24
pairs, and humans having 23 pairs. the chromosomes in that every one of
1,000 nonheterochromatic G-bands has been accounted for both the species.
That means that each non-heterochromatic band has been located in each
species.
KARYOTYPIC DIFFERENTIATION
• Chromosomes 4 and 17 are different among the species.
• Include nine pericentrc inversions and one fusion of ancestral
chromosomes (homologous chimpanzee chromosomes 12 and 13) that gave
rise to human chromosome 2.
• Most of the chromosomal differences among the species involve inversions
- localities on the chromosome that have been inverted, or swapped end for
end.
• Other types of rearrangements include a few translocations (parts swapped
among the chromosomes), and the presence or absence of nucleolar
organizers.
• The biggest single chromosomal rearrangement among the four species is
the unique number of chromosomes (23 pairs) found in humans as opposed
to the chimpanzee (24 pairs)
Karyotypic differences between human and
chimpanzee genomes
Conclusion
– variation, between cells, individual organisms, or groups of organisms
of any species caused either by genetic differences (genotypic
variation) or by the effect of environmental factors on the expression of
the genetic potentials (phenotypic variation). Variation may be shown
in physical appearance, metabolism, fertility, mode of reproduction,
behaviour, learning and mental ability, and other obvious or measurable
characters.
– When one looks at the chromosomes of humans and the living great
apes ( chimpanzee), it is immediately apparent that there is a great
deal of similarity between the number and overall appearance of the
chromosomes across the species. But there is great difference
between them due to which they differ in morphology and other gene
expressions. The difference include inversion,rearrangements e.t.c.
– Also in case of different species of drosophila ,even though they fall
under the same species but they differ in chromosomal morphology
and thus differ in difference in the gene expression.
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