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.

THANK YOU