Polytene chromosomes are giant chromosomes found in the salivary glands of Chironomus larvae. They have a series of dark bands that stain intensely alternating with clear interbands. The bands are highly condensed with no transcription, while the interbands are lightly condensed and contain mRNA. The positioning and distribution of bands is consistent between homologous chromosomes. The chromosomes appear as fine strands attached to a central chromocenter when flattened, with the short chromosomes representing the fused fourth chromosomes. Theories on polytene chromosome structure include that they result from repeated chromosome replication, incorporate extra material, or consist of bundled chromonema that accumulate extra material or grow in length.

1. POLYTENE CHROMOSOMES
Study of polytene chromosomes in salivary glands of
Chironomous larvae

2. Balbiani in 1881 was the first to observe salivary gland chromosomes in the salivary glands of the
Chironomus larva. This kind of giant chromosome is strictly confined to certain type of somatic tissues in
the insects belonging to the order Diptera.

3. STRUCTURE
• Along the entire length of the chromosome there is a series of dark bands
alternating into other clear zones called interbands.
• The dark bands stain intensely and are Feulgen positive.
• They absorb ultraviolet light at 600 A°.
• These bands may be considered as disks, since they occupy the whole
diameter of the chromosome.
• Highly condensed, no transcription occur.

4. STRUCTURE(2)
• The longer bands have more complicated structure.
• They often form doublets, two bands located next to each other and of
identical thickness and shape.

5. STRUCTURE(3)
• The Interbands are of fibrillar aspect and do not stain with basic dyes.
• They are Feulgen negative and absorb very little ultraviolet light.
• They present a greater elasticity than the regions of the bands.
• The constancy in situation and distribution of the discs or bands in two
homologous (paired) chromosomes is notable.
• Lightly condensed, presence of mRNA

6. In the case of Drosophila melanogaster, the chromosomes of each polytene nucleus, when flattened, appear as
fine long strands and one quite short attached to a central mass known as the chromo center, to which single
large nucleolus is also attached.

7. EXPLANATION
• (1) The two members of each pair of chromosomes are closely fused
throughout their length.
• (2) The centromeres of all the chromosomes together with the
heterochromatic segments adjacent to them are all joined together to form
the chromo center.

8. Showing a giant chromosome of the salivary gland of the larvae of a fruit fly.
Of the six strands the short one represent the two fused IV th chromosomes.
longer one represents the X chromosomes, while the remaining four are the limbs of the ‘V’- shaped second and
third chromosomes.
In salivary gland nuclei from female larvae, the strand representing the ‘X’ is double, like the others, while in
nuclei from male individual it is single. The V being quite small and almost completely included in chromocenter.

9. Puffis and Balbiani Right and Gene
activity.
• The most important morphological peculiarity of polytene chromosome is
the presence of bands and inter bands.
• These enlarged bands are considered as ultimate units of heredity-the genes
at work.
• These active genes take the form of puffs scattered here and there along the
salivary gland chromosomes.

10. THEORIES EXPLAINING
STRUCTURE OF POLYTENE
CHROMOSOME

11. THEORY(1)
• Polytene chromosomes are the result of several cycle of intracellular
chromosomal reproduction and consists of bundles of the folded ordinary
chromosomes.
• This is the polytene theory sponsored by Her-twig (1935), Cooper (1938)
Painter (1939) and Beerrnann (1952).

12. THEORY(2)
• Polytene chromosomes are paired chromosomes which have enormous
length and width by addition or incorporation of extra material not present
in ordinary chromosomes.
• This is the earlier alveolar concept of Metz (193 5) and proposed by Kodani
(1942) and Darlington (1949).

13. THEORY(3)
• Polytene chromosomes consist of bundles of chromonema, their size is due
at least in part to the accumulation of extra material in the center of
chromosomes, and or to an actual growth in length of the chromonema .
• (Koltzof, 1934; Painter, 1934; Calvin et al, 1940; Ris and Course, 1954;
White, 1945)

14. POLYTENE
CHROMOSOMES
IN CHIRONOMOUS
LARVAE
IMAGE CAPTURED IN LAB