Polytene chromosomes are large chromosomes found in certain specialized cells of Drosophila larvae. They form through repeated DNA replication without cell division, causing the chromosomes to be 100-200 times larger than normal chromosomes. Polytene chromosomes exhibit a banded pattern with dark bands containing mostly DNA and light interbands containing RNA and proteins. The bands can form puffs or Balbiani rings during active transcription, marking sites of mRNA, rRNA, and tRNA synthesis. Polytene chromosomes help organisms conserve energy during development by increasing DNA content without cell division.

1. POOJA SINGH

2. POLYTENE CHROMOSOMES
• First reported by E.G. Balbiani 1881
• Found in larvae of Drosophila (fruitfly)
• Also called “salivary gland chromosomes”

3. Drosophila Larva

4. Development of Polytene chromosomes
• Polytene chromosomes are developed from
standard chromosomes
• Some Specialized cells undergo repeated
DNA replication without cell division
process is known as ENDOMITOSIS

5. • Chromosomes are 100-200 times larger
• than normal chromosomes
• There is an increase in cell volume
• forming polytene chromosomes
Salivary gland cell

6. STRUCTURE OF
POLYTENE CHROMOSOMES
• These show dark bands and light interbands
of different sizes

7. • Dark bands have large amount of DNA , small
amount of RNA and basic proteins
• Interbands have small amount of RNA and
acidic proteins

8. FORMATION OF PUFF
The band of Polytene
chromosomes first
swell up and then extend
out at right angle to main
axis of the Chromosome.
The enlarged
band is called PUFF

9. Puffs are the active site of mRNA, rRNA
and tRNA synthesis
Puffs seems to have concentration of
fibres, granules, RNA molecules
and proteins

10. BALBIANI RING
Chromonemata (Chromosomes material) of
polytene chromosomes give out a number of
loops at certain places.
Such loops are called BALBIANI RING. These
are rich in m RNA

11. Chromonemata undergo coiling and
supercoiling which results in thickness of
chromosomes

12. BAND INTERBAND
Give dark appearance Give light appearance
DNA strands are coiled
up in bands region
DNA strands are nearly
straight in interbands
region
Bands have large
amount of DNA, small
amount of RNA and
basic proteins
Interbands have small
amount of RNA and
acidic proteins
Bands in some tissue
swell up to form puff
or may give out loops
called Balbiani ring
Puff or Balbiani ring
are not formed in
interband regions

13. IMPORTANCE OF POLYTENE CHROMOSOMES
• Polyteny arises in tissues, organs and at
developmental stages when there is need for the
rapid development of an organ.
• Organs containing cells with polytene
chromosomes are involved in intense secretory
functions for rapid growth
• Polytinization helps the organism to conserve the
energy during the development where the cells
only increases the DNA content and cell division
phase is eliminated, there by conserving the
energy and time