Question

How does polyteny occur?

Answer 1

Polytene chromosomes are created by the repetitive replication of homologous chromosomes that do not segregate the replicated individual sister chromatid strands. Polytene chromosomes feature about 1000 identical DNA molecules arranged laterally inside the structure.

Complete answer:
Multiple rounds of replication produce many sister chromatids that remain fused together, resulting in polytene chromosomes.

These chromosomal strands are produced when the chromosome is divided repeatedly without cytoplasmic division. Endomitosis is the name for this form of division.

To expand cell volume, specialised cells conduct repeated rounds of DNA replication without cell division, resulting in the formation of a gigantic polytene chromosome.
The thick and thin banding patterns on polytene chromosomes are distinct.

As the cell remains in S phase indefinitely, the DNA of the chromosome continues to duplicate many times. Polyteny occurs when a chromosome generates a large number of chromatids as a result of repeated replication.

Many (=poly) threads/strands (=tene) are referred to as polytene. Instead of the usual two chromatids, a polytene chromosome has numerous (perhaps thousands) parallelly organised chromatids.

ENDODUPLICATION is a phenomenon in which a chromosome forms after numerous cycles of DNA replication, but there is no separation of chromatids (as occurs during cell division).

Polytene chromosomes are found in larval diptera such as Chironomous (midge), Drosophila (fruit fly), and others. Salivary gland cells, Malpighian tubules, and gut epithelium cells all have enormous banded chromosomes.

Polytene chromosome cells never divide and are lost during the larval to adult insect metamorphosis.

The banding pattern on the polytene chromosome is caused by differential chromatin coiling: tightly coiled regions look black (=band) under the microscope, whilst loosely coiled portions (=interband) appear light.
Some dark areas' coiled chromatin may uncoil, forming a 'puff' where active RNA transcription may occur. Protein synthesis is then dictated by these RNA molecules. As a result, particular genes found on polytene chromosomes are activated.

In a diploid cell, homologous polytene chromosomes are still paired. Please keep in mind that centromeric heterochromatin does not replicate like the rest of the chromosome, and the centromeric sections of all polytene chromosomes in a cell can combine to form a CHROMOCENTER.

Note:
Polytene chromosomes are found in the salivary glands and other tissues of insects, whereas lampbrush chromosomes are found in the oocytes of vertebrates excluding mammals and some invertebrates. Polytene chromosomes are detected in the interphase nuclei of some tissue in flies' larvae.