Question

Polytene chromosomes are formed due to
A. Endoreplication
B. Duplication without separation
C. Replication of DNA without cell division
D. All of the above

Answer 1

Hint:-Polytene chromosomes have thousands of DNA strands which makes the polytene chromosomes giant/large chromosomes. They are found in the nucleus interphase of cells of tissues like salivary glands in insects like flies and their larvae which is why they are also called salivary chromosomes.

Complete Answer:- Polytene chromosomes are giant chromosomes that can be a thousand times or so larger than the length of DNA. They were initially reported in the salivary glands of Chironomus by Balbiani in 1881.

Polytene chromosomes develop by successive duplication of chromosomes of the diploid nuclei. The major differences between this type of cells and mitotic cells are the elimination of the terminal mitotic division steps and the lack of DNA segregation. As a consequence of these two, the cell cycle of these consists of only the S and G phases and they also remain multi-stranded, making them giant or “polytene”.

Among the given options, options B and C are direct explanations for the phenomena occurring in polytene chromosome formation and option A is the term for this type of replication resulting in polyploidy due to the absence of mitosis.

Thus, the correct answer is D. All of the above.

Note:- Lampbrush chromosomes are the other type of giant chromosomes similar to polytene chromosomes. However, as their names suggest, they have different appearances and the lampbrush chromosomes are larger than the polytene chromosomes. They are also found in most animals in the growing oocytes while polytene is found only in dipteron species.