Most chromosomes in living cells are incredibly tiny. However, in Biology, there is a special type of unusually large chromosome known as the Polytene Chromosome (also called a Giant Chromosome).
Polytene: Means 'many threads'.
Cause: Repeated DNA replication without cell division (Endoreduplication).
Location: Famously found in the salivary glands of Drosophila (fruit flies).
Size: They are called 'Giant Chromosomes' because they are 100 times larger than normal chromosomes.
Polytene Chromosomes are oversized, giant chromosomes. Normally, when a cell prepares to divide, its DNA replicates (doubles) and then the cell immediately splits. However, in polytene chromosomes, the DNA replicates over and over again, but the cell never divides.
As a result, thousands of copies of DNA strands stay attached together side-by-side like a thick bundle of wires. Because there are so many strands, the chromosome becomes massive and can be easily seen under a standard microscope.
Why do insects have them? The salivary glands of these insects need to produce massive amounts of sticky proteins (like glue for making pupae) very quickly. Having thousands of copies of the same DNA allows the cell to produce these proteins at incredibly high speeds.
Polytene chromosomes are giant chromosomes formed when DNA replicates multiple times without the cell dividing, creating a thick bundle of DNA threads. They are commonly found in the salivary glands of insects like the fruit fly (Drosophila).
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