Polytene chromosomes

Definition

Giant, cable-like chromosomes formed by repeated rounds of DNA replication without cell division (endoreduplication), resulting in thousands of aligned sister chromatids that remain physically paired and visible as distinct, reproducible banding patterns. These structures permit high transcriptional output in specialized tissues and have served as foundational tools for physical gene mapping and chromosomal aberration detection in cytogenetics.

Etymology

From Greek poly- (many) + tainia (band, ribbon), referring to the multiple parallel strands.

Example

In Drosophila melanogaster, the polytene chromosomes of larval salivary gland cells display ~5,000 discernible bands that correspond to euchromatic gene regions; inversions and deletions in these chromosomes were used to construct the first physical maps of the genome.

Synonyms

• giant chromosomes
• salivary gland chromosomes

Related Terms

• chromosome puff
• endoreduplication
• salivary gland
• chromosomal inversion
• cytogenetics
• Drosophila

Usage Notes

The term specifically denotes the morphological outcome of endoreduplication, not merely any large chromosome. Banding patterns are reproducible enough to identify individual chromosomal regions, but interband regions are transcriptionally active, not inert. Polytene chromosomes are best developed in Diptera but occur in other arthropods and some plants; their utility for mapping is largely restricted to species where they can be reliably prepared.