Polytene Chromosomes

Polytene chromosomes are giant interphase chromosomes formed when cells undergo multiple rounds of DNA replication without cell division. In Drosophila salivary gland cells, hundreds to thousands of identical DNA copies remain aligned side by side instead of segregating, creating these massive structures. This process produces unique chromosomes visible under a microscope with distinctive banding patterns.

Polytene chromosomes display alternating dark bands and light interbands because thousands of identical DNA sequences align side by side, magnifying chromatin compaction differences. Bands constitute 95% of the DNA and contain condensed, transcriptionally inactive chromatin. Interbands make up 5% and contain less condensed, transcriptionally active chromatin, making these features easily observable under a microscope.

In polytene chromosomes, bands represent transcriptionally inactive regions with condensed chromatin, while interbands are transcriptionally active regions with loosely organized chromatin. This relationship demonstrates how chromatin compaction directly affects gene expression. The visible banding pattern thus reflects the underlying transcriptional state of different chromosome regions.

Puffs are expanded structures that form when interband regions de-condense to allow active RNA synthesis during development. Balbiani rings are exceptionally large puffs found in salivary glands of certain species with highly decondensed chromatin supporting very high transcription rates. Both structures represent sites of intense gene activity and serve as excellent models for studying transcription.

Polytene chromosomes form in cells that lack cell division after DNA replication, causing chromatids to accumulate without segregating. Unlike mitotically dividing cells, polytene cells maintain an intact nuclear membrane and nucleolus throughout consecutive replication cycles. This results in thousands of aligned chromatids arranged side by side, creating the characteristic giant chromosome structure.

Polytene chromosomes are found in salivary glands, intestines, muscles, and other tissues of dipteran insects like Drosophila larvae. They also occur in certain organs of mammals and in synergids and antipodes of flowering plants. These chromosomes were first discovered in 1881 in salivary glands of Chironomus plumosus larvae, earning them the alternate name salivary gland chromosomes.

Chromomere patterns are species-specific, representing multiple compact dark bands formed by identical chromatin compaction in aligned homologous chromosomes. The number and size of chromomeres can change during an organism's lifetime as developmental stages progress. These variable patterns reflect dynamic changes in chromatin organization and gene activity across different developmental periods.