Typically, when a diploid cell divides, it first duplicates each of its chromosomes. Then, the cell distributes a copy of each chromosome between the two daughter cells so that each daughter cell receives a complete set of chromosomes.
Certain cells undergo multiple cycles of chromosome duplication without undergoing any cell division. Such cells, called polyploid cells, contain multiple sets of each chromosome.
In the salivary gland cells of Drosophila larvae, this process is taken to an extreme. Here, hundreds to thousands of copies of each chromosome are generated without any cell division.
This results in the formation of unique, giant chromosomes, called polytene chromosomes.
As thousands of identical DNA sequences lay side by side, like crayons in a box, certain features in the chromatin become visible. These features exist in ordinary interphase chromosomes but are difficult to observe.
When viewed under a microscope, an alternating pattern of dark bands and lighter interbands can be seen in the polytene chromosomes.
The bands constitute 95% of the DNA, whereas the interbands constitute 5%.
Where the chromatin in the bands is more condensed and transcriptionally inactive, the chromatin in the interbands is less condensed and transcriptionally active.
As Drosophila larvae progress from one developmental stage to another, specific bands and interbands greatly increase in diameter, producing structures called ‘Puffs’ or ‘Balbiani’ rings.
These puffs arise from the de-condensation of chromatin and are sites of active DNA transcription. The chromatin in puffs is arranged in looped structures, much like those observed in ‘lampbrush’ chromosomes.
7.10:
Polytene Chromosomes
Polytene chromosomes are giant interphase chromosomes with several DNA strands placed side by side. They were discovered in the year 1881 by Balbiani in salivary glands, intestine, muscles, malpighian tubules, and hypoderm of larvae Chironomus plumosus. Hence, these are also called "Salivary gland chromosomes." These are found in insects of the order Diptera and Collembola; in certain organs of mammals; and synergids, antipodes of flowering plants. Polytene chromosomes are also regularly observed in cells of salivary gland in Drosophila.
There are certain differences between cells with polytene chromosomes and mitotically dividing cells. First is the absence of cell division after DNA replication that results in the accumulation of a large number of chromatids. Second is the failure of DNA strands to segregate after each round of DNA replication, resulting in several thousand chromatids arranged side by side. Third is the intact nuclear membrane and nucleolus during consecutive DNA replication cycles.
Banding patterns
Variation in the chromatin compaction can result in different concentrations of chromatin along the length of the polytene chromosome. Since the homologous chromosomes have identical chromatin compaction and are arranged side by side, it results in multiple compact dark bands called chromomeres. The regions between the bands are called interband or inter-chromomere regions. Interbands are lightly stained and are made up of decondensed chromatin. The chromomere patterns are specific for a species; although the number and size of chromomeres can change during the organism's lifetime.
Puffs
In certain instances, the interband region forms expanded structures called puffs that are loosely coiled, to allow RNA synthesis. Therefore, the puffs are excellent models to study the process of transcription. Exceptionally large puffs called “Balbiani rings” are found in salivary glands of certain species. These have decondensed chromatin that supports a very high rate of transcription.
Suggested Reading
- Molecular Biology of Cell, Alberts, 6th edition, Pages 208-210
- Zhimulev, I. F., E. S. Belyaeva, V. F. Semeshin, D. E. Koryakov, S. A. Demakov, O. V. Demakova, G. V. Pokholkova, and E. N. Andreyeva. "Polytene chromosomes: 70 years of genetic research." International review of cytology 241 (2004): 203-275.