To increase cell

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 volume, some specialized cells undergo repeated rounds of DNA replication

DNA replication

DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...

 without cell division

Cell division

Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...

 (endomitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

), forming a giant polytene chromosome. Polytene chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s form when multiple rounds of replication produce many sister chromatid

Chromatid

A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division . They are called sister chromatids so long as they are joined by the centromeres...

s that remain synapsed together.

In addition to increasing the volume of the cells' nuclei and causing cell expansion, polytene cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene expression

Gene expression

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

. In Drosophila melanogaster

Drosophila melanogaster

Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

, for example, the chromosomes of the larval salivary glands undergo many rounds of endoreplication, to produce large amounts of glue before pupation.

Polytene chromosomes have characteristic light and dark banding patterns that can be used to identify chromosomal rearrangements and deletions. Dark banding frequently corresponds to inactive chromatin, whereas light banding is usually found at areas with higher transcriptional activity. The banding patterns of the polytene chromosomes of Drosophila melanogaster were sketched in 1935 by Calvin B. Bridges, in such detail that his maps are still widely used today. The banding patterns of the chromosomes are especially helpful in research, as they provide an excellent visualization of transcriptionally active chromatin

Chromatin

Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...

 and general chromatin structure.

Chromosome puffs are diffused uncoiled regions of the polytene chromosome that are sites of RNA

RNA

Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 transcription

Transcription (genetics)

Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

. A Balbiani ring is a large chromosome puff.

Polytene chromosomes were originally observed in the larval salivary gland

Salivary gland

The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose...

s of Chironomus

Chironomidae

Chironomidae are a family of nematoceran flies with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae...

midges by Balbiani in 1881, but the hereditary nature of these structures was not confirmed until they were studied in Drosophila melanogaster

Drosophila

Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

in the early 1930s by Emil Heitz and Hans Bauer. They are known to occur in secretory tissues of other dipteran insects such as the Malpighian tubules of Sciara

Sciara (genus)

Sciara is a genus of fungus gnats that feed on decaying organic matter and fungi, and are often found in greenhouses. Their larvae are up to 6 mm long, white, slender and legless, with a black head and smooth semi-transparent skin which reveals the contents of the digestive tract.The adult fly...

and also in protists, plants, mammals, or in cells from other insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s. Some of the largest polytene chromosomes described thus far (see scale bar in figure below) occur in larval salivary gland cells of the Chironomid

Chironomidae

Chironomidae are a family of nematoceran flies with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae...

 genus Axarus

Axarus

The genus Axarus is widely distributed with records from the Holarctic, the Neotropics and Australasia . There are currently 5 described nearctic species . Erected as a subgenus of Xenochironomus , Axarus was subsequently renamed and elevated to generic status ...

.
Polytene chromosomes are about 200micron in length. The chromonema of these chromosomes divide but do not separate. Therefore, they remain together to become large in size.
Another form of chromosomal enlargement that provides for increased transcription is the lampbrush chromosome

Lampbrush chromosome

Lampbrush chromosomes are a special form of chromosomes that are found in the growing oocytes of most animals, except mammals. Lampbrush chromosomes of tailed and tailless amphibians, birds and insects are described best of all...

.

Polytene chromosomes are also used to identify the species of Chironomid larvae that are notoriously difficult to identify. Each morphologically distinct group of larvae consists of a number of morphologically identical (sibling) species that can only be identified by rearing adult males or by cytogenetic analysis of the polytene chromosomes of the larvae. Karyotypes are used to confirm the presence of specific species and to study genetic diversity in species with a wide range.

Other References

• PAVAN, C. and BREUER, M. E. 1952. Polytene chromosomes in different tissues of Rhynchosciara. Journal of Heredity. vol. 63, p. 151-157.

• PAVAN, C. 1967. Chromosomal changes induced by infective agents Triangle. Sandoz J. Med. Sci. vol. 8, p. 42-48.

• PAVAN, C., BIESELE, J., RIESS, R. W. and WERTZ, A. V. 1971. XIII. Changes in the ultrastructure of Rhynchosciara cells infected by Microsporidia. Studies in Genetics. vol. VI, p. 7103.

• PAVAN, C., DA CUNHA, A. B. and MORSOLETTO, C. 1971. Virus-chromosome relationships in cells of Rhynchosciara (Diptera, Sciaridae). Caryologia. vol. 24, p. 371-389.

External links

http://commons.wikimedia.org/wiki/File:87AC_polytene_heatshock_puffs-1-.jpg#filelinks (87A&C Heat-shock puffs)

http://www.natureprotocols.com/2007/05/09/highresolution_light_microscop.php (High resolution spreads)

• Phaseolus Polytene chromosomes (Plants)