POLYTENE CHROMOSOMES

• These are over-sized chromosomes.
• They have developed from standard chromosomes.
• They are commonly found in the salivary glands of Drosophila melanogaster.

Salivary glands in Drosophila (people.hsc.edu)

• Polytene chromosomes are formed when specialized cells undergo repeated rounds of DNA replication without any cell division occurring in the cell.
• This in turn increases the cell volume forming a giant chromosome such as the polytene chromosome.
• Multiple rounds of replication produce many sister chromatids that remain synapsed together forming polytene chromosomes.
• Other than simply increasing the nuclei's volume and causing cell expansion, there is also a certain metabolic advantage in the formation of polytene chromosomes.
• The advantage is the result of a high level of gene expression in the presence of multiple copies of genes.

  

  A polytene chromosome in Drosophila melanogaster. (www.mun.ca)

• Example: In Drosophila melanogaster the chromosomes of the larval salivary glands undergo many rounds of endoreduplication in order to produce large amounts of glue before pupation.
• Example: Again in Drosophila melanogaster, within the organism there is tandem duplication of various polytene bands located near the centromere of the X chromosome. This results in the Bar phenotype of kidney-shaped eyes.

Structure

• These chromosomes have characteristic light and dark banding patterns. The banding patterns can be used to identify chromosomal rearrangements and deletions.
• Dark banding frequently corresponds to inactive chromatin, whereas the light banding is usually found at areas with higher transcriptional activity.
• The banding patterns of D. melanogaster was sketched by Calvin B Bridges in 1935.
• The polytene chromosomes in Drosophila have been used to support the “Theory of genomic equivalence”. This theory suggests that all the cells in the body maintain the same genome.
• Chromosomal Puffs are diffused uncoiled regions of the polytene chromosomes that are sites of RNA transcription.

Chromosomal puff (www.tutorsglobe.com)

• A Balbiani ring is a large chromosome puff.
• Polytene chromosomes are usually about 200 um in length.
• The chromonema of these chromosomes divide but do not separate. This makes them stay together in a large size.
• Another such type of chromosomes is the Lampbrush chromosome.
• Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Balbiani in 1881.

Chironomus Larvae (mmp.vfu.cz)

• Their hereditary nature was however confirmed only in Drosophila melanogaster I the early 1930s by Emil Heitz and Hans Bauer.
• They are known to occur in the secretory tissues of other dipteran insects such as Malpighian tubules of Sciara and also in protists, plants, mammals or in the cells from other insects.
• The largest polytene chromosomes described thus far occur in larval salivary gland cells of the Chironomid genus Axarus.
• Polytene chromosomes are also used to identify the species of Chironomid larvae that are notoriously difficult to identify.