They are also known as polytene chromosomes. They commonly occur in the dipteran species of the flies such as Drosophila. First, they look like normal chromosomes. However, later on, these regions undergo repeated rounds of DNA replication. They look longer than the normal chromosomes. Hence, they are known as the giant chromosomes. The process of repeated DNA replication without the cell division is known as endoreplication. However, the centromere does not undergo endoreplication. The reason remains unclear. The process of replication requires the removal of the internal eliminated sequences (IES). The non-coding genes get removed or excised. The polytene chromosomes become larger and appear banded. A rare event occurs in the cells with the polytene chromosome. The centromeres come together and bundle up. The structure so formed is known as a chromocenter. Dissecting the Drosophila larvae and observing their cells under the microscope reveals polytene chromosomes. They commonly occur in the larvae as compared with the adult flies. Balbiani first reported these chromosomes in the year 1881. Thousands of DNA strands occupy the polytene chromosomes. They play a crucial role in the salivary glands. A polytene band represents the vertical stripes on the polytene chromosomes. It results from the specified association between the homologous chromomeres. A somatically paired bundle of chromosomes consists of these chromomeres at the same level. The polytene chromosome is also known as the Balbianin chromosome due to the presence of large puffs known as Balbiani rings. The RNA puffs rarely exist.
The large size of the polytene chromosomes occurs due to the presence of chromonemata (longitudinal strands). They form after repeated cytoplasmic divisions known as endomitosis. The polytene chromosomes consist of two main types of bands. The dark bands involve a dark stain. The light bands stain light. They are known as interbands. The dark bands possess more DNA and less RNA. The light bands contain more of RNA.
Image: Giant Chromosome or Polytene Chromosome
Functions of giant chromosomes:
The polytene bands get enlarged and form swellings. These structures are known as puffs. They consist of active genes. The chromonemata uncoil in the puffs. Thus, it opens up and forms many loops. The individual chromomeres also uncoil. These puffs indicate RNA synthesis. The loops appear like the ring structures. The scientist Balbiani discovered the rings. Hence, they are known as Balbiani rings. They consist of nucleic acids and proteins. These sites are known as transcriptionally active sites. Thus, they have RNA polymerases and ribonucleoproteins actively involved in the transcription process. The protozoan puffs consist of only DNA. Thus, they do not undergo transcription in the puffs. Multiple gene copies in the polytene chromosome permit a very high level of gene expression. Many DNA replication rounds resulting in the endoreplication in the cells of the salivary glands secrete a mucoprotein. It possesses a sticky glue-like characteristic useful for the pupae stage. The regions near the Centromeres possess polytene bands. These bands undergo a tandem duplication. Mainly, they occur near the centromere of the X chromosome. The flies showing tandem duplication in this region possess a phenotype of bar eyes with kidney shape. The interband get involved in the interaction with the important structures. They include active chromatin proteins, nucleosome remodeling complexes, and origin recognition complexes. They help in initiating the process of replication. The interphase replication involved in producing giant chromosome with many stranded cables (polytenes) is known as polyteny.
The polytene chromosomes have a larger size in comparison with the metaphase chromosomes. Both light and electron microscopy reveal the structure of the polytene bands. The tight association also takes place between the chromatids of one single chromosome. Ectopic pairing also involves the polytene chromosome. The pairing occurs between the nonhomologous chromosomes in the ectopic pairing. It occurs between the single chromosome bands. However, it is a site-specific feature.
Role of chromocenter in polytenization:
It is a diffuse region. It consists of fused pericentric regions of all the chromosomes. It mainly consists of the heterochromatic region. The heterochromatic region surrounds the mitotic chromosome. The chromocenter mainly contains two types of heterochromatin such as the alpha and the beta-heterochromatin. The former one does not participate in the polytenization. The latter on, the beta-heterochromatin gets involved in the polytenization. It contains sequences capable of cross-hybridization with many mobile genetic elements. The telomeric heterochromatin also plays a crucial role in polytenization. The DNA rich banded regions of the polytene chromosomes consist of histone proteins. These proteins help in various hybridization assays. Immunostaining technique helps in identifying the proteins present abundantly for the polytene chromosomes.
Control of genes present in the polytene chromosomes:
The insect hormone known as ecdysone plays a crucial role in controlling the genes in the polytene chromosome. The larval development involves a periodic rise and fall of the ecdysone hormone. The larva requires certain proteins for each of its molt and pupation stage. The levels of the ecdysone hormone control the expression of the genes involved in producing these proteins. The activation and deactivation of the transcriptional events lead to the receding of the old puffs. Also, the new puffs arise. It happens when the organism switches from one developmental stage to the other. The polytene chromosomes roughly contain about 5000 genes.
Polytene chromosomes in mammals:
Mammalian trophoblast cells also contain polytene chromosomes. They show a ploidy between the 32C and 2048C. However, these polytene chromosomes do not have a banded structure like that of the Drosophila polytene chromosomes. These chromosomes have a transitory existence. Later on, they break down into smaller fragments.
References:
[1] Biology: A Functional Approach, M. B. V. Roberts, Giant chromosome, Page 241
[2] Laboratory Manual for Biotechnology, Verma, Ashish S./ Das Surajit & Singh Anchal, Page 190-192
[3] Advanced Topics In Zoology, H V Kashyap, Page 16-17
[4] Polytene Chromosome –Wikipedia
Copyright, 2019. All Rights Reserved
.png)
