Salivary Gland Chromosomes

 

 

1. Discovery: First identified in Dipteran insects by E.G. Balbiani.
2. Chromosomal Appearance: These chromosomes appear somatically paired (n), not as diploid (2n).
3. Banding Pattern: They exhibit a distinct pattern of transverse bands, which include both chromatic and achromatic regions.
4. Band Forms: The bands form reversible puffs, chromosomal puffs, and Balbiani rings, which are associated with different gene activations.
5. Also Known As: These chromosomes are referred to as polytene chromosomes.
6. Endo-Replication: They undergo repeated cycles of endo-replication (endomitosis).
7. Chromosomal Structure: They consist of five long and one short strand, forming a chromocenter at the center.
8. Chromosomal Types: Includes the long X chromosome and four chromosomes (II and III).
9. Y Chromosome: In male flies, the Y chromosome is centrally located and not visibly distinct.
10. Puff Sites: Puffs on these chromosomes are sites of RNA synthesis.
11. Ecdysone Effect: Ecdysone induces puffs in specific bands.
12. Phenotypic Impact: Changes in the number or position of bands can affect the phenotype, such as the 16A region in the X chromosome of Drosophila.