Polytene activity

Figure 36-4. Illustration of the tight correlation between the presence of RNA polymerase II and RNA synthesis. A number of genes are activated when Chirono-mus tentans larvae are subjected to heat shock (39 °C for 30 minutes). A Distribution of RNA polymerase II (also called type B) in isolated chromosome IV from the salivary gland (at arrows). The enzyme was detected by immunofluorescence using an antibody directed against the polymerase. The 5C and BR3 are specific bands of chromosome IV, and the arrows indicate puffs. B Autoradiogram of a chromosome IV that was incubated in H-uridine to label the RNA. Note the correspondence of the immunofluorescence and presence of the radioactive RNA (black dots). Bar = 7 pm. (Reproduced, with permission, from Sass H RNA polymerase B in polytene chromosomes. Cell 1982 28 274. Copyright 1982 by the Massachusetts Institute of Technology.)...

Like the puffs of polytene chromosomes (Chapter 28), which may have a similar structure, lampbrush chromosomes appear to be actively engaged in transcription. Approximately 3% of the DNA may be functional in producing mRNA that is accumulated within the oocyte and is used as a template for protein synthesis during early embryonic development.90... 

Ashbumer M., Chihara C., Meltzer P. and Richards G. (1973) Temporal control of puffing activity in polytene chromosomes. Cold Spring Harbor Symp. Quant. Biol. 38, 655-662. 

Figure 5.1 illustrates the process of assembly of the Heat Shock Factors labeled green in the free nucleoplasm of the nucleus of the polytene Drosophila nucleus as the temperature is rapidly raised from 23 to 37 °C. Its segregation to the genes it activates in the red-labeled chromatin ensemble is seen as the short stripes of yellowish green on the chromatin. 

Taken as a whole, this factual evidence makes the hypothesis of puffing as a cytological expression of gene activity very alluring. In some cases, however, it was demonstrated that the processes of transcription and translation in the salivary glands of Drosophila occurred before puff formation and before a change in the puffing distribution of polytenic chromosomes (Kress, 1972 Korochkin et al.. 

The majority of hereditary material in polytenic chromosomes is inactive. No more than 10%-15% of the 5000 bands in D. melanogaster and 1900 bands in Chironomus formed puffs. The maximum proportion of bands which are simultaneously active is smaller and is approximately equal to 6% (Felling, 1966). 

What are the principles of regulation in nonpolytenic chromosomes, if the activation of genetic units forms puffs and the synthesis of RNA is regulated by the number of polytenic chromosomes ... 

The best arguments for possible differential activity of autosomal genes are cases of heteromorphism of chromosomal regions for which the relation between cytological and functional expressions have been clearly demonstrated. These include puffs of polytene chromosomes, lateral loops of lampbrush chromosomes and nucleoli. 

The differences in the characteristics of DNA replication could play an important role in differential transcriptional activity of homologous chromosomes, especially in interspecific hybrids. It was demonstrated that in polytene chromosomes of D. melanogaster there is positive correlation between transcription intensity and the rate of replication of the corresponding X chromosomes (Berendes, 1966a,b Ananiev and Gvozdev, 1974 a, b). 

The lateral loops of lampbrush chromosomes are active chromosomal regions, similar to puffs. These chromosomes have been studied less than polytene chromosomes from thp point of view of the differential activity of homologous chromosomes. However, there are examples of the functional heteromorphism in the behavior of homologous loops (Callan, 1963, 1965). 

The specificity of the alternation of the heterochromatic and the actively transcribed chromosomal segments in various tissues and cellular types could be the physical basis for the maintenance of the differential conditions. The puffing in polytenic chromosomes could be considered to be a demonstration of such heterochromatin effect on the transport of RNA molecules from the nucleus into the cytoplasm by the regulation of the transportation time for specific templates. 

Zhimulev IF (1974) Cytological aspects of transcriptional activity of polytenic chromosomes in D. melanogaster. Autoreferate candidate dissertation. Novosibirsk (Russ)... 

The puff activation of polytene chromosomes in the salivary glands of Diptera larvas occurs under the control of the steroid hormone ecdysone. The genes which control the formation of steroid metabolic enzymes are supposedly active in the gland cells where this hormone is synthesized. We can consider the regulation of salivary gland functions as a result of the indirect interactions... 

Morphological, and Genetic Differentiation of Chromosomes. The Study of Puffs on Polytene Chromosomes and Dynamics of Distribution in Connection with Morphogenesis and Specialization of Zones of Active RNA Synthesis in Chromosomes... 

This pattern is shown in Fig. 56 for only one chromosome, but in the paper cited similar results were obtained for all four polytene chromosomes of this insect. Experiments by Kiknadze and Filatova (1960, 1963) and Kiknadze (1965) showed that during larval development of C. dorsalis a gradual increase in the number of actively functioning zones first takes place at the beginning of age IV there are about 100 puffs on the complete set of chromosomes, while at the end of the number is about 160. In the late pupal stage the puffs disappear just before lysis of the salivary gland. 

Berghella L. and Dimitri P. 1996. The heterochromatic rolled gene of Drosophila melanogaster is extensively polytenized and transcriptionally active in the salivary gland chromocenter. Genetics 144 117-125. 

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