Chromosomes giant

Gertain cells of insects, eg, Chironomus, contain giant chromosomes that have been repficated for ten cycles without separation of daughter chromatids. These copies of DNA fine up side by side in precise register and produce a banded chromosome containing regions of condensed chromatin and fighter bands of... 

Nuclear Differentiation Starts in Early Development Chromosome Structure Varies with Gene Activity Giant Chromosomes Permit Direct Visualization of Active Genes... 

Giant Chromosomes Permit Direct Visualization of Active Genes... 

Chromosome puff. A swollen region of a giant chromosome the swelling reflects a high degree of transcription activity. 

Nuclear receptors for the steroid hormone ecdysone in Drosophila deserve special consideration, for several reasons. First, the insect hormone ecdysone (Fig. 11.2) was the first steroid hormone shown to act at the level of the gene, because it induced puffs in the giant chromosomes of the fruitfly. Secondly, ecdysone activates the developmental programme of Drosophila, and, finally, the sequences of the DNA-binding domain (DBD) of the Drosophila nuclear ecdysone receptor and of receptor homologues (for example, the COUP-TFs chicken ovalbumin upstream promoter transcription factors) are highly conserved and nearly identical in vertebrates and humans. 

E. DNA Puffing Differential Gene Amplification in Giant Chromosomes of Dipterans. 116... 

A further level of organization is represented by Giant chromosomes (see) or polytene chromosomes, whose bands probably correspoiid to the looped do-... 

A puff in a giant chromosome deft) and a diagram of the course of the individual chromatids in the region of the puff (right). 

There is only little evidence of correlation between individual bands and cystrons (Welshons, 1965). The middle-sized chromomere in the giant chromosomes of Chironomus salivary glands contains sufficient DNA for the coding of 100 intermediate-size protein molecules (Edstrom, 1965). 

Differential gene activity can be demonstrated on the chromosomes themselves provided suitable objects, such as giant chromosomes and lampbrush chromosomes, are chosen and adequate methods are used. 

Giant chromosomes are found, inter alia, in the salivary glands of Dip-/emsuch as Drosophila, Chironomus, Acricotopus, and Sciara. They are bundles of a few thousand homologous chromatids laid down in parallel and, at the same time, slightly skewed like the strands of a rope (Fig. 143). Each chromatid appears to be divided lengthwise chromomeres appear to be strung on the chromonema like a set of beads. 

Puffs are not fixed structures. In the course of development puffs that are already in existence on a given giant chromosome contract and new ones are formed. Differences in puff formation on any given giant chromosome are also found between one tissue and another. In puff patterns of this kind, which are specific for the tissue and stage of development, differential gene activity becomes microscopically visible. 

A giant chromosome from the suspensor of Phaseolus coccineus. The local loosenings of the structure correspond to the formations of puffs (from Nagl 1970). 

It is safe to assume that gene activity is one of those metabolic processes that are influenced by temperature. This is supported by the fact that, for example, the RNA production on plant giant chromosomes (page 176) can be curbed by lower temperatures. However, experimental evidence of the same precision as that which has been obtained for the relationship between light and gene activity is not yet available. 

Large decrease in mitotic index (0.1% at 18 hr) increases in abnormal metaphases (80%), anaphases (60%), gradual recovery with time chromatid breaks, exchanges giant chromosomes tripolarity... 

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