increase of ploidity (4, 8, 16, 32, 64, etc.), the chromatids remain bound to
one another and the so-called polythene chromosomes are obtained.
Sometimes the chromatid number in them is so high, that it is almost
impossible to differentiate between the individual chromosomes in the
nucleus.
Polythene chromosomes represent a particular case of polyploidy
since they permanently exist in an interphase, that is unfolded state which
renders them ideal for the study of their structural organization and
functions. They consist of consecutively arranged intensively stained discs
and light interdisc spaces. At certain moments some of the discs swell and
form spheric swellings denoted as puffs, the most typical of them called
Balbiani rings after the name of their discoverer (Fig. 2–25). The
processes taking place in them are related to intensive RNA synthesis and
specific enzymes.
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Figure 2–24. Set of four giant chromosomes from a cell of the salivary gland of Chironomus tentans (After Beermann and Clever, 1964).
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Figure 2–25. Chromosome puffs in giant chromosomes from a salivary gland cell of Chironomus tentans. The larger of them are called Balbiani rings (After Beermann and Clever, 1964).
The lampbrush chromosomes (Fig. 2–26) are observed during the
meiosis in oocytes of fish, crabs, etc. Recently they have been provoking
certain interest from an evolutionary point of view after it has been
established that they are devoid of DNA or its content is very low in them.