Special types of chromosomes or Giant Chromosomes
In some organisms, the giant chromosomes assume special structures in some specific tissues, e.g.,
These were described by Ruckert (1892). These are present in primary oocyte nuclei of vertebrates as well as invertebrates. These are diplotene bivalent chromosomes joined at certain points called chiasmata.
Their main axis is formed by DNA. Each of the bivalent bear rows of large number of chromosomes. Many of the chromosomes give out lateral projections or loops.
Loops are extended parts of chromosomes, participating in transcription, the hairs on loops are actually nascent RNA molecules. These hairs are bound to protein, giving it a fibrillar granular appearance. Some of these are stored as informosomes (mRNA + proteins) for future use (development of embryo).
Salivary gland chromosomes or Polytene chromosomes
In salivary gland cells of insects of order Diptera (dipteran insects), some special chromosomes were reported by E. G. Balbiani (1881). It is due to the presence of these giant chromosomes that maximum cytological studies have been made in Drosophila. Another example is Chironomus.
In drosophila, salivary gland chromosomes upto 2000 µm (2mm) have been observed. The name polytene chromosome has been given to them as there occurs polyteny i.e., number of chromonemata or fibrils increases upto 2000 or more per chromosome.