Amplification of ribosomal DNA in Acheta

At late pachytene, in the oocytes of Acheta , the major chromomere of chromosome 6, a nucleolar structure, becomes a “puff”, in which the amplified ribosomal DNA copies occupy the central core and protrude to the periphery, the RNA being mainly located in the outer region of the “puff”. As meiosis progresses (late diplotene) the rDNA copies start to be released from the chromomere, and two types of nucleolar structures are formed. Type 1, the main nucleolar component ( MNC ), consists of hundreds of small irregularly shaped minor nucleoli (0.3–1.0 μ in diameter). Later in development appears type 2, the secondary nucleolar component ( SNC ), which is represented by a few (1 to 6) large spherical bodies (3–6 μ in diameter) located at the periphery of the nucleolar mass. These two types of structures were identified in the light‐ and electron‐microscopes and their DNA and RNA components were determined by cytochemical tests. The particles present in the MNC are 150–200Å in diameter. The SNC consists of three zones: Zone A contains electron‐dense fibrils and granules, the size of these granules increasing from the center to the periphery. This zone is seen to contain DNA in the light microscope. Zone B is formed by an assembly of 350–450Å particles regularly spaced. Zone C is an area consisting of granules and fibrils (50Å or less) and is known to contain RNA. The relation of these structures to the biochemical processes involved in preribosome formation is discussed.