substances are transferred. Elements of the endoplasmic reticulum are
connected with the outer membrane of nuclear envelope. In nucleoplasm
there are located parts of the chromatin, consisting of DNA, RNA and
proteins. During cell division chromatin is organized in chromosomes whose
number, as it was already mentioned, is precisely fixed and specific for each
species. The nucleoplasm of nuclei also contains numerous enzymes and
co-factors participating in the processes of phosphorylation and acetylation of
nuclear proteins, glycolytic enzymes, etc.
In the nucleus there is a nucleolus (in some cases 2 or more). It can be
seen easily under electron, as well as under light microscope. It is formed
in certain parts of DNA, called nucleolus organizer. It is considered to be
the place of ribosomal RNA (rRNA) synthesis.
image
Figure 2–60. Electron micrograph of a prokaryotic cell of Bacillus subtilis (After Stent, 1974).
The nucleus is a depot and distributor of the genetic
information of the cell. It is the place of most important genetic
processes related with its development and reproduction —
replication and transcription of DNA, as well as the synthesis of
the different kinds of RNA. Also here is the place of preparing the
start of translation. It is realized in the cytoplasm from already
synthesized information or messinger RNA (mRNA) by
means of different transport RNA (tRNA) molecules. Thus, on the
ribosomes the polypeptide chains are built. In this way, in a close
interaction with cytoplasm, the nucleus participates in realizing
the gene expression and in regulating the metabolic
processes running in the cell. In prokaryotic cells which do
not possess a clearly defined nuclei, the nuclear equivalent, i.e.
nucleoid, is located “freely” in the cytoplasm and is connected in some way
with the cell membrane (Fig. 2–60). It is considered that transcription and
protein synthesis are realized almost simultaneously in them. Here, as it was
already mentioned, the main unsolved question is the manner of uncoiling the
circular DNA-molecule during the replication. Clarifying this question will shed
more light on the structural organization of genome in prokaryotic cells.