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Most of the functions of the nucleus are intimately related to events that occur in the cytoplasm; for example, translation of mRNA by cytoplasmic ribosomes is the end result of the transcription of DNA in the nucleus and the processing of the resulting RNA.
Structurally too, the nucleus and cytoplasm are interrelated.
The outer membrane of the nuclear envelope is continuous with membranes of the endoplasmic reticulum.
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Events in the cytoplasm also affect nuclear function, initiating or terminating specific nuclear activities. Therefore, whereas the nucleus is perhaps the single dominant structural feature of most eukaryotic cells, it is not to be viewed as an isolated organelle.
Structure of the Nucleus:
The nucleus of a eukaryotic cell (Fig. 20-1) is delimited by a pair of membranes called the nuclear envelope. The outer and inner membranes of this envelope are separated by a narrow space called the perinuclear space but fuse with each other at the margins of pores.
The fluid of the cytoplasm (the cytosol) is continuous through the nuclear pores with the fluid of the nucleus, called the nucleoplasm. Although ribosomes may be attached to the cytoplasmic surface of the outer nuclear membrane, these ribosomes are not considered nuclear structures. The nucleoplasm contains a number of discrete structures including one or more nucleoli, chromosomes (as many as several hundred in some eukaryotic cells), and other structures or regions that appear at various times depending on nuclear activity.
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Prokaryotic cells do not have a “true” nucleus; indeed, identification of a cell as prokaryotic (“pre- nucleus”) or eukaryotic (“true-nucleus”) is based on the presence or absence of this membrane-enclosed organelle. In prokaryotic cells, most of the genetic material is confined to an area of the cell called the nucleoid, but the nucleoid is not delimited by a membrane.
