The nucleus

The nucleus contains, immersed in the so-called nuclear juice, or "carioplasma", DNA (chromatin, chromosomes), RNA (especially in the nucleolus), different proteins and metabolites. The spiraling of DNA in the chromosomes is not simple, but can be imagined as a spiral of spirals. In the intercinetic nucleus the higher spiralization is not sufficient to allow the individual chromosomes to be identified under the microscope. Single traits, however, can be spiraled, therefore visible, constituting the «masses» of chromatin. The less spiraled areas seem to be the most metabolically active.

The stages of activity of the chromosomal DNA are: autosynthetic and allosynthetic. In the first case the DNA molecule is reduplicated through a semi-conservative process, in the second case it synthesizes the three types of RNA.

The nucleus is a large spherical body, usually the most evident intracellular structure, and is surrounded by two unitary membranes that together constitute the nuclear envelope. The surface of the envelope is crossed by nuclear pores, closed by a single thin membrane, which only apparently allow the passage of large specific molecules, thus maintaining the composition of the nuclear material different from that of the cytoplasm. The membrane behaves as if it were semipermeable, where, through it, RNA and ribosomes pass.

Chromosomes, which are composed of DNA and proteins, are found inside the nucleus. When a cell is not in division, the chromosomes are visible as a confused cluster of thin filaments, called chromatin.

The most evident body inside the nucleus is the nucleolus, composed, like chromosomes, from DNA and proteins and formed from a part of a chromosome; this is the site where a particular type of RNA is formed, ribosomal RNA. For this reason the nucleolus (or nuclear organelle) constitutes the main ribosomal RNA deposit.

Certain areas of the endoplasmic reticulum are dynamically connected to the nucleoplasmic membrane. This supports the hypothesis that the nuclear envelope originates from cell membranes.

The functions of the nucleus

The nucleus performs two crucial functions for the cell. First of all it brings to the cell the hereditary information, the instructions that determine if a particular organism will have to develop as paramecium, oak or human being; and not any paramecium, an oak or any human being, but one that resembles the parent or parents of that particular single organism.

Secondly, the nucleus directs the activities of the cell, ensuring that the complex molecules that the cell requires are in the number and type necessary. These molecules are involved in the implementation of various cellular activities and in the formation of organelles and other structures.