The basic theoretical items of information

The nucleus. The discovery of the cell nucleus probably dates back to 1710 when Van Leeuwenhoek noted a centrally located "clear" area in living blood cells of birds and amphibians. Although credit is usually given to Fontana, who observed (in 1781) an ovoid structure in isolated eel skin cells. However, the current concept of the nucleus has its origins in the work of the botanist Robert Brown, who gave the structure its name (nucleus) and provided the first precise morphological description in 1831. Since then, much has been learned about its morphology, biochemistry, and function of nucleus.

The nucleus is the central device of a cell(figure 21.1). The functions of nucleus are:

1. Storage of the genetic information

2. Transfer of the genetic information from cell to cell or from generation to next generation

3. Realization of the genetic information in processes of synthesis of several kinds of DNA and proteins.

4. Reduplication of the genetic information.

Nucleus consists of double nuclear membrane, nucleoplasm, nucleolus and chromatin.

There are two biological membranes in the nuclear membrane and there is a perinuclear space between these membranes.

The nuclear membranes have numerous pores ensuring selective connection with cytoplasm. Selective transport of proteins to and from the nucleus occurs through nucleus pore. Each pore has a nuclear pore complex. The complex consists of an assembly of eight spokes attached to ring on the cytoplasmic and nuclear sides of the nuclear envelope. The spoke-ring assembly surrounds a central transporter. Cytoplasmic filaments extend from the cytoplasmic ring, and filaments forming the nuclear basket extend from the nuclear ring.

Under the nuclear membrane a nuclear lamina is laid. It is a fibrous meshwork that provides structural support to the nucleus. The nuclear lamina is composed of one or more proteins caleed lamins. Most mammalian cells, for example, contain four different lamins, designated A, B1, B2, and C. Like other intermediate filament proteins, the lamins associate with each other to form filaments. Two nuclear membranes, nuclear lamina and nuclear pore complexes form a nuclear envelope.

Nucleoplasm is a very complex solution of organic and inorganic materials. It is environment for passing of biochemical reactions and existence of another parts of nucleus.

The important component of a core is one or several nucleoluses. It is round body with the high contents RNA and protein. Nucleolus is the place of ribosome’s RNA synthesis and processing, and assembly of the ribosomal subunits. Following each cell division, nucleoli form around chromosomal regions that contain the 5.8S, 18S, and 28S rRNA genes, which are therefore called nucleolar-organizing regions.

Morphologically, nucleoli consist of three distinguishable regions: the fibrillar center, dense fibrillar component, and granular component. These different regions are thought to represent the sites of progressive stages of rRNA transcription, processing and ribosome assembly. In the granular component the rRNA is assembled with ribosomal proteins to form nearly completed preribosomal subunits, ready for export to the cytoplasm. Eukariotic ribosomes contain four types of rRNA, designated the 5S, 5.8S, 18S, and 28S. The 5.8S, 18S, and 28S rRNAs are transcribed as a singl unit within the nucleolus by RNA polymerase I. Transcription of the 5S rRNA takes place outside of nucleolus and is catalyzed by RNA polymerase III.

Figure 21.1a Schematic representation of the arrangement of nucleus:

1 - nuclear membrane, 2 - pore, 3 - nucleoplasm, 4 - chromatin, 5 - nucleolus, 6 - endoplasmatic reticulum.

Дата добавления: 2015-11-16