Molecular Biology of the Cell - Volume 2 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1993

Intracellular sorting of macromolecules and maintenance of cellular compartments
Cell nucleus

Almost all of the DNA in a Introduction/5.html">Eukaryotic Cell is housed within The Nucleus, which accounts for approximately 10% of the total cell volume. The nucleus is surrounded by a nuclear envelope consisting of two concentric membranes. The nuclear membranes are perforated at intervals by nuclear pores, which play a crucial role in mediating The transport of specific molecules into and out of the Cytoplasm. The nuclear envelope is directly continuous with The Endoplasmic reticulum. Both of its surfaces are lined by mesh-like networks of Intermediate filaments. The network lining the inner nuclear membrane appears as a thin sheet known as the nuclear lamina, whereas the network surrounding the outer nuclear membrane is much less compact (Fig. 9-1).

Like living prokaryotic organisms, the ancestors of Eukaryotic Cells presumably lacked a nucleus (see Section 8.1.2), and one can only speculate on why such a distinct Structure evolved. Clues to the possible reasons for sequestering DNA away from the cytoplasm come from two specific properties of eukaryotic cells. One of these is the existence of a Cytoskeleton composed mainly of microtubules and Actin filaments, which are involved in eukaryotic cell motility (see Chapter 11). Bacteria, whose DNA is in direct contact with the cytoplasm, lack such filaments and move via external appendages. It has been suggested that one of the Functions of the eukaryotic nuclear envelope is to protect their long, fragile DNA molecules from the mechanical stresses exerted by the cytoskeleton.

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Fig. 9-1. Cross-section of a typical Cell Nucleus. The nuclear envelope consists of two membranes, with the outer membrane being continuous with the membrane of the endoplasmic reticulum (see also Fig. 8-19). The lipid bilayers of the inner and outer nuclear membranes join at the nuclear pores. Two networks of filamentous intermediate fibers (colored lines) provide mechanical stability to the nuclear envelope, with the fibers lining the interior forming the nuclear lamina.

Fig. 9-2. Protein Synthesis in eukaryotes (DNA -> RNA -> protein). Due to the nuclear envelope, active Ribosomes are segregated from the nucleus; consequently, RNA transcripts undergo Processing before being exported to the cytoplasm, where Translation takes place. Thus, RNA Processing and transport take place between DNA METABOLISM/31.html">Transcription and RNA translation.

The second major feature of eukaryotic cells is the occurrence of RNA processing, which takes place before the RNA is translated into protein. During the evolution of The Eukaryotic Cell, numerous membrane-enclosed compartments arose, each dedicated to a distinct set of Chemical Reactions, and the nucleus can be viewed as one such compartment. In Prokaryotic Cells, by contrast, RNA Synthesis (transcription) and protein synthesis (translation) occur simultaneously: ribosomes begin translating a protein from the 5' end of an RNA molecule while transcription is still ongoing at the 3' end. Consequently, there is little opportunity to modify RNA transcripts prior to protein translation. In eukaryotes, however, transcription and translation are segregated both in time and in space: transcription takes place in the nucleus, whereas translation occurs in the cytoplasm. RNA transcripts are immediately packaged into ribonucleoprotein complexes, which facilitate their subsequent processing. During processing, specific nucleotide sequences are removed from the RNA, and the remaining segments are spliced together. This step in the flow of Genetic information in eukaryotes is of paramount importance. Only after splicing is complete are the packaging Proteins stripped off, and the mature RNA molecules are exported from the nucleus to the cytoplasm, where ribosomes initiate protein translation from the RNA (Fig. 9-2).

RNA splicing allows a single Gene to encode multiple distinct proteins (see Section 10.4.2) and confers several other evolutionary advantages upon The Cell (see Section 10.5.3). It is quite plausible that the primary biological driving force behind the Evolution of the nucleus in eukaryotic cells was precisely The Need for a nuclear compartment that allows efficient RNA splicing (Fig. 9-3). A hypothetical Scheme for the Water/144.html">Origin of the nucleus is presented in Fig. 8-4.

The structure of the nuclear envelope is discussed in detail in Chapter 8 in the context of selective macromolecular transport (see Section 8.3). In this chapter, we explore how proteins package DNA into Chromosomes, how chromosomes are folded and organized within the nucleus, and how they are replicated during the S phase of each Cell Cycle. Particular attention will be devoted to RNA synthesis and splicing—the most prominent events occurring in the interphase nucleus. Mechanisms controlling Gene Expression are discussed in Chapter 10.

Fig. 9-3. The nuclear envelope separates the nucleus from cytoplasmic Organelles. This electron micrograph shows a thin section of a sea urchin oocyte, whose nucleus stains remarkably evenly while the cytoplasm is densely packed with organelles. (Courtesy of David Begg and Tim Hunt.)