BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.10. Cell Structures

5.10.1. The Nucleus

Nuclei are present in all Eukaryotic Cells, with the exception of mature sieve tube elements in the phloem and mature mammalian erythrocytes. Some protists, notably Paramecium, possess two nuclei—a micronucleus and a macronucleus. Generally, however, cells contain only a single nucleus. When observed under a Light Microscope, nuclei immediately stand out because they are the largest of all Cell Organelles. For this reason, they were also the first cellular structures to be described in early microscopical studies. The diameter of a nucleus is typically around 10 µm.

The Nucleus is essential for cell survival because it regulates all cellular activity. This is because the nucleus carries genetic (hereditary) information encoded in DNA. DNA has The ability to replicate, and its Replication precedes nuclear division, ensuring that daughter nuclei also receive DNA. Nuclear division, in turn, precedes Cell Division, so that all daughter cells likewise contain nuclei. The nucleus is surrounded by a nuclear envelope and contains Chromatin, as well as one or more nucleoli.

Nuclear Envelope and Nuclear Pores

Under the light microscope, the membrane surrounding the nucleus appears to be single, but in reality, it is a nuclear envelope consisting of two membranes. The outer membrane is continuous with The Endoplasmic reticulum (ER), as shown in Figs. 5.10, 5.11, and 5.24, and like the ER, it may be studded with Ribosomes engaged in Protein Synthesis. The nuclear envelope is perforated by nuclear pores (Fig. 5.24). These are particularly prominent in preparations obtained by freeze-etching (Fig. 5.25). Various substances are exchanged between the nucleus and the Cytoplasm through nuclear pores, such as the export of Messenger RNA (mRNA) and ribosomal subunits into the cytoplasm, or the import of ribosomal Proteins, NUCLEOTIDES, and molecules regulating DNA activity (e.g., certain Hormones) into the nucleus. The pores have a definite Structure resulting from the fusion of the outer and inner membranes of the nuclear envelope. This structure regulates the passage of molecules through the pore.

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Fig. 5.24. Transmission electron micrograph of a Cell Nucleus. Note that the endoplasmic reticulum forms a continuous structure with the nuclear envelope.

Fig. 5.25. Electron micrograph of a nucleus showing nuclear pores. (Preparation obtained by freeze-etching.) ×30,000. In preparing such specimens, rapidly frozen cells are fractured with a metal blade. The fracture plane follows the line of least resistance, frequently passing through the membrane. After the ice is removed, the etched surface is exposed.

Chromatin

Chromatin consists of numerous DNA coils attached to Histones, which are basic proteins. DNA strands are so exceptionally long (in each human somatic cell nucleus, their average length is about 1 m!) that they must be packaged in an orderly fashion; otherwise, they would become tangled like an uncoiled ball of twine. Histones and DNA are assembled into bead-like structures called nucleosomes. The packing of nucleosomes within chromatin is also regular in nature.

The word "chromatin" translates to "colored material," and it was so named because it stains readily during preparation for light Microscopy. During nuclear division, chromatin stains more intensely and thus becomes more conspicuous due to its Condensation—The formation of tightly coiled (spiraled) threads known as Chromosomes. During interphase (the period between two nuclear divisions), chromatin reverts to a more dispersed state. However, a portion of it remains tightly coiled and continues to stain intensely. This fraction is called heterochromatin; heterochromatin appears as characteristic dark patches, usually located closer to the nuclear envelope (Figs. 5.10–13 and 5.24). The remaining, more loosely coiled chromatin is termed euchromatin. It is believed to harbor the DNA that is genetically active during interphase.

The Nucleolus

The nucleolus is a prominent, rounded structure located within the nucleus (Figs. 5.12 and 5.24); it is the site of ribosome formation. A nucleus may contain one or more nucleoli. The nucleolus stains intensely because it contains large amounts of DNA and RNA. RNA is structurally similar to DNA since it is "copied" (transcribed) from DNA. Within the nucleolus, There is a distinct dense region containing the DNA of one or more chromosomes. This area concentrates numerous copies of Genes encoding ribosomal RNA (rRNA). During nuclear division, the nucleolus becomes invisible because the DNA disperses. Upon completion of division, the nucleolus reappears.

The central region of the nucleolus is surrounded by a less dense peripheral region where the coiling of ribosomal RNA begins and ribosome assembly takes place—RNA combines with protein. Partially assembled ribosomes pass through the nuclear pores from the nucleus into the cytoplasm, where their assembly is completed.