An eukaryotic cell’s nucleus directly or indirectly controls virtually all cellular physiological activities, including initiation, regulation, and termination of enzymatic events.
It is also the repository of genetic information (the genome), housing and protecting the chromosomes and the genes that they carry. In all eukaryotic cells the nucleus is generally the largest and most centrally located cell structure, although in plant storage cells certain kinds of vacuoles may be larger and more conspicuous.
The nucleus, from the Greek word nucle (meaning “pit” or “kernel”) is the command and control center of the cell. The six basic functions of the nucleus are, first, to protect and store genes, ultimately protecting the deoxyribonucleic acid (DNA) on which the genes are organized from the rest of the cell.
Second, to organize genes into chromosomes to facilitate their movement and distribution during cell division; third, to organize the uncoiling of DNA during the copying of genes for the production of thousands of proteins.
Fourth, to manufacture and transport regulatory molecules, mostly enzymes and other gene products, into the cytoplasm; fifth, to manufacture subunits of ribosomes; and sixth, to respond to hormones and other chemical signals received via the nuclear pores.
Structurally, the nucleus consists of several distinct parts: a nuclear envelope, nucleoplasm, chromatin, and one or more sub organelles called nucleoli. The nuclear envelope forms a protective barrier that isolates the nucleus from the cytoplasm of the cell.
The envelope consists of two unit membranes (a double-unit membrane) which are structurally similar to other membranes of the cell. The outer membrane is closely associated with the cell’s endoplasmic recticulum (ER) and may be continuous with it. Like the rough ER of the cytoplasm, the outer nuclear membrane has ribosomes embedded in it.
|Cell nucleus diagram|
At intervals, the nuclear envelope is perforated by small pores which function as communication channels for the controlled exchange of materials between the nucleus and the cytoplasm. Collectively, the nuclear pores cover about 10 percent of the surface of the nucleus.
Each nuclear pore is a complex consisting of a central pore that has been estimated at 30-100 nanometers in diameter. The selectively permeable nuclear pores function as entry and exit ways for a variety of water-soluble molecules, mostly nuclear products, such as ribosome subunits, messenger RNA (ribonucleic acid) molecules, and chromosomal proteins.
The protoplasm within the nucleus is called nucleoplasm. Like cytoplasm, it consists of a jelly like mix of substances and organelles but differs in having a higher concentration of nucleotides and other organic molecules that are used in the synthesis of DNA and RNA.
Major structures within the nucleoplasm include the DNA and usually one organelle—but sometimes several—called the nucleolus. Except during cell division, the molecules of DNA occur as a network of unwound fibers called chromatin. During cell division molecular strands of DNA coil and supercoil around histone proteins to condense and form the chromosomes.
The number of chromosomes found within the nucleus are specific for each species of plant and animal. Humans, for example, have forty-six chromosomes, tobacco has forty-eight, corn has twenty, carrots have eighteen, and peas have fourteen chromosomes.
The nucleolus is the largest visible organelle within the nucleus. It is typically associated with specific regions of chromosomes, called nuclear organizer regions, which contain genes that direct synthesis of ribosomal subunits.
The main products of nucleolus activity are the units of ribosomal RNA (rRNA). These subunits are eventually complexed with ribosomal proteins and transported from the nucleus into the cytoplasm by special carrier proteins.
Other sites within the nucleus called functional domains control the synthesis of messenger (pre-mRNA), transfer (tRNA) molecules. Once formed, these molecules are then complexed with proteins and transported as nucleoproteins to the cytoplasm.
Although seemingly both stable and durable, the nucleus disappears from normal view and is reformed during cell division in almost all plants except yeasts, which retain a clearly defined nucleus throughout the division process.
In other eukaryotic plant cells the nucleus disappears early during the prophase of mitosis, when the nuclear envelope is enzymatically fragmented into small, nearly invisible vesicles. These are not reassembled until the final events of telophase, when they reform around the chromosomes and are controlled by the lamina of the daughter cells.