Nucleus
The nucleus is a vital organelle found in eukaryotic cells, serving as the command center that regulates cellular functions. Its primary roles include protecting and storing genetic material, organizing genes into chromosomes for cell division, and facilitating the synthesis of proteins by copying DNA. Structurally, the nucleus is encapsulated by a nuclear envelope, which consists of two membranes and is interconnected with the endoplasmic reticulum. Inside, the nucleoplasm houses DNA, chromatin, and nucleoli, the latter playing a crucial role in ribosomal RNA synthesis and ribosome assembly. The nuclear envelope features pores that allow selective exchange of molecules between the nucleus and the cytoplasm, ensuring efficient communication within the cell. During cell division, the nucleus temporarily disassembles and re-forms around the chromosomes, a process crucial for the accurate distribution of genetic material. The number of chromosomes varies among species, highlighting the diversity of life at the cellular level. Understanding the nucleus is essential for comprehending cellular biology and the fundamental processes that govern life.
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Nucleus
Categories: Anatomy; cellular biology; physiology
The nucleus, from the Latin word nucleum (meaning “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.
Components
Structurally, the nucleus consists of several distinct parts: a nuclear envelope, nucleoplasm, chromatin, and one or more suborganelles 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) that 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. Some scientists, in fact, suggest that the nuclear envelope is just a localized and specialized version of the ER. The inner nuclear membrane is lined with a fibrous layer, called the nuclear lamina, which provides strength and structure to the nucleus shape and may also serve as a binding site for some chromatin.
At intervals, the nuclear envelope is perforated by small pores that 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 thirty to one hundred 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 jellylike 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 the 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.
Cell Division
Although seemingly both stable and durable, the nucleus disappears from normal view and is reformed during cell division in almost all eukaryotic cells. (An exception is yeasts, which retain a clearly defined nucleus throughout the division process.) In eukaryotic 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.
Bibliography
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