n. (pl. nucleoli) a dense spherical structure within the cell *nucleus that disappears during cell division. The nucleolus contains *RNA for the synthesis of *ribosomes and plays an important part in RNA and protein synthesis.
The nucleus of a living cell is a roughly spherical unit at the centre of the cell. It contains the chromosomes (composed mainly of nucleic acid), which are responsible for directing the cell’s activities, and is surrounded by a membrane. The membrane has small pores through which various substances can pass between the nucleus and the cytoplasm, a thick fluid that forms the bulk of the cell. Usually, the nucleus has 1 nucleolus, a smaller dense region with no membrane that is concerned with protein manufacture.
A nerve nucleus is a group of neurons (nerve cells) within the brain and spinal cord that work together to perform a particular function.
The nucleus of an atom, composed of protons and neutrons, accounts for almost the total mass of the atom but only a tiny proportion of its volume. Nuclear energy is produced through changes in atomic nuclei.... nucleus
Every cell consists essentially of a cell-body of soft albuminous material called cytoplasm, in which lies a kernel or nucleus which seems to direct all the activities of the cell. Within the nucleus may be seen a minute body, the nucleolus; and there may or may not be a cell-envelope around all. (See also MITOCHONDRIA.) Each cell nucleus carries a set of identical CHROMOSOMES, the body’s genetic instructions.
Cells vary much in size, ranging in the human body from 0·0025 mm to about 0·025 mm.
All animals and plants consist at ?rst of a single cell (the egg-cell, or ovum), which begins to develop when fertilised by the sperm-cell derived from the opposite sex. Development begins by a division into two new cells, then into four, and so on till a large mass is formed. These cells – among them stem cells (see STEM CELL) which have the potential to develop into a variety of specialised cells – then arrange themselves into layers, and form various tubes, rods, and masses which represent in the embryo the organs of the fully developed animal. (See FETUS.)
When the individual organs have been laid down on a sca?olding of cells, these gradually change in shape and in chemical composition. The cells in the nervous system send out long processes to form the nerves; those in the muscles become long and striped in appearance; and those which form fat become ?lled with fat droplets which distend the cells. Further, they begin to produce, between one another, the substances which give the various tissues their special character. Thus, in the future bones, some cells deposit lime salts and others form cartilage, while in tendons they produce long white ?bres of a gelatinous substance. In some organs the cells change little: thus the liver consists of columns of large cells packed together, while many cells, like the white blood corpuscles, retain their primitive characters almost entire.
Thus cells are the active agents in forming the body, and they have a similar function in repairing its wear and tear. Tumours, and especially malignant tumours, have a highly cellular structure, the cells being of an embryonic type, or, at best, forming poor imitations of the tissues in which they grow (see TUMOUR).... cells
All cells, except red blood cells, have a nucleus, a control centre that governs all major cell activities by regulating the amount and types of proteins made in the cell. Inside the nucleus are the chromosomes, which are made of the nucleic acid DNA. This contains the instructions for protein synthesis, which are carried into the cytoplasm by a type of RNA, another nucleic acid, and are decoded in particles called ribosomes. The nucleus also contains a spherical structure called the nucleolus, which plays a role in the production of ribosomes.
The cell also contains various organelles, each with a specific role.
Energy is generated from the breakdown of sugars and fatty acids by mitochondria.
Substances that would damage the cell if they came into contact with the cytoplasm are contained in particles called lysosomes and peroxisomes.
A system of membranes in the cytoplasm called the endoplasmic reticulum transports materials through the cell.
Flattened sacs called the Golgi complex receive and process proteins dispatched by the endoplasmic reticulum.
Products for export, such as enzymes and hormones, are secreted by vesicles at the cell surface.
Other materials, water, and waste products are transported and stored in the cytoplasm by vacuoles.
The cytoplasm itself has a network of fine tubes (microtubules) and filaments (microfilaments) known as the cytoskeleton, which gives the cell a definite shape.... cell
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