Nucleus

The nucleus is the control center that directs the activities of the cell. Most important is its control of cell reproduction and the construction of materials like proteins. The nucleus also functions as the cell's main repository of genetic information in the form of deoxyribonucleic acid (DNA).

A eukaryotic cell is a cell that contains a separate nucleus. Plants, animals, fungi, and some forms of single-celled life are eukaryotic or eukaryotes. Those living things, like bacteria, whose cells do not have a distinct nucleus are called prokaryotes. A eukaryotic cell contains many structures called organelles, each of which has a separate function. The most important organelle in a cell is its nucleus (named after a Latin word meaning "kernel" because it looks like a seed in the center of a fruit). If a cell can be described as a miniature factory in which conditions are carefully regulated, then the nucleus in the cell is the factory's main office or control center. The nucleus is a very busy place as it simultaneously controls many cellular activities, responds to changes in its environment, and helps make ribonucleic acid (RNA). As the heart of every cell, the nucleus is easily seen when a cell is observed with a microscope. Because of its large size, the nucleus is by far the outstanding feature of every cell. It usually appears as a rounded structure near the center of the cell. Besides its size, what makes the nucleus so distinct is that it is surrounded by a double membrane called the nuclear envelope. This keeps the nucleus separate from the rest of the cell's living material called the cytoplasm. This envelope has many tiny openings known as pores that allow certain substances to pass in and out of the nucleus to the rest of the cell.

Inside the nucleus are two important structures: threadlike structures called chromosomes and small, round structures called nucleoli. The chromosomes contain the cell's genetic material called DNA, which contains all the instructions needed to make a cell work. DNA also contains a cell's genes, which are the basic units of heredity. The number of chromosomes a nucleus contains will change from species to species (humans have forty-six chromosomes). Also found in the nucleus are one or more small, round nucleoli (singular, nucleolus) which help the cell make ribosomes. Ribosomes are organelles that play an important role in the manufacture of proteins. The nucleolus sends RNA to the ribosomes, which use amino acids to make proteins. Ribosomes are outside the nucleus. The nucleus of a cell

is therefore the control center of the cell because it directs the cell's activities by controlling the synthesis of production of proteins. It is with these chemicals called proteins that the nucleus actually runs the cell. Proteins are also a key ingredient in the material out of which cells, are made.

[See alsoCell; Cell Division; Chromosome; DNA; Protein ]