Delphinidin

The nucleus is the control center of the cell. In some ways, it functions like a combination of a computer program and a dispatcher that sends coded messages or “blueprints” originating from DNA in the nucleus with information that will ultimately be used in other parts of the cell. In other words, the DNA in the nucleus provides the original information needed to fulfill the cell’s needs. This nuclear information contributes toward growth, differentiation, and the myriad activities of the complex cell “factory.” The nucleus also stores hereditary information, which is passed from cell to cell as new cells are formed.

The nucleus often is the most conspicuous object in a living cell, although in green cells, chloroplasts may obscure it. In living cells without chloroplasts, the nucleus may appear as a grayish, spherical to ellipsoidal lump, sometimes lying against the plasma membrane to one side of the cell or toward a corner. Some nuclei are irregular in form, and they can vary greatly in size. They are, however, generally from 2 to 15 micrometers or larger in diameter. Certain fungi and algae have numerous nuclei within a single extensively branched cell, but the cells of more complex plants usually have a single nucleus.

Each nucleus is bounded by two membranes, which together constitute the nuclear envelope. Structurally complex pores, about 50 to 75 nanometers apart, occupy up to onethird of the total surface area of the nuclear envelope. Proteins that act as channels for molecules are embedded within the pores. The pores apparently permit only certain kinds of molecules (for example, proteins being carried into the nucleus and RNA being carried out) to pass between the nucleus and the cytoplasm.

The nucleus contains a granular-appearing fluid called nucleoplasm, which is packed with short fibers that are about 10 nanometers in diameter; several different larger bodies are suspended within it. Of the larger nuclear bodies, the most noticeable are one or more nucleoli (singular: nucleolus), which are composed primarily of RNA and associated proteins.

Other important nuclear structures, which are not apparent with light microscopy unless the cell is stained or is in the process of dividing, include thin strands of chromatin. When a nucleus divides, the chromatin strands coil, becoming shorter and thicker, and in their condensed condition, they are called chromosomes. Chromatin is composed of protein and DNA. Each cell of a given plant or animal species has its own fixed number and composition of chromosomes; the cells involved in sexual reproduction have half the number found in other cells of the same organism.

The number of chromosomes present in a nucleus normally bears no relation to the size and complexity of the organism. Each body cell of a radish, for example, has 18 chromosomes in its nucleus, while a cell of one species of goldenweed has 4, and a cell of a tropical adder’s tongue fern has over 1,000. Humans have 46 chromosomes in each body cell.