The Nucleus

The largest and most easily seen of all the organelles within a eukaryotic cell is the nucleus. The word “nucleus” is derived from the Greek word for a nut. A cell may be mononucleate, binucleate or multinucleate. In animal cells the nucleus is typically located in the central region. It controls all the activities of the cell. A typical nucleus is about 10 mille micron in diameter. Nucleus consists of nuclear membrane, nucleoplasm, nucleolus and chromosomes.

Nuclear membrane or nuclear envelope

A double membrane bounds the surface of the nucleus. The outer membrane is continuous with ER. It is believed that it has been formed by ER. It is covered with ribosomes. The nuclear membrane has many nuclear pores. The pores are embedded with many proteins, permitting certain molecules to pass into and out of the nucleus, i.e. nuclear pore allows exchange of substances between the nucleus and the cytoplasm.

                                       The Nucleus

Nucleoplasm

The fluid inside the nucleus is called nucleoplasm.

Nucleolus

The dark staining region in the nucleus is called nucleolus. (Plural: nucleoli). A cell may have one or more nucleoli. Nucleolus consists of ribosomal ribonucleic acid and some ribosomal proteins. It stores RNA synthesized by DNA. During nuclear division nucleoli seem to disappear. The rRNA and proteins make ribosomes. The partly assembled ribosomes move out through the nuclear pores into the cytoplasm where assembly is completed.

Chromatin and Chromosome

Chromatin (Chroma: color, and teino stretch) Looks grainy, but actually it is a threadlike material that undergoes coiling into rod like structures called chromosomes (Gk, Chroma, color, soma, body) just before cell division.

Chromosomes

Chromosomes are separate thread like structures in nucleus. During cell division they stain heavily, so they are visible only during cell division. At other times they lose their ability to stain. They maintain their structural integrity at all times. Each chromosome is bounded by delicate membrane. The centromere is a constriction functionally related to the movement of chromosomes during cell division. Each centromere has two plaques of proteins called kinetochores that are oriented on the opposite sides of the constriction. Each kinetochore forms the site of attachment for a single microtubule during cell division. E.M. studies reveal that chromosomes are composed of lengthwise microfibrils. Each species of animals and plants has a characteristic number of chromosome e.g. human 46, frog 26, chimpanzee 48 and fruit fly (Drosophila) 8. The number varies from 2 to 100 in other species. Individual chromosomes can be identified by their size and shape. Chemically chromosomes consist of DNA and histone proteins. DNA is the genetic material. It is transferred from one generation to the next.

Chromosome

Cytoplasmic Organelles and Membrane System

The protoplasm outside the nucleus is called cytoplasm. Various organelles are suspended within the fluid component of the cytoplasm called cytosol. Therefore, the term cytoplasm includes both, the cytosol and all the organelles other than the nucleus. Membranes have unique properties that enable membranous organelles to carry out a variety of functions. The membrane-bounded compartments allow certain cellular activities to be localized within specific enclosed regions of the cell.