An organelle is any specialized structure of the cell, especially one for which a specific function can be assigned. as we mentioned earlier ,the name means “little organ” and is based on the analogy that cells have their organelles just as complex multicellular organisms have their livers,kidneys,stomaches,and other organs.
The nucleus is the most prominent organelle of the cell. It is also the most easily stained, so it was described early in the history of cell study. In fact, the word, which means “kernel”, was first used to describe cell nuclei in 1831, about the time of the emergence of the cell theory. Nuclei have since been found in virtually every type of eukaryotic cell.
The nucleus is where the chromosomes are and chromosomes contain the organism’s genes, which control its heredity. People did not always know that, of course, but it was realized quite early that the nucleus was somehow important .in no dividing cells, basic stains will reveal a dark nucleus laden with the DNA of chromosomes. In many cells, darker bodies can be seen inside the nuclei. These are the nucleoli. The nucleoli are rich in a different nucleic acid, RNA; each nucleolus is a great lump of material from which new ribosomes will be made at about the time of the next cell division.
The nucleus of an undiving cell is almost as featureless in an electron micrograph as it is under the light micrograph as it is under the light microscope .however, its membrane is interesting. We find that the nucleus is enclosed in a double membrane. Each of the two parts of the double membrane is a regular lipid bilayer, so altogether there are four layers of lipid molecules and the usual associated proteins and carbohydrates. The inner and outer nuclear membranes pinch together in scattered places over the nuclear surface to form nuclear pores. Under the electron microscope these pores look just like holes communicating between the nucleoplasm inside the nucleus and the cytoplasm outside the nucleus, these pores will be helpful for the travel of some large molecules like ribosomes and messenger RNA molecule from nucleus to cytoplasm.
The nucleus has two important functions. First it contains the hereditary information of the cell. All the genetic instructions for development,metabolism,and behavior of the species are found in the DNA within the nucleus .each cell nucleus of a multicellualar organism has a complete copy of all the information needed to produce the organism .In its role as guardian and carrier of genetic information, the nucleus has the capability of preparing extract duplicates of this information for transmission to new generations of cells .Secondly the nucleus has the machinery for controlling various cellular activities .It does this by directing the synthesis of enzymes that are present in the cytoplasm through various mechanisms.
The endoplasmic reticulum:
The endoplasmicreticulum was unknown in the days when we were restricted to light microscopes. in fact in those days the contents of cells were thought to be formless, soupy”protoplasm” but electron micrographs reveals a complex membrane system that takes up a large part of the cytoplasm of eukaryote cells ,especially those cells that are engaged in significant protein synthesis .Although the ER system was first seen in 1940s it wasn’t until 1953 that Keith Porter of the Rockefeller Institute first suggested the name .close study of serial sections reveal the ER to be a system of broad sheets forming channels and that the ER membrane is often continuous with both the cell membrane and the outer membrane of the nucleus .this suggest that the ER may be involved in the transportation of materials between the cells environment and its nucleus.